MAX1771C/D [MAXIM]
12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller; 12V或可调，高效率，低IQ ，升压型DC- DC控制器
| 型号： | MAX1771C/D |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller |
| 文件： | 总16页 (文件大小：197K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-0263; Rev 1; 7/95
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
MAX71
_______________Ge n e ra l De s c rip t io n
____________________________Fe a t u re s
♦ 90% Efficiency for 30mA to 2A Load Currents
♦ Up to 24W Output Power
The MAX1771 step-up switching controller provides
90% efficiency over a 30mA to 2A load. A unique cur-
rent-limited pulse-frequency-modulation (PFM) control
scheme gives this device the benefits of pulse-width-
modulation (PWM) converters (high efficiency at heavy
loads), while using less than 110µA of supply current (vs.
2mA to 10mA for PWM converters).
♦ 110µA Max Supply Current
♦ 5µA Max Shutdown Current
♦ 2V to 16.5V Input Range
This controller uses miniature external components. Its
high switching frequency (up to 300kHz) allows sur-
face-mount magnetics of 5mm height and 9mm diame-
ter. It accepts input voltages from 2V to 16.5V. The
output voltage is preset at 12V, or can be adjusted
using two resistors.
♦ Preset 12V or Adjustable Output Voltage
♦ Current-Limited PFM Control Scheme
♦ Up to 300kHz Switching Frequency
♦ Evaluation Kit Available
The MAX1771 optimizes efficiency at low input voltages
and reduces noise by using a single 100mV current-limit
threshold under all load conditions. A family of similar
devices, the MAX770–MAX773, trades some full-load
efficiency for greater current-limit accuracy; they provide
a 200mV current limit at full load, and switch to 100mV
for light loads.
______________Ord e rin g In fo rm a t io n
PART
TEMP. RANGE
0°C to +70°C
PIN-PACKAGE
8 Plastic DIP
8 SO
MAX1771CPA
MAX1771CSA
MAX1771C/D
MAX1771EPA
MAX1771ESA
MAX1771MJA
The MAX1771 drives an external N-channel MOSFET
switch, allowing it to power loads up to 24W. If less power
is required, use the MAX756/MAX757 or MAX761/MAX762
step-up switching regulators with on-board MOSFETs.
0°C to +70°C
0°C to +70°C
Dice*
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
8 Plastic DIP
8 SO
An evaluation kit is available. Order the MAX1771EVKIT-SO.
8 CERDIP**
________________________Ap p lic a t io n s
Positive LCD-Bias Generators
* Contact factory for dice specifications.
** Contact factory for availability and processing to MIL-STD-883B.
Flash Memory Programmers
High-Power RF Power-Amplifier Supply
Palmtops/Hand-Held Terminals
Battery-Powered Applications
__________________P in Co n fig u ra t io n
Portable Communicators
__________Typ ic a l Op e ra t in g Circ u it
TOP VIEW
INPUT
2V TO V
OUT
OUTPUT
12V
1
2
3
4
8
7
6
5
EXT
V+
CS
N
EXT
CS
MAX1771
SHDN
GND
AGND
MAX1771
ON/OFF
FB
REF
SHDN
REF
V+
DIP/SO
FB AGND GND
________________________________________________________________ Maxim Integrated Products
1
Ca ll t o ll fre e 1 -8 0 0 -9 9 8 -8 8 0 0 fo r fre e s a m p le s o r lit e ra t u re .
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
ABSOLUTE MAXIMUM RATINGS
Supply Voltage
Operating Temperature Ranges
V+ to GND ...............................................................-0.3V, 17V
EXT, CS, REF, SHDN, FB to GND ...................-0.3V, (V+ + 0.3V)
GND to AGND.............................................................0.1V, -0.1V
MAX1771C_A .....................................................0°C to +70°C
MAX1771E_A ..................................................-40°C to +85°C
MAX1771MJA ................................................-55°C to +125°C
Junction Temperatures
Continuous Power Dissipation (T = +70°C)
A
Plastic DIP (derate 9.09mW/°C above +70°C) ............727mW
SO (derate 5.88mW/°C above +70°C).........................471mW
CERDIP (derate 8.00mW/°C above +70°C).................640mW
MAX1771C_A/E_A.......................................................+150°C
MAX1771MJA ..............................................................+175°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
MAX71
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V+ = 5V, I
= 0mA, T = T
to T , unless otherwise noted. Typical values are at T = +25°C.)
MAX A
LOAD
A
MIN
PARAMETER
SYMBOL
CONDITIONS
MIN
2.0
3.0
3.1
TYP
MAX
12.5
16.5
16.5
2.0
UNITS
MAX1771 (internal feedback resistors)
MAX1771C/E (external resistors)
MAX1771MJA (external resistors)
Input Voltage Range
V
Minimum Start-Up Voltage
Supply Current
1.8
85
2
V
µA
V+ = 16.5V, SHDN = 0V (normal operation)
V+ = 10.0V, SHDN ≥ 1.6V (shutdown)
V+ = 16.5V, SHDN ≥ 1.6V (shutdown)
110
5
µA
V
Standby Current
4
V+ = 2.0V to 12.0V, over full load range,
Circuit of Figure 2a
Output Voltage (Note 1)
11.52
12.0
5
12.48
Output Voltage Line Regulation
(Note 2)
V+ = 5V to 7V, V
= 12V
OUT
mV/V
I
= 700mA, Circuit of Figure 2a
LOAD
Output Voltage Load Regulation
(Note 2)
V+ = 6V, V
500mA, Circuit of Figure 2a
= 12V, I
= 0mA to
OUT
LOAD
20
mV/A
µs
µs
Maximum Switch On-Time
Minimum Switch Off-Time
t
(max)
12
16
20
ON
t
(min)
1.8
2.3
2.8
OFF
V+ = 5V, V
Circuit of Figure 2a
= 12V, I
LOAD
= 500mA,
OUT
Efficiency
92
%
MAX1771C
MAX1771E
MAX1771M
MAX1771C/E
MAX1771M
1.4700
1.4625
1.4550
1.5
1.5
1.5
4
1.5300
1.5375
1.5450
10
I
0µA
Reference Voltage
V
V
REF =
REF
REF Load Regulation
REF Line Regulation
0µA ≤ I
≤ 100µA
mV
REF
4
15
3V ≤ V+ ≤ 16.5V
MAX1771C
40
1.5
1.5
1.5
100
µV/V
1.4700
1.4625
1.4550
1.5300
1.5375
1.5450
FB Trip Point Voltage
V
MAX1771E
V
FB
MAX1771M
_______________________________________________________________________________________
2
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
MAX71
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, I
= 0mA, T = T
to T , unless otherwise noted. Typical values are at T = +25°C.)
MAX A
LOAD
A
MIN
PARAMETERS
SYMBOL
CONDITIONS
MIN
TYP
MAX
±20
±40
±60
UNITS
MAX1771C
MAX1771E
MAX1771M
FB Input Current
I
FB
nA
SHDN Input High Voltage
SHDN Input Low Voltage
SHDN Input Current
V
V+ = 2.0V to 16.5V
1.6
V
V
IH
V
IL
V+ = 2.0V to 16.5V
0.4
±1
µA
V+ = 16.5V, SHDN = 0V or V+
MAX1771C/E
MAX1771M
85
75
100
100
0.01
55
115
125
±1
Current-Limit Trip Level
V
CS
V+ = 5V to 16V
mV
µA
ns
CS Input Current
EXT Rise Time
EXT Fall Time
V+ = 5V, 1nF from EXT to ground
V+ = 5V, 1nF from EXT to ground
55
ns
Note 1: Output voltage guaranteed using preset voltages. See Figures 4a–4d for output current capability versus input voltage.
Note 2: Output voltage line and load regulation depend on external circuit components.
__________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
(T = +25°C, unless otherwise noted.)
A
LOAD CURRENT vs.
MINIMUM START-UP INPUT VOLTAGE
EFFICIENCY vs. LOAD CURRENT
(BOOTSTRAPED MODE)
EFFICIENCY vs. LOAD CURRENT
(NON-BOOTSTRAPED MODE)
700
600
100
100
95
V
= 12V, CIRCUIT OF FIGURE 2a
OUT
95
90
V
IN
= 10V
V
IN
=10V
EXTERNAL FET THRESHOLD
LIMITS FULL-LOAD START-UP
BELOW 3.5V
90
V
= 8V
IN
500
400
300
200
V
= 8V
IN
85
80
85
80
V
IN
= 3V
V
= 5V
IN
75
70
75
70
V
= 5V
IN
V
= 12V
V
= 12V
OUT
OUT
CIRCUIT OF
FIGURE 2a
CIRCUIT OF
FIGURE 2b
100
0
65
60
65
60
1
10
1000
10,000
1
10
100
1000
10,000
2.00 2.25 2.50 2.75 3.00 3.25 3.50
MINIMUM START-UP INPUT VOLTAGE (V)
100
LOAD CURRENT (mA)
LOAD CURRENT (mA)
_______________________________________________________________________________________
3
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(T = +25°C, unless otherwise noted.)
A
SUPPLY CURRENT vs. TEMPERATURE
EXT RISE/FALL TIME vs. SUPPLY VOLTAGE
250
SUPPLY CURRENT vs. SUPPLY VOLTAGE
4
0.8
0.6
0.4
V
OUT
= 12V, V = 5V
IN
V
OUT
= 12V
CIRCUIT OF FIGURE 2a
BOOTSTRAPPED MODE
200
150
3
2
C
= 2200pF
= 1000pF
= 446pF
= 100pF
BOOTSTRAPPED
CIRCUIT OF
FIGURE 2a
EXT
MAX71
ENTIRE
CIRCUIT
C
EXT
C
EXT
C
EXT
100
50
0
SCHOTTKY DIODE
LEAKAGE EXCLUDED
1
0
0.2
0
NON-BOOTSTRAPPED
CIRCUIT OF FIGURE 2b
12
-50 -25
0
10
-75
25 50 75 100 125
12
2
6
8
10
4
2
6
8
4
TEMPERATURE (°C)
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
REFERENCE OUTPUT RESISTANCE vs.
TEMPERATURE
MAXIMUM SWITCH ON-TIME vs.
TEMPERATURE
REFERENCE vs. TEMPERATURE
16.5
16.0
250
200
1.506
1.504
1.502
10µA
150
100
50
1.500
1.498
1.496
1.494
1.492
50µA
100µA
15.5
0
-30
0
30
60
120 150
-60
90
-60 -40 -20
0
20 40 60 80 100 120 140
-60 -40 -20
0
20 40 60 80 100 120 140
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
MAXIMUM SWITCH ON-TIME/
MINIMUM SWITCH OFF-TIME RATIO
vs. TEMPERATURE
MINIMUM SWITCH OFF-TIME vs.
TEMPERATURE
SHUTDOWN CURRENT vs. TEMPERATURE
4.0
3.5
3.0
2.5
2.30
2.25
8.0
7.5
7.0
2.0
1.5
1.0
0.5
0
V+ = 15V
V+ = 8V
6.5
6.0
V+ = 4V
2.20
-30
0
30
60
120 150
-30
0
30
60
120 150
-60 -40 -20
0
20 40 60 80 100 120 140
-60
90
-60
90
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
_______________________________________________________________________________________
4
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
MAX71
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(Circuit of Figure 2a, T = +25°C, unless otherwise noted.)
A
HEAVY-LOAD SWITCHING WAVEFORMS
MEDIUM-LOAD SWITCHING WAVEFORMS
V
V
OUT
OUT
A
B
C
A
B
0V
0V
I
LIM
I
LIM
0A
0A
C
2µs/div
= 5V, I = 900mA, V = 12V
10µs/div
V
IN
V = 5V, I = 500mA, V = 12V
IN OUT OUT
OUT
OUT
A: EXT VOLTAGE, 10V/div
A: EXT VOLTAGE, 10V/div
B: INDUCTOR CURRENT, 1A/div
B: INDUCTOR CURRENT, 1A/div
C: V RIPPLE, 50mV/div, AC-COUPLED
C: V RIPPLE, 50mV/div, AC-COUPLED
OUT
OUT
LINE-TRANSIENT RESPONSE
LOAD-TRANSIENT RESPONSE
A
7V
5V
500mA
0A
A
B
0V
B
5ms/div
5ms/div
I
= 700mA, V = 12V
OUT
V
IN
= 6V, V = 12V
OUT
OUT
A: V , 5V to 7V, 2V/div
IN
A: LOAD CURRENT, 0mA to 500mA, 500mA/div
B: V RIPPLE, 100mV/div, AC-COUPLED
OUT
B: V RIPPLE, 100mV/div, AC-COUPLED
OUT
_______________________________________________________________________________________
5
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(Circuit of Figure 2a, T = +25°C, unless otherwise noted.)
A
ENTERING/EXITING SHUTDOWN
MAX71
A
B
0V
5V
0V
2ms/div
I
= 500mA, V = 5V
IN
OUT
A: SHDN, 5V/div
B: V , 5V/div
OUT
______________________________________________________________P in De s c rip t io n
PIN
NAME
FUNCTION
1
2
EXT
V+
Gate Drive for External N-Channel Power Transistor
Power-Supply Input. Also acts as a voltage-sense point when in bootstrapped mode.
Feedback Input for Adjustable-Output Operation. Connect to ground for fixed-output operation.
Use a resistor divider network to adjust the output voltage. See Setting the Output Voltage section.
3
4
5
FB
Active-High TTL/CMOS Logic-Level Shutdown Input. In shutdown mode, V
is a diode drop
OUT
below V+ (due to the DC path from V+ to the output) and the supply current drops to 5µA
maximum. Connect to ground for normal operation.
SHDN
REF
1.5V Reference Output that can source 100µA for external loads. Bypass to GND with 0.1µF.
The reference is disabled in shutdown.
6
7
AGND
GND
Analog Ground
High-Current Ground Return for the Output Driver
Positive Input to the Current-Sense Amplifier. Connect the current-sense resistor between CS
and GND.
8
CS
_______________________________________________________________________________________
6
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
MAX71
range of load currents; and 3) the maximum supply
current is only 110µA.
_______________De t a ile d De s c rip t io n
The MAX1771 is a BiCMOS, step-up, switch-mode pow-
er-supply controller that provides a preset 12V output,
in addition to adjustable-output operation. Its unique
control scheme combines the advantages of pulse-fre-
quency modulation (low supply current) and pulse-
width modulation (high efficiency with heavy loads),
providing high efficiency over a wide output current
range, as well as increased output current capability
over previous PFM devices. In addition, the external
sense resistor and power transistor allow the user to tai-
lor the output current capability for each application.
Figure 1 shows the MAX1771 functional diagram.
The device has a shutdown mode that reduces the
supply current to 5µA max.
Bo o t s t ra p p e d /No n -Bo o t s t ra p p e d Mo d e s
Figure 2 shows the standard application circuits for
bootstrapped and non-bootstrapped modes. In boot-
strapped mode, the IC is powered from the output
(V , which is connected to V+) and the input voltage
OUT
range is 2V to V . The voltage applied to the gate of
OUT
the external power transistor is switched from V
to
OUT
ground, providing more switch gate drive and thus
reducing the transistor’s on-resistance.
The MAX1771 offers three main improvements over
prior pulse-skipping control solutions: 1) the converter
operates with miniature (5mm height and less than
9mm d ia me te r) s urfa c e -mount ind uc tors d ue to its
300kHz switching frequency; 2) the current-limited PFM
control scheme allows 90% efficiencies over a wide
In non-bootstrapped mode, the IC is powered from the
input voltage (V+) and operates with minimum supply
current. In this mode, FB is the output voltage sense
point. Since the voltage swing applied to the gate of the
external power transistor is reduced (the gate swings
from V+ to ground), the power transistor’s on-resistance
REF
FB
DUAL-MODE
COMPARATOR
SHDN
MAX1771
50mV
BIAS
1.5V
CIRCUITRY
REFERENCE
ERROR
COMPARATOR
MIN OFF-TIME
ONE-SHOT
Q
TRIG
2.3µs
V+
N
F/F
R
S
Q
LOW-VOLTAGE
OSCILLATOR
2.5V
MAX ON-TIME
ONE-SHOT
TRIG
16µs
Q
EXT
CURRENT-SENSE
AMPLIFIER
0.1V
CS
Figure 1. Functional Diagram
_______________________________________________________________________________________
7
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
VIN = 5V
VIN = 5V
C1
C2
68µF
0.1µF
C2
0.1µF
2
2
L1
22µH
V+
V+
V
= 12V
@ 0.5A
D1
1N5817-22
OUT
5
4
6
C1
68µF
5
4
3
6
REF
REF
C3
0.1µF
C3
0.1µF
L1
22µH
C4
300µF
MAX1771
MAX1771
1
8
D1
1N5817-22
V
= 12V
@ 0.5A
OUT
SHDN
AGND
EXT
CS
N
SHDN
MAX71
MTD20N03HDL
1
8
N
FB
EXT
CS
R
SENSE
R2
127k
Si9410DY/
40mΩ
MTD20N03HDL
3
FB
AGND
GND
7
R
40mΩ
C4
300µF
SENSE
R1
18k
GND
7
C5
100pF
R2 = (R1) (VV -1)
OUT
REF
V
REF
= 1.5V
Figure 2a. 12V Preset Output, Bootstrapped
Figure 2b. 12V Output, Non-Bootstrapped
for fixed-output operation. External resistors must
be used to set the output voltage. Use 1% external
feedback resistors when operating in adjustable-output
mode (Figures 2b, 2c) to achieve an overall output volt-
age accuracy of ±5%. To achieve highest efficiency,
operate in bootstrapped mode whenever possible.
V
= 4V
IN
C2
0.1µF
C1
47µF
2
L1
22µH
V+
D1
5
REF
V
OUT
= 9V
1N5817-22
C3
0.1µF
Ex t e rn a l P o w e r-Tra n s is t o r
Co n t ro l Circ u it ry
MAX1771
1
8
3
C4
200µF
4
6
SHDN
AGND
EXT
CS
N
Si9410DY/
MTD20N03HDL
PFM Control Scheme
The MAX1771 uses a proprietary current-limited PFM
control scheme to provide high efficiency over a wide
range of load currents. This control scheme combines the
ultra-low supply current of PFM converters (or pulse skip-
pers) with the high full-load efficiency of PWM converters.
R
SENSE
R2
140k
40mΩ
FB
GND
7
R1
28k
C5
100pF
Unlike traditional PFM converters, the MAX1771 uses a
sense resistor to control the peak inductor current. The
device also operates with high switching frequencies
(up to 300kHz), allowing the use of miniature external
components.
R2 = (R1) (VV -1)
OUT
REF
V
= 1.5V
REF
Figure 2c. 9V Output, Bootstrapped
As with traditional PFM converters, the power transistor
is not turned on until the voltage comparator senses
the output is out of regulation. However, unlike tradition-
al PFM converters, the MAX1771 switch uses the com-
bination of a peak current limit and a pair of one-shots
that set the maximum on-time (16µs) and minimum off-
time (2.3µs); there is no oscillator. Once off, the mini-
mum off-time one-shot holds the switch off for 2.3µs.
After this minimum time, the switch either 1) stays off if
the output is in regulation, or 2) turns on again if the
output is out of regulation.
increases at low input voltages. However, the supply
current is also reduced because V+ is at a lower volt-
a g e , a nd b e c a us e le s s e ne rg y is c ons ume d while
charging and discharging the external MOSFET’s gate
capacitance. The minimum input voltage is 3V when
using external feedback resistors. With supply voltages
below 5V, bootstrapped mode is recommended.
Note: When using the MAX1771 in non-boot-
strapped mode, there is no preset output operation
because V+ is also the output voltage sense point
_______________________________________________________________________________________
8
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
MAX71
The control circuitry allows the IC to operate in continu-
ous-conduction mode (CCM) while maintaining high
efficiency with heavy loads. When the power switch is
turned on, it stays on until either 1) the maximum on-
time one-shot turns it off (typically 16µs later), or 2) the
switch current reaches the peak current limit set by the
current-sense resistor.
R2
V
OUT
FB
MAX1771
GND
R1
C5*
The MAX1771 switching frequency is variable (depend-
ing on load current and input voltage), causing variable
switching noise. However, the subharmonic noise gen-
erated does not exceed the peak current limit times the
filter capacitor equivalent series resistance (ESR). For
example, when generating a 12V output at 500mA from
a 5V input, only 100mV of output ripple occurs using
the circuit of Figure 2a.
R1 = 10k TO 500k
V
OUT
R2 = R1
-1
)
(
V
REF
V
= 1.5V
REF
* SEE TEXT FOR VALUE
Figure 3. Adjustable Output Circuit
Low-Voltage Start-Up Oscillator
The MAX1771 features a low input voltage start-up oscil-
lator that guarantees start-up with no load down to 2V
when operating in bootstrapped mode and using inter-
nal feedback resistors. At these low voltages, the supply
voltage is not large enough for proper error-comparator
operation and internal biasing. The start-up oscillator
has a fixed 50% duty cycle and the MAX1771 disre -
gards the error-comparator output when the supply volt-
age is less than 2.5V. Above 2.5V, the error-comparator
and normal one-shot timing circuitry are used. The low-
voltage start-up circuitry is disabled if non-bootstrapped
mode is selected (FB is not tied to ground).
R1 a nd R2 c onfig ure d a s s hown in Fig ure 3. For
adjustable-output operation, select feedback resistor
R1 in the 10kΩ to 500kΩ range. R2 is given by:
V
OUT
R2 = (R1) ––––– -1
(
)
V
REF
where V
equals 1.5V.
REF
For p re s e t-outp ut op e ra tion, tie FB to GND (this
forces bootstrapped-mode operation.
Figure 2 shows various circuit configurations for boot-
strapped/non-bootstrapped, preset/adjustable operation.
S h u t d o w n Mo d e
When SHDN is high, the MAX1771 enters shutdown
mod e . In this mod e , the inte rna l b ia s ing c irc uitry is
De t e rm in in g R
S ENS E
Use the theoretical output current curves shown in
Figures 4a–4d to select R . They were derived
turned off (including the reference) and V
falls to a
OUT
d iod e d rop b e low V (d ue to the DC p a th from the
SENSE
IN
using the minimum (worst-case) current-limit compara-
tor thre s hold va lue ove r the e xte nd e d te mp e ra ture
range (-40°C to +85°C). No tolerance was included for
inp ut to the outp ut). In s hutd own mod e , the s up p ly
current drops to less than 5µA. SHDN is a TTL/CMOS
log ic -le ve l inp ut. Conne c t SHDN to GND for norma l
operation.
R
. The volta g e d rop a c ros s the d iod e wa s
SENSE
assumed to be 0.5V, and the drop across the power
__________________De s ig n P ro c e d u re
switch r
0.3V.
and coil resistance was assumed to be
DS(ON)
S e t t in g t h e Ou t p u t Vo lt a g e
To set the output voltage, first determine the mode of
operation, either bootstrapped or non-bootstrapped.
Boots tra p p e d mod e p rovid e s more outp ut c urre nt
capability, while non-bootstrapped mode reduces the
supply current (see Typical Operating Characteristics).
If a decaying voltage source (such as a battery) is
used, see the additional notes in the Low Input Voltage
Operation section.
De t e rm in in g t h e In d u c t o r (L)
Practical inductor values range from 10µH to 300µH.
22µH is a good choice for most applications. In appli-
cations with large input/output differentials, the IC’s
output current capability will be much less when the
inductance value is too low, because the IC will always
operate in discontinuous mode. If the inductor value
is too low, the current will ramp up to a high level before
the current-limit comparator can turn off the switch.
The MAX1771’s output voltage can be adjusted from
very high voltages down to 3V, using external resistors
The minimum on-time for the s witc h (t (min)) is
ON
_______________________________________________________________________________________
9
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
3.5
3.0
2.5
3.5
3.0
2.5
V
= 5V
V
= 12V
OUT
OUT
L = 22µH
L = 22µH
R
SENSE
= 20mΩ
R
= 20mΩ
SENSE
R
= 25mΩ
SENSE
R
SENSE
= 25mΩ
R
SENSE
= 35mΩ
2.0
1.5
1.0
2.0
1.5
1.0
R
= 35mΩ
SENSE
MAX71
R
SENSE
= 50mΩ
R
= 50mΩ
SENSE
0.5
0
0.5
0
R
SENSE
= 100mΩ
R
SENSE
= 100mΩ
2
3
4
5
2
4
6
8
10
12
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 4a. Maximum Output Current vs. Input Voltage
(V = 5V)
Figure 4b. Maximum Output Current vs. Input Voltage
(V = 12V)
OUT
OUT
0.8
3.5
3.0
2.5
V
= 15V
OUT
V
= 24V
OUT
L = 22µH
L =150µH
R
SENSE
= 20mΩ
0.6
0.4
0.2
R
SENSE
= 25mΩ
R
= 50mΩ
SENSE
R
SENSE
= 35mΩ
R
SENSE
= 100mΩ
2.0
1.5
1.0
R
SENSE
= 50mΩ
0.5
0
R
= 200mΩ
SENSE
R
= 100mΩ
SENSE
0
2
4
6
8
10
12
14
16
2
6
10
INPUT VOLTAGE (V)
14
INPUT VOLTAGE (V)
Figure 4c. Maximum Output Current vs. Input Voltage
(V = 15V)
Figure 4d. Maximum Output Current vs. Input Voltage
(V = 24V)
OUT
OUT
approximately 2µs; select an inductor that allows the cur-
rent to ramp up to I
Inductors with a ferrite core or equivalent are recom-
mended; powder iron cores are not recommended for
use with high switching frequencies. Make sure the
inductor’s saturation current rating (the current at which
the core begins to saturate and the inductance starts to
.
LIM
The standard operating circuits use a 22µH inductor.
If a different inductance value is desired, select L such
that:
fall) exceeds the peak current rating set by R
.
SENSE
However, it is generally acceptable to bias the inductor
into saturation by approximately 20% (the point where
the inductance is 20% below the nominal value). For
highest efficiency, use a coil with low DC resistance,
preferably under 20mΩ. To minimize radiated noise,
use a toroid, a pot core, or a shielded coil.
V
(max) x 2µs
IN
L ≥ —————----—--
I
LIM
Larger inductance values tend to increase the start-up
time slightly, while smaller inductance values allow the
c oil c urre nt to ra mp up to hig he r le ve ls b e fore the
switch turns off, increasing the ripple at light loads.
Table 1 lists inductor suppliers and specific recom-
mended inductors.
______________________________________________________________________________________
10
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
MAX71
P o w e r Tra n s is t o r S e le c t io n
Use an N-channel MOSFET power transistor with the
MAX1771.
Dio d e S e le c t io n
The MAX1771’s high switching frequency demands a
hig h-s p e e d re c tifie r. Sc hottky d iod e s s uc h a s the
1N5817–1N5822 are recommended. Make sure the
Schottky diode’s average current rating exceeds the
To ensure the external N-channel MOSFET (N-FET) is
turne d on ha rd , us e log ic -le ve l or low-thre s hold
N-FETs when the input drive voltage is less than 8V. This
applies even in bootstrapped mode, to ensure start-up.
N-FETs provide the highest efficiency because they do
not draw any DC gate-drive current.
peak current limit set by R
, and that its break-
SENSE
down voltage exceeds V . For high-temperature
OUT
applications, Schottky diodes may be inadequate due
to the ir hig h le a ka g e c urre nts ; hig h-s p e e d s ilic on
diodes such as the MUR105 or EC11FS1 can be used
instead. At heavy loads and high temperatures, the
benefits of a Schottky diode’s low forward voltage may
outweigh the disadvantages of its high leakage current.
When selecting an N-FET, three important parameters
are the total gate charge (Q ), on-resistance (r
),
g
DS(ON)
and reverse transfer capacitance (C
).
RSS
Q
takes into account all capacitances associated with
g
Ca p a c it o r S e le c t io n
charging the gate. Use the typical Q value for best
g
results; the maximum value is usually grossly over-
specified since it is a guaranteed limit and not the mea-
sured value. The typical total gate charge should be
50nC or less. With larger numbers, the EXT pins may
not be a ble to a de q ua te ly drive the ga te . The EXT
rise/fall time varies with different capacitive loads as
shown in the Typical Operating Characteristics.
Output Filter Capacitor
The primary criterion for selecting the output filter capac-
itor (C4) is low effective series resistance (ESR). The
product of the peak inductor current and the output filter
capacitor’s ESR determines the amplitude of the ripple
seen on the output voltage. Two OS-CON 150µF, 16V
output filter capacitors in parallel with 35mΩ of ESR each
typically provide 75mV ripple when stepping up from 5V
to 12V at 500mA (Figure 2a). Smaller-value and/or high-
er-ESR capacitors are acceptable for light loads or in
applications that can tolerate higher output ripple.
The two most sig nific a nt losse s c ontributing to the
2
N-FET’s power dissipation are I R losses and switching
losses. Select a transistor with low r
and low
DS(ON)
C
to minimize these losses.
RSS
Since the output filter capacitor’s ESR affects efficien-
cy, use low-ESR capacitors for best performance. See
Table 1 for component selection.
Determine the maximum required gate-drive current
from the Q specification in the N-FET data sheet.
g
The MAX1771’s maximum allowed switching frequency
during normal operation is 300kHz; but at start-up, the
maximum frequency can be 500kHz, so the maximum
c urre nt re q uire d to c ha rg e the N-FET’s g a te is
Input Bypass Capacitors
The input bypass capacitor (C1) reduces peak currents
drawn from the voltage source and also reduces noise
at the voltage source caused by the switching action of
the MAX1771. The input voltage source impedance
determines the size of the capacitor required at the V+
input. As with the output filter capacitor, a low-ESR
capacitor is recommended. For output currents up to
1A, 68µF (C1) is adequate, although smaller bypass
capacitors may also be acceptable.
f(max) x Q (typ). Use the typical Q number from the
g
g
transistor data sheet. For example, the Si9410DY has a
Q (typ) of 17nC (at V = 5V), therefore the current
g
GS
required to charge the gate is:
I
= (500kHz) (17nC) = 8.5mA.
GATE (max)
The bypass capacitor on V+ (C2) must instantaneously
furnish the gate charge without excessive droop (e.g.,
less than 200mV):
Bypass the IC with a 0.1µF ceramic capacitor (C2)
placed as close to the V+ and GND pins as possible.
Q
g
∆V+ = ——
C2
Reference Capacitor
Byp a s s REF with a 0.1µF c a p a c itor (C3). REF c a n
source up to 100µA of current for external loads.
Continuing with the example, ∆V+ = 17nC/0.1µF = 170mV.
Figure 2a’s application circuit uses an 8-pin Si9410DY
Feed-Forward Capacitor
In adjustable output voltage and non-bootstrapped
modes, parallel a 47pF to 220pF capacitor across R2,
as shown in Figures 2 and 3. Choose the lowest capac-
itor value that insures stability; high capacitance values
may degrade line regulation.
surface-mount N-FET that has 50mΩ on-resistance with
4.5V V , and a guaranteed V of less than 3V. Figure
GS
TH
2b’s application circuit uses an MTD20N03HDL logic-
level N-FET with a guaranteed threshold voltage (V
)
TH
of 2V.
______________________________________________________________________________________
11
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
Table 1. Component Suppliers
PRODUCTION
INDUCTORS
CAPACITORS
TRANSISTORS
Siliconix
Si9410DY
Si9420DY (high voltage)
Motorola
DIODES
Central Semiconductor
CMPSH-3
CMPZ5240
Sumida
CD54 series
CDR125 series
Coiltronics
CTX20 series
Coilcraft
DO3316 series
DO3340 series
Matsuo
267 series
Sprague
595D series
AVX
TPS series
Nihon
MTP3055EL
Surface Mount
EC11 FS1 series (high-
speed silicon)
Motorola
MBRS1100T3
MMBZ5240BL
MTD20N03HDL
MMFT3055ELT1
MTD6N1O
MMBT8099LT1
MMBT8599LT1
MAX71
Sanyo
OS-CON series
Nichicon
Motorola
Sumida
RCH855 series
RCH110 series
1N5817–1N5822
MUR115 (high voltage)
MUR105 (high-speed
silicon)
Through Hole
PL series
SUPPLIER
PHONE
FAX
AVX
USA: (803) 448-9411
(803) 448-1943
Central
Semiconductor
USA: (516) 435-1110
(516) 435-1824
Coilcraft
USA: (708) 639-6400
USA: (407) 241-7876
(708) 639-1469
(407) 241-9339
Coiltronics
USA: (714) 969-2491
Japan: 81-6-337-6450
(714) 960-6492
81-6-337-6456
Matsuo
Motorola
Nichicon
Nihon
USA: (800) 521-6274
USA: (708) 843-7500
USA: (805) 867-2555
(602) 952-4190
(708) 843-2798
(805) 867-2556
USA: (619) 661-6835
Japan: 81-7-2070-1005
(619) 661-1055
81-7-2070-1174
Sanyo
Siliconix
Sprague
USA: (800) 554-5565
USA: (603) 224-1961
(408) 970-3950
(603) 224-1430
USA: (708) 956-0666
Japan: 81-3-3607-5111
(708) 956-0702
81-3-3607-5144
Sumida
______________________________________________________________________________________
12
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
MAX71
La yo u t Co n s id e ra t io n s
Due to high current levels and fast switching wave-
forms, which radiate noise, proper PC board layout is
C1
2.2µF
V *
3V TO 11V
IN
essential. Protect sensitive analog grounds by using a
star ground configuration. Minimize ground noise by
connecting GND, the input bypass capacitor ground
lead, and the output filter capacitor ground lead to a
single point (star ground configuration). Also, minimize
lead lengths to reduce stray capacitance, trace resis-
tance, and radiated noise. Place input bypass capaci-
tor C2 as close as possible to V+ and GND.
D2
1N5817
2
L1
20µH
1 CTX20-4
V+
V
OUT
5V
500mA
4
D1
1N5817
SHDN
3V = OFF
C2
47µF
16V
MAX1771
5
C4
0.1µF
1
8
L2
C3
220µF
10V
EXT
CS
Q1**
REF
Excessive noise at the V+ input may falsely trigger the
timing circuitry, resulting in short pulses at EXT. If this
occurs it will have a negligible effect on circuit efficien-
cy. If desired, place a 4.7µF directly across the V+ and
GND pins (in parallel with the 0.1µF C2 bypass capaci-
tor) to reduce the noise at V+.
R1
0.1Ω
GND AGND FB
7
6
3
†
R3
†
R2
C5
47pF
Ot h e r Ap p lic a t io n Circ u it s
SEE TEXT FOR FURTHER COMPONENT INFO
4 Cells to 5V (or 3 Cells to 3.3V), 500mA
Step-Up/Down Converter
**V MAY BE LOWER THAN INDICATED IF THE SUPPLY IS NOT
IN
**REQUIRED TO START UNDER FULL LOAD
**MOTOROLA MMFT3055ELT1
The circuit shown in Figure 5 generates 5V (or 3.3V) at
500mA with 85% efficiency, from an input voltage that
varies above and below the output. The output couples
to the switching circuitry via a capacitor. This configu-
ration offers two advantages over flyback-transformer
and step-up linear-regulator circuits: smooth regulation
as the input passes through the output, and no output
current in shutdown.
†
FOR 5V: R2 = 200kΩ, R3 = 470kΩ
3.3V: R2 = 100kΩ, R3 = 20kΩ
Figure 5. Step-Up/Down for a 5V/3.3V Output
__________Ap p lic a t io n s In fo rm a t io n
This circuit requires two inductors, which can be wound
on one core with no regard to coupling since they do
not work as a transformer. L1 and L2 can either be
wound together (as with the Coiltronics CTX20-4) or
kept as two separate inductors; both methods provide
equal performance. Capacitors C2 and C3 should be
low-ESR typ e s for b e s t e ffic ie nc y. A 1µF c e ra mic
capacitor will work at C2, but with about 3% efficiency
loss. C2’s voltage rating must be greater than the maxi-
mum input voltage. Also note that the LX switch must
withstand a voltage equal to the sum of the input and
output voltage; for example, when converting 11V to
5V, the switch must withstand 16V.
Lo w In p u t Vo lt a g e Op e ra t io n
Whe n us ing a p owe r s up p ly tha t d e c a ys with time
(such as a battery), the N-FET transistor will operate in
its linear region when the voltage at EXT approaches
the threshold voltage of the FET, dissipating excessive
power. Prolonged operation in this mode may damage
the FET. This effect is much more significant in non-
bootstrapped mode than in bootstrapped mode, since
bootstrapped mode typically provides much higher
V
GS
voltages. To avoid this condition, make sure V
EXT
is above the V of the FET, or use a voltage detector
TH
(such as the MAX8211) to put the IC in shutdown mode
once the input supply voltage falls below a predeter-
mined minimum value. Excessive loads with low input
voltages can also cause this condition.
LX switch pulses are captured by Schottky diode D2 to
boost V+ to (V
+ V ). This improves efficiency with
OUT
IN
a low input voltage, but also limits the maximum input
supply to 11V. If the input voltage does not fall below 4V
and if a 3V logic threshold FET is used for Q1, you may
omit D2 and connect V+ directly to the input supply.
S t a rt in g Up Un d e r Lo a d
The Typical Operating Characteristics show the Start-
Up Voltage vs. Load Current graph for bootstrapped-
mod e op e ra tion. This g ra p h d e p e nd s on the typ e
of power switch used. The MAX1771 is not designed to
start up under full load in bootstrapped mode with low
input voltages.
12V Output Buck/Boost
The circuit in Figure 6 generates 12V from a 4.5V to
16V input. Higher input voltages are possible if you
______________________________________________________________________________________
13
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
tries to use internal feedback and looks to V+ for its
V
feedback signal. However, since V+ may be greater
than the internally set feedback (12V for the MAX1771),
the IC may think the output is sufficiently high and not
start. D2 ensures start-up by pulling FB above ground
and forcing the external feedback mode. In a normal
(not AC-coupled) boost circuit, D2 isn’t needed, since
the outp ut a nd FB ris e a s s oon a s inp ut p owe r is
applied.
IN
4.5V TO 15V
C1
33µF
†
L1
16V
2
20µH
V
12V
250mA
OFF
ON
OUT
V+
4
D1
1N5819
C2*
1µF
SHDN
MAX71
MAX1771
1
8
Transformerless -48V to +5V at 300mA
The circuit in Figure 7 uses a transformerless design to
supply 5V at 300mA from a -30V to -75V input supply.
The MAX1771 is biased such that its ground connec-
tions are made to the -48V input. The IC’s supply volt-
age (at V+) is set to about 9.4V (with respect to -48V)
by a zener-biased emitter follower (Q2). An N-channel
FET (Q1) is driven in a boost configuration. Output reg-
ulation is achieved by a transistor (Q3), which level
shifts a feedback signal from the 5V output to the IC’s
FB input. Conversion efficiency is typically 82%.
Q1**
EXT
CS
L2*
20µH
C4
100µF
16V
C3
100µF
16V
6
7
AGND
R1
0.1Ω
5
GND
FB
3
REF
C5
0.1µF
NOTE: HIGH-
CURRENT GND
R3
28k
1%
R2
200k
1%
D2*
1N4148
*SEE TEXT FOR FURTHER
COMPONENT INFORMATION
**Q1 = MOTOROLA MMFT3055ELT1
NOTE: KEEP ALL TRACES CONNECTED
TO PIN 3 AS SHORT AS POSSIBLE
When selecting components, be sure that D1, Q1, Q2,
Q3, and C6 are rated for the full input voltage plus a
reasonable safety margin. Also, if D1 is substituted, it
†
L1 + L2 = ONE COILTRONICS CTX20-4
should be a fast-recovery type with a t less than 30ns.
rr
R7, R9, C8, and D3 are optional and may be used to
soft start the circuit to prevent excessive current surges
at power-up.
Figure 6. 12V Buck/Boost from a 4.5V to 15V Input
carefully observe the component voltage ratings, since
some components must withstand the sum of the input
and output voltage (27V in this case). The circuit oper-
ates as an AC-coupled boost converter, and does not
change operating modes when crossing from buck to
b oos t. The re is no ins ta b ility a round a 12V inp ut.
Efficiency ranges from 85% at medium loads to about
82% at full load. Also, when shutdown is activated
(SHDN high) the output goes to 0V and sources no cur-
rent. A 1µF ceramic capacitor is used for C2. A larger
capacitor value improves efficiency by about 1% to 3%.
Battery-Powered LCD Bias Supply
The circuit in Figure 8 boosts two cells (2V min) to 24V
for LCD b ia s or othe r p os itive outp ut a p p lic a tions .
Output power is boosted from the battery input, while
V+ voltage for the MAX1771 is supplied by a 5V or 3.3V
logic supply.
5V, 1A Boost Converter
The circuit in Figure 9 boosts a 2.7V to 5.5V input to a
regulated 5V, 1A output for logic, RF power, or PCMCIA
applications. Efficiency vs. load current is shown in the
adjacent graph.
D2 ensures start-up for this AC-coupled configuration
by overriding the MAX1771’s Dual-Mode feature, which
allows the use of preset internal or user-set external
feedback. When operating in Dual-Mode, the IC first
______________________________________________________________________________________
14
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
MAX71
L1
D03340
220µH–680µH
D1
+5V
300mA
MBRS1100T3
3
R2
C1
220µF
10V
Q1
MTD6N10
C3
0.33µF
C8
1µF
C7
220pF
C2
220µF
10V
4
5
3
7
1
8
2
6
2
47k
1%
EXT
SHDN
REF
R9
R7
C5
0.1µF
1
5.1k
200Ω
CS
V+
Q3
D3
CMPSH-3
MMBT8599LT1
MAX1771
FB
Q2
MMBT8099LT1
-48V
R4
AGND
GND
C4
2.2µF
100k
20V
R6
200k
R5
1k
R1
R3
C6
10µF
100V
D2
0.15Ω
16k
1%
CMPZ5240/
MMBZ5240BL
Figure 7. -48V Input to 5V Output at 300mA, Without a Transformer
BATTERY
INPUT
2V TO 12V
OUTPUT
Adj. = 12V TO 24V
(AS SHOWN)
L1
22µH
3.3V OR 5V
LOGIC
1N5817
0.1µF
SUPPLY
2
V+
1
8
N
EXTL
CS
MMFT3055ELT1
47µF
OFF
4
SHDN
ON
R2
150k
R
SENSE
0.2Ω
MAX1771
3
FB
REF GND
R3
10k
5
6, 7
0.1µF
10k
Figure 8. 2V Input to 24V Output LCD Bias
______________________________________________________________________________________
15
1 2 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Up DC-DC Co n t ro lle r
Q
INPUT
2.7V TO 5.5V
22µH
EFFICIENCY vs. LOAD CURRENT
100
OUTPUT
5V
1A
150µF
1N5820
90
80
70
60
1
8
EXT
CS
MTD20N03HDL
4
V
= 4V
IN
SHDN
OFF
MAX71
ON
V
= 3V
330µF
IN
0.04Ω
232k
100k
MAX1771
2
5
V+
FB
REF
0.1µF
100pF
3
50
GND AGND
1m
10m
100m
1
7
6
LOAD CURRENT (A)
0.1µF
Figure 9. 5V/1A Boost Converter
___________________Ch ip To p o g ra p h y
EXT
V+
CS
0. 126"
(3. 200mm)
GND
AGND
FB
SHDN
REF
0. 080"
(2. 032mm)
TRANSISTOR COUNT: 501
SUBSTRATE CONNECTED TO V+
______________________________________________________________________________________
16
相关型号：
MAX1771CPA+
Switching Controller, Current-mode, 0.5A, 300kHz Switching Freq-Max, BICMOS, PDIP8, PLASTIC, DIP-8
MAXIM
©2020 ICPDF网 联系我们和版权申明