MAX1745EUB [MAXIM]
High-Voltage, Step-Down DC-DC Controller in レMAX; 高电压，降压，μMAX封装DC- DC控制器
| 型号： | MAX1745EUB |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | High-Voltage, Step-Down DC-DC Controller in レMAX |
| 文件： | 总14页 (文件大小：256K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-1776; Rev 1; 10/01
High-Voltage, Step-Down DC-DC
Controller in µMAX
General Description
____________________________Features
The MAX1744/MAX1745 are step-down DC-DC con-
trollers capable of handling up to 36V inputs. These
parts use a proprietary current-limited control scheme
for excellent light- and full-load efficiency, while their
330kHz (max) switching frequency permits small exter-
nal components for space-critical applications.
Operation to 100% duty cycle permits the lowest possi-
ble dropout voltage.
o High-Voltage Operation (up to 36V IN)
o Efficiency >90%
o Output Power Capability Exceeds 50W
o 10-Pin µMax Package
o Low-Dropout Voltage
o 100% (max) Duty Cycle
The MAX1744 contains an internal feedback network
that provides a pin-selectable output voltage of either
3.3V or 5V. The MAX1745 uses an external feedback
network to generate an adjustable output voltage
between 1.25V and 18V.
o 90µA Quiescent Current
o 4µA Shutdown Current
o Up to 330kHz Switching Frequency
o Output Voltage
The MAX1744/MAX1745 are available in a space-sav-
ing 10-pin µMAX package.
5V or 3.3V (MAX1744)
Adjustable 1.25V to 18V (MAX1745)
o Current-Limited Control Scheme
________________________Applications
Automotive Electronics
Telecom Systems
Ordering Information
Wall-Cube-Powered Devices
Industrial Control Systems
Firewire/IEEE1394
PART
TEMP RANGE
-40°C to +85°C
-40°C to +125°C
-40°C to +85°C
-40°C to +125°C
PIN-PACKAGE
10 µMAX
MAX1744EUB
MAX1744AUB
MAX1745EUB
MAX1745AUB
10 µMAX
10 µMAX
10 µMAX
Typical Operating Circuit
Pin Configuration
TOP VIEW
IN
4.5V TO 36V
IN VH
SHDN
EXT
P
GND
VL
1
2
3
4
5
10 IN
ON
5V
OFF
9
8
7
6
EXT
VH
MAX1744
MAX1745
3/5
3.3V
REF
3/5 (FB)
OUT
SHDN
CS
MAX1744
VL
CS
REF
OUT
µMAX
OUT
3.3V
GND
OR 5V
( ) ARE FOR MAX1745 ONLY.
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
High-Voltage, Step-Down DC-DC
Controller in µMAX
ABSOLUTE MAXIMUM RATINGS
Operating Temperature Range
IN, EXT, SHDN to GND...........................................-0.3V to +38V
VH to GND..............................................................-0.3V to +34V
VH, EXT to IN............................................................-7V to +0.3V
CS, OUT to GND ....................................................-0.3V to +20V
FB, 3/5, REF to GND.....................................-0.3V to (VL + 0.3V)
VL to GND...................................................................-0.3V to 6V
MAX174_EUB ..................................................-40°C to +85°C
MAX174_AUB................................................-40°C to +125°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s) ................................+300°C
Continuous Power Dissipation (T = +70°C)
A
10-Pin µMAX (derate 5.6mW/°C above 70°C) .............444mW
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 = SHDN = 5.5V to 36V, 3/5 = GND, I
= 0, T = 0°C to +85°C, unless otherwise noted. Typical values at V = SHDN =
IN
LOAD
A
IN
36V, T = +25°C.)
A
PARAMETER
CONDITIONS
MIN
TYP
MAX
36
UNITS
V
Input Voltage Range
4.5
Supply Current into IN
Shutdown Supply Current
SHDN = V = 5.5V to 36V
90
4
140
12
µA
IN
SHDN = GND
3/5 = VL
µA
4.85
3.20
5.00
3.30
28
5.15
3.40
44
Output Voltage (MAX1744)
V
3/5 = GND
OUT Input Current (MAX1744)
FB Threshold Voltage (MAX1745)
FB Input Current (MAX1745)
VH Output Voltage with Respect to IN
VL Output Voltage
3/5 = VL, V
= 5V
µA
V
OUT
Falling edge, hysteresis = 8mV
1.22
-50
-6.0
4.5
2.0
85
1.25
1.28
50
nA
V
V
V
= 5.5V to 36V, I = 100µA to 20mA
-5.3
5.0
3.0
100
110
15
-4.3
5.5
4.1
115
150
25
IN
IN
VH
= 5.5V to 36V, I = 100µA to 2mA
V
VL
VL Undervoltage Lockout
V
V
V
V
V
V
V
= V
= V
= V
= V
= 2.5V to 18V
= GND
CS
CS
CS
CS
IN
OUT
OUT
OUT
OUT
CS Threshold Voltage
CS Input Current
mV
µA
80
= 2.5V to 18V
= GND
0
-25
2.4
0
SHDN, 3/5 Logic-High Threshold
SHDN, 3/5 Logic-Low Threshold
3/5 Input Current
= 4.5V to 36V
= 4.5V to 36V
V
V
0.4
1
IN
SHDN = GND
3/5 = GND
µA
1
SHDN Input Current
µA
SHDN = 36V
12
20
2.5
1.5
EXT Resistance
8
2.0
1.0
5
Ω
µs
Minimum EXT Off-Time
Minimum EXT On-Time
Output Line Regulation
Output Load Regulation
Reference Voltage
1.5
0.7
µs
Figure 1, 5.5V < V < 36V, I
= 1A
mV/V
mV/A
V
IN
LOAD
Figure 1, V = 12V, 30mA < I
< 2A
15
1.25
4
IN
LOAD
I
= 0
REF
1.22
1.28
10
REF
REF Load Regulation
REF Line Regulation
0 ≤ I
≤ 100µA
mV
µV/V
V
= 4.5V to 36V, I
= 0
30
60
IN
REF
2
_______________________________________________________________________________________
High-Voltage, Step-Down DC-DC
Controller in µMAX
ELECTRICAL CHARACTERISTICS
(V = SHDN = 5.5V to 36V, 3/5 = GND, I
= 0, T = -40°C to +85°C, unless otherwise noted.) (Note 1)
A
IN
LOAD
PARAMETER
Input Voltage Range
CONDITIONS
MIN
MAX
36
UNITS
V
4.5
Supply Current into IN
SHDN = V = 5.5V to 36V
140
12
µA
IN
Shutdown Supply Current
SHDN = GND
3/5 = VL
µA
4.85
3.20
5.15
3.40
44
Output Voltage (MAX1744)
V
3/5 = GND
OUT Input Current (MAX1744)
FB Threshold Voltage (MAX1745)
FB Input Current (MAX1745)
VH Output Voltage with Respect to IN
VL Output Voltage
3/5 = VL, V
= 5V
µA
V
OUT
Falling edge, hysteresis = 8mV
1.22
-50
-6.0V
4.5
2.0
85
1.28
50
nA
V
V
V
= 5.5V to 36V, I = 100µA to 20mA
-4.3V
5.5
4.1
115
150
25
IN
IN
VH
= 5.5V to 36V, I = 100µA to 2mA
V
VL
VL Undervoltage Lockout
V
V
V
V
V
V
V
= V
= V
= V
= V
= 2.5V to 18V
= GND
CS
CS
CS
CS
IN
OUT
OUT
OUT
OUT
CS Threshold Voltage
CS Input Current
mV
µA
80
= 2.5V to 18V
= GND
0
-25
2.4
0
SHDN, 3/5 Logic-High Threshold
SHDN, 3/5 Logic-Low Threshold
3/5 Input Current
= 4.5V to 36V
= 4.5V to 36V
V
V
0.4
1
IN
SHDN = GND
3/5 = GND
µA
1
SHDN Input Current
µA
SHDN = 36V
12
20
2.5
1.5
1.28
10
60
EXT Resistance
Ω
µs
Minimum EXT Off-Time
Minimum EXT On-Time
Reference Voltage
REF Load Regulation
REF Line Regulation
1.5
0.7
µs
I
= 0
REF
1.22
V
REF
0 ≤ I
≤ 100µA
mV
µV/V
V
= 4.5V to 36V, I
= 0
IN
REF
_______________________________________________________________________________________
3
High-Voltage, Step-Down DC-DC
Controller in µMAX
ELECTRICAL CHARACTERISTICS
(V = SHDN = 5.5V to 36V, 3/5 = GND, I
= 0, T = -40°C to +125°C, unless otherwise noted.) (Note 1)
A
IN
LOAD
PARAMETER
Input Voltage Range
CONDITIONS
MIN
MAX
36
UNITS
V
4.5
Supply Current into IN
SHDN = V = 5.5V to 36V
140
15
µA
IN
Shutdown Supply Current
SHDN = GND
3/5 = VL
µA
4.85
3.20
5.15
3.40
44
Output Voltage (MAX1744)
V
3/5 = GND
OUT Input Current (MAX1744)
FB Threshold Voltage (MAX1745)
FB Input Current (MAX1745)
VH Output Voltage with Respect to IN
VL Output Voltage
3/5 = VL, V
= 5V
µA
V
OUT
Falling edge, hysteresis = 8mV
1.22
-50
-6.0V
4.5
1.6
85
1.28
50
nA
V
V
V
= 5.5V to 36V, I = 100µA to 20mA
-4.3V
5.5
4.1
115
150
25
IN
IN
VH
= 5.5V to 36V, I = 100µA to 2mA
V
VL
VL Undervoltage Lockout
V
V
V
V
V
V
V
= V
= V
= V
= V
= 2.5V to 18V
= GND
CS
CS
CS
CS
IN
OUT
OUT
OUT
OUT
CS Threshold Voltage
CS Input Current
mV
µA
80
= 2.5V to 18V
= GND
0
-25
2.4
0
SHDN, 3/5 Logic-High Threshold
SHDN, 3/5 Logic-Low Threshold
3/5 Input Current
= 4.5V to 36V
= 4.5V to 36V
V
V
0.4
1
IN
SHDN = GND
3/5 = GND
µA
1
SHDN Input Current
µA
SHDN = 36V
12
20
2.5
1.5
1.28
10
80
EXT Resistance
Ω
µs
Minimum EXT Off-Time
Minimum EXT On-Time
Reference Voltage
REF Load Regulation
REF Line Regulation
1.5
0.7
µs
I
= 0
REF
1.22
V
REF
0 ≤ I
≤ 100µA
mV
µV/V
V
= 4.5V to 36V, I
= 0
IN
REF
Note 1: Specifications to -40°C are guaranteed by design, not production tested.
4
_______________________________________________________________________________________
High-Voltage, Step-Down DC-DC
Controller in µMAX
Typical Operating Characteristics
(Circuit of Figure 1, T = +25°C, unless otherwise specified.)
A
EFFICIENCY vs. LOAD CURRENT
IN PIN QUIESCENT CURRENT
vs. INPUT VOLTAGE (5.5V TO 36V)
EFFICIENCY vs. LOAD CURRENT
(V
= +3.3V)
OUT
(V
= +5.0V)
OUT
100
80
60
40
20
0
110
105
100
95
100
80
60
40
20
0
B
B
A
A
D
C
C
D
90
A: V = +5.5V
IN
IN
A: V = +7.2V
IN
IN
IN
B: V = +12.0V
B: V = +12.0V
85
C: V = +24.0V
IN
C: V = +24.0V
D: V = +36.0V
IN
D: V = +36.0V
IN
80
0.0001 0.001
0.01
0.1
1
10
0
10
20
30
40
0.0001 0.001
0.01
0.1
1
10
LOAD CURRENT (A)
INPUT VOLTAGE (V)
LOAD CURRENT (A)
IN PIN QUIESCENT CURRENT
vs. INPUT VOLTAGE (3.5V TO 5.5V)
IN PIN QUIESCENT CURRENT
vs. TEMPERATURE
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
6
140
95
94
93
92
91
90
89
88
87
86
85
120
100
5
4
3
2
1
80
60
40
V
OUT
= 3.3V
= 2.0A
20
0
OUT
I
V
OUT
= 3.3V
0
3.5
4.5
5.5
0
10
20
30
40
-50 -25
0
25
50
75 100 125
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
TEMPERATURE (°C)
CURRENT-SENSE TRIP LEVEL
vs. TEMPERATURE
EXT RISE AND FALL TIMES
vs. CAPACITANCE
EXT RISE AND FALL TIMES
vs. TEMPERATURE
120
100
80
60
40
20
0
115
110
105
100
95
50
45
40
35
30
25
20
15
10
5
V
IN
= +5V
V
= +5V
C = 1000pF
IN
L
t
RISE
t
FALL
t
FALL
90
t
RISE
85
0
2000
3000
0
1000
4000
5000
-50 -25
0
25
50
75 100 125
-50 -25
0
25
50
75 100 125
CAPACITANCE (pF)
TEMPERATURE (°C)
TEMPERATURE (°C)
_______________________________________________________________________________________
5
High-Voltage, Step-Down DC-DC
Controller in µMAX
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T = +25°C, unless otherwise specified.)
A
MAX1744
ENTERING/EXITING SHUTDOWN
REFERENCE OUTPUT VOLTAGE CHANGE
vs. TEMPERATURE
MAX1744/5toc11
5
R = 3.3Ω
L
4
3
2
V
OUT
2V/div
1
0
-1
-2
-3
-4
-5
SHUTDOWN
PULSE
5V/div
2ms/div
-50 -25
0
25
50
75 100 125
TEMPERATURE (°C)
LOAD-TRANSIENT RESPONSE
LINE-TRANSIENT RESPONSE
A
A
B
B
50µs/div
4ms/div
V
= 7.2V, V
OUT
= 3.3V, LOAD CURRENT = 0.1A TO 2A
V
= 5V, LOAD CURRENT = 1A
OUT
IN
IN
OUT
OUT
A: V , 50mV/div, 3.3V AC-COUPLED
A: V , 100mV/div, AC-COUPLED
B: LOAD CURRENT, 1A/div
B: V , 6V TO 12V, 5V/div
6
_______________________________________________________________________________________
High-Voltage, Step-Down DC-DC
Controller in µMAX
Pin Description
PIN NAME
PIN
FUNCTION
MAX1744
MAX1745
1
2
GND
VL
GND
Ground
5V Linear Regulator Output. VL provides power to the internal circuitry and can supply up
to 1mA to an external load. Bypass VL to GND with 4.7µF or greater capacitor.
VL
REF
—
1.25V Reference Output. REF can supply up to 100µA to an external load. Bypass REF to
GND with a 0.1µF or greater ceramic capacitor.
3
4
REF
3/5
3.3V or 5V Selection. Connect 3/5 to GND to set the output voltage to 3.3V. Connect 3/5 to
VL to set the output voltage to 5V.
Feedback Input for Adjustable Output Operation. Connect to an external voltage-divider
between the output and FB to set the output voltage. The regulation voltage threshold is
1.25V.
4
5
6
7
8
—
OUT
CS
FB
OUT
CS
Sense Input for Fixed 5V or 3.3V Output Operation (MAX1744) and Negative Current-Sense
Input (MAX1744/5). OUT is connected to an internal voltage-divider (MAX1744). OUT does
not supply current.
Current-Sense Input. Connect the current-sense resistor between CS and OUT. External
MOSFET is turned off when the voltage across the resistor is equal to or greater than the
current limit trip level (100mV).
Active-Low Shutdown Input. Connect SHDN to IN for normal operation. Drive SHDN to low
to shut the part off. In shutdown mode, the reference, output, external MOSFET, and
internal regulators are turned off.
SHDN
VH
SHDN
VH
High-Side Linear Regulator Output. VH provides a regulated output voltage that is 5V below
IN. The external P-channel MOSFET gate is driven between IN and VH. Bypass VH to IN
with a 4.7µF or greater capacitor (see the Capacitor Selection section).
9
EXT
IN
EXT
IN
Gate Drive for External P-Channel MOSFET. EXT swings between IN and VH.
10
Positive Supply Input. Bypass IN to GND with a 0.47µF or greater ceramic capacitor.
Operating Modes
Detailed Description
When delivering low output currents, the MAX1744/
MAX1745 operate in discontinuous-conduction mode.
Current through the inductor starts at zero, rises as
high as the current limit, then ramps down to zero dur-
ing each cycle (Figure 3). The switch waveform exhibits
ringing, which occurs at the resonant frequency of the
inductor and stray capacitance, due to residual energy
trapped in the core when the commutation diode (D1 in
Figure 1) turns off.
The MAX1744/MAX1745 are high-voltage step-down
DC-DC converter controllers. These devices offer high
efficiency over a wide range of input/output voltages
and currents, making them optimal for use in applica-
tions such as telecom, automotive, and industrial con-
trol. Using an external P-channel MOSFET and
current-sense resistor allows design flexibility and
improved efficiency. The MAX1744/MAX1745 automati-
cally switch from PWM operation at medium and heavy
loads to pulse-skipping operation at light loads to
improve light-load efficiency. The low 90µA quiescent
current further optimizes these parts for applications
where low input current is critical. Operation to 100%
duty cycle allows the lowest possible dropout voltage,
which allows a wider input voltage variation. The small
size, high switching frequency, and low parts count
minimize the required circuit board area and compo-
nent cost. Figure 1 shows the MAX1744 typical applica-
tion circuit.
When delivering medium-to-high output currents, the
MAX1744/MAX1745 operate in PWM continuous-con-
duction mode (Figure 4). In this mode, current always
flows through the inductor and never ramps to zero.
The control circuit adjusts the switch duty cycle to
maintain regulation without exceeding the peak switch-
ing current set by the current-sense resistor.
_______________________________________________________________________________________
7
High-Voltage, Step-Down DC-DC
Controller in µMAX
INPUT
4.5V TO 36V
C2
C3
4.7µF
0.47µF
4.7µF
LOW ESR
IN
SHDN
VH
ON
5V
M1
FAIRCHILD
NDS9407
P
OFF
3.3V
EXT
3/5
L1
22µH
R
SENSE
40mΩ
OUT
3.3V OR 5V
2A
MAX1744
CS
OUT
C1
220µF
VL
C5
4.7µF
D1
REF
NIHON
C4
0.1µF
GND
EC2IQ506
Figure 1. Typical Application Circuit
✕
approximately f ≈ 1MHz D, where D is the duty cycle.
If the duty cycle is greater than 33%, the off-time sets the
frequency; and the frequency is approximately f ≈ 500kHz
100% Duty Cycle and Dropout
The MAX1744/MAX1745 operate with a duty cycle up to
100%. This feature extends the input voltage range by
turning the MOSFET on continuously when the supply
voltage approaches the output voltage. This services
the load when conventional switching regulators with
less than 100% duty cycle would fail. Dropout voltage is
defined as the difference between the input and output
voltages when the input is low enough for the output to
drop out of regulation. Dropout depends on the
MOSFET drain-to-source on-resistance, current-sense
resistor, and inductor series resistance, and is propor-
tional to the load current:
✕
(1 - D).
In both cases, the voltage is regulated by the error
comparator. For low duty cycles (<33%), the MOSFET
is turned on for the minimum on-time, causing fixed-on-
time operation. During the MOSFET on-time, the output
voltage rises. Once the MOSFET is turned off, the volt-
age drops to the regulation threshold (set by the inter-
nal voltage-divider for the MAX1745 and by the external
voltage-divider for the MAX1744), at which time another
cycle is initiated. For high duty cycles (>33%), the
MOSFET remains off for the minimum off-time, causing
fixed-off-time operation. In this case, the MOSFET
remains on until the output voltage rises to the regula-
tion threshold. Then the MOSFET turns off for the mini-
mum off-time, initiating another cycle.
Dropout voltage=
I
x R
[
+ R
+ R
OUT
DS(ON)
SENSE INDUCTOR
]
Regulation Control Scheme
By switching between fixed-on-time and fixed-off-time
operation, the MAX1744/MAX1745 can operate at high
input-output ratios, yet still operate up to 100% duty
cycle for low dropout. Note that when transitioning from
fixed-on-time to fixed-off-time operation, the output volt-
age drops slightly due to the output ripple voltage. In
fixed-on-time operation, the minimum output voltage is
regulated, but in fixed-off-time operation, the maximum
output voltage is regulated. Thus, as the input voltage
drops below approximately three times the output volt-
age, a decrease in line regulation can be expected.
The MAX1744/MAX1755 have a unique operating
scheme that allows PWM operation at medium and high
current, with automatic switching to pulse-skipping
mode at lower currents to improve light-load efficiency.
Figure 2 shows the simplified block diagram.
Under medium- and heavy-load operation, the inductor
current is continuous and the part operates in PWM
mode. In this mode, the switching frequency is set by
either the 1µs minimum on-time or the 2µs minimum off-
time, depending on the duty cycle. The duty cycle is
approximately the output voltage divided by the input
voltage. If the duty cycle is less than 33%, the minimum
on-time controls the frequency; and the frequency is
The drop in voltage is approximately V
≈ V
/ 2.
DROP
RIPPLE
8
_______________________________________________________________________________________
High-Voltage, Step-Down DC-DC
Controller in µMAX
EXT
IN
REF
VH
VH
SHDN
VL
LINEAR
REGULATOR
VL
LINEAR
REGULATOR
1.25
REFERENCE
Q
Q
TRIG
OUT
(FB)
MINIMUM
ON-TIME
ONE SHOT
ERROR
COMPARATOR
TRIG
MINIMUM
OFF-TIME
ONE SHOT
3/5
Q
R
SHDN
S
-
+
CS
( ) MAX1745 ONLY
- - - MAX1744 ONLY
100mV
Figure 2. Simplified Functional Diagram
At light output loads, the inductor current is discontinu-
ous, causing the MAX1744/MAX1745 to operate at
lower frequencies, reducing the MOSFET gate drive
and switching losses. In discontinuous mode, under
most circumstances, the on-time will be the fixed mini-
mum on-time of 1µs. If the inductor value is small, or
the current-sense resistor large, the current limit will be
tripped before the minimum on-time, terminating the
on-time and thus setting the fixed on-time.
where I
is the inductor ripple current, and can be
RIPPLE
determined by:
✕
I
= (V - V
)
t
/ L
RIPPLE
IN
OUT
ON(MIN)
where t
mum on-time-control, or:
is the minimum on-time (1µs) for mini-
ON(MIN)
✕
I
= (V
)
t
/ L
RIPPLE
OUT
OFF(MIN)
If the inductance is too large, or the output capacitance
high and equivalent series resistance (ESR) low, then
the MOSFET remains on longer than the minimum on-
time, until the output capacitor charges beyond the
✕
where t
mum off-time-control.
is the minimum off-time (2µs) for mini-
OFF(MIN)
error comparator’s (V
/ 1.25V) 8mV hysteresis,
OUT
causing the part to operate in hysteretic mode.
Operating in hysteretic mode results in lower frequency
operation. The transition to hysteretic mode occurs at
the critical output capacitor ESR:
✕
ESR
= (V
/ 1.25V) 8mV / I
CRITICAL
OUT
RIPPLE
_______________________________________________________________________________________
9
High-Voltage, Step-Down DC-DC
Controller in µMAX
A
B
C
A
B
C
10µs/div
10µs/div
CIRCUIT OF FIGURE 1, V = 18V, V
IN
A: MOSFET DRAIN, 10V/div
= 3.3V, I
= 100mA
CIRCUIT OF FIGURE 1, V = 18V, V
IN
A: MOSFET DRAIN, 10V/div
= 3.3V, I
= 1.5A
OUT
LOAD
OUT
LOAD
B: OUT, 50mV/div, 3.3V DC OFFSET
C: INDUCTOR CURRENT, 1A/div
B: OUT, 50mV/div, 3.3V DC OFFSET
C: INDUCTOR CURRENT, 1A/div
Figure 3. Discontinuous-Conduction Mode, Light-Load-Current
Waveform
Figure 4. Continuous-Conduction Mode, Heavy-Load-Current
Waveform
VL Linear Regulator
The MAX1744/MAX1745 contain a 5V low-side linear reg-
ulator (VL) that powers the internal circuit and can supply
up to 1mA to an external load. This allows the
MAX1744/MAX1745 to operate up to 36V input, while
maintaining low quiescent current and high switching fre-
quency. When the input voltage goes below 5.5V, this
regulator goes into dropout and the IN pin quiescent cur-
rent will rise. See the Typical Operating Characteristics.
Bypass VL with a 4.7µF or greater capacitor.
Shutdown Mode
When SHDN is low, the device enters shutdown mode. In
this mode, the internal circuitry is turned off. EXT is
pulled to IN, turning off the external MOSFET. The shut-
down supply current drops to less than 10µA. SHDN is a
logic-level input. Connect SHDN to IN for normal opera-
tion.
Reference
The 1.25V reference is suitable for driving small external
loads. It has a guaranteed 10mV maximum load regula-
tion while sourcing load currents up to 100µA. The refer-
ence is turned off during shutdown. Bypass the
reference with 0.1µF for normal operation. Place the
bypass capacitor within 0.2in (5mm) of REF, with a direct
trace to GND.
VH Linear Regulator
The MAX1744/MAX1745 contain a high-side linear reg-
ulator (VH) that regulates its output to 5V below IN (the
positive supply input voltage). This regulator limits the
external P-channel MOSFET gate swing (EXT), allowing
high input voltage operation without exceeding the
MOSFET gate-source breakdown. Bypass VH with a
4.7µF or greater capacitor between IN and VH.
Design Information
Setting the Output Voltage
The MAX1744’s output voltage can be selected to 3.3V
or 5V under logic control by using the 3/5 pin. Connect
the 3/5 pin to GND to ensure a 3.3V output, or connect
Quiescent Current
The devices’ typical quiescent current is 90µA.
However, actual applications draw additional current to
supply MOSFET switching currents, OUT pin current,
external feedback resistors (if used), and both the diode
and capacitor leakage currents. For example, in the cir-
the 3/5 pin to V to ensure a 5V output.
L
The MAX1745’s output voltage is set using two resis-
tors, R2 and R3 (Figure 5), which form a voltage-divider
between the output and FB. R2 is given by:
cuit of Figure 1, with IN at 30V and V
at 5V, typical
OUT
no-load supply current for the entire circuit is 100µA.
V
V
OUT
R2= R3 x
−1
REF
10 ______________________________________________________________________________________
High-Voltage, Step-Down DC-DC
Controller in µMAX
where V
= 1.25V. Since the input bias current at FB
REF
FROM
OUTPUT
has a maximum value of 50nA, large values (10kΩ to
200kΩ) can be used for R3 with no significant accuracy
loss. For 1% error, the current through R2 should be at
least 100 times FB’s input bias current.
R2
TO FB
Current-Sense-Resistor Selection
The current-sense comparator limits the peak switching
R3
current to V /R
, where R
is the value of
SENSE
CS SENSE
the current-sense resistor and V
is the current-sense
CS
threshold. V
is typically 100mV. Minimizing the peak
CS
switching current will increase efficiency and reduce
the size and cost of external components. However,
since available output current is a function of the peak
switching current, the peak current limit must not be set
too low.
Figure 5. Adjustable-Output Operation Using the MAX1745
External Switching Transistor
The MAX1744/MAX1745 drive a P-channel enhance-
ment-mode MOSFET. The EXT output swings from VH
to IN. Be sure that the MOSFET’s on-resistance is spec-
ified for 5V gate drive or less. Table 1 recommends
MOSFET suppliers.
Set the peak current limit to 1.3 times the maximum
load current by setting the current-sense resistor to:
V
CS(MIN)
R
=
CS
1.3 x I
OUT(MAX)
Four important parameters for selecting a P-channel
MOSFET are drain-to-source breakdown voltage, cur-
rent rating, total gate charge (Q ), and R . The
DS(ON)
Inductor Selection
g
drain-to-source breakdown voltage rating should be at
The essential parameters for inductor selection are
inductance and current rating. The MAX1744/MAX1745
operate with a wide range of inductance values. In many
applications, values between 4.7µH and 100µH take
best advantage of the controller’s high switching fre-
quency.
least a few volts higher than V . Choose a MOSFET
IN
with a maximum continuous drain-current rating higher
than the peak current limit:
V
CS(MAX)
I
≥ I
=
D(MAX)
LIM(MAX)
R
SENSE
Calculate the minimum inductance value as follows:
The Qg specification should be 80nC or less to ensure
fast drain voltage rise and fall times, and reduce power
V
- V
x 1µs
(
=
)
IN
OUT
L
(MIN)
V
CS(MIN)
losses during transition through the linear region. Q
g
R
specifies all of the capacitances associated with charg-
ing the MOSFET gate. EXT pin rise and fall times vary
with different capacitive loads, as shown in the Typical
CS
where 1µs is the minimum on-time. Inductor values
between 2 and 10 times L are recommended. With
high inductor values, the MAX1744/MAX1745 begin
continuous-conduction operation at a lower fraction of
the full load (see the Detailed Description section).
(MIN)
Operating Characteristics. R
should be as low as
DS(ON)
practical to reduce power losses while the MOSFET is
on. It should be equal to or less than the current-sense
resistor.
The inductor’s saturation and heating current ratings
must be greater than the peak switching current to pre-
vent overheating and core saturation. Saturation occurs
when the inductor’s magnetic flux density reaches the
maximum level the core can support, and inductance
starts to fall. The heating current rating is the maximum
DC current the inductor can sustain without overheating.
For optimum efficiency, the inductor windings’ resis-
tance should be less than the current-sense resistance.
If necessary, use a toroid, pot-core, or shielded-core
inductor to minimize radiated noise. Table 1 lists induc-
tor types and suppliers for various applications.
______________________________________________________________________________________ 11
High-Voltage, Step-Down DC-DC
Controller in µMAX
Capacitor Selection
Table 1. Component Suppliers
Choose filter capacitors to service input and output
peak currents with acceptable voltage ripple. ESR in
the output capacitor is a major contributor to output rip-
ple, so low-ESR capacitors are recommended. Low-
ESR tantalum, polymer, or ceramic capacitors are best.
Low-ESR aluminum electrolytic capacitors are tolera-
ble, but standard aluminum electrolytic capacitors are
not recommended.
COMPANY COUNTRY
PHONE
FAX
803-946-0690
or
AVX
USA
803-626-3123
800-282-4975
847-639-6400
516-241-7876
402-564-3131
408-986-0424
USA
USA
USA
USA
Coilcraft
847-639-1469
516-241-9339
402-563-6418
408-986-1442
Coiltronics
Dale/Vishay
Kemet
Voltage ripple is the sum of contributions from ESR and
the capacitor value:
International
Rectifier
V
≈ V
+ V
RIPPLE
RIPPLE,ESR RIPPLE,C
USA
310-322-3331
310-322-3332
For tantalum capacitors, the ripple is determined by the
ESR, but for ceramic capacitors, the ripple is mostly
due to the capacitance. Voltage ripple as a conse-
quence of ESR is approximated by:
USA
USA
IRC
512-992-7900
602-303-5454
512-992-3377
602-994-6430
Motorola
USA
Japan
847-843-7500
847-843-2798
Nichicon
Nihon
81-7-5231-8461 81-7-5256-4158
805-867-2555 805-867-2698
81-3-3494-7411 81-3-3494-7414
619-661-6835 619-661-1055
V
≈ (R
)∆ I
RIPPLE,ESR
ESR p−p
USA
Japan
The ripple due to the capacitance is approximately:
2
USA
Japan
Sanyo
LI
2CV
PEAK
81-7-2070-6306 81-7-2070-1174
V
≈
RIPPLE,C
O
408-988-8000
Siliconix
USA
USA
408-970-3950
or
Estimate input and output capacitor values for given
voltage ripple as follows:
800-554-5565
603-224-1961
Sprague
Sumida
603-224-1430
847-956-0702
81-3-3607-5111 81-3-3607-5144
2
1
2
LI
USA
Japan
847-956-0666
∆L
C
C
=
IN
V
V
RIPPLE,CIN IN
United
Chemi-Con
2
1
2
USA
714-255-9500 714-255-9400
LI
V
∆L
IN
=
OUT
V
V
V − V
IN OUT
RIPPLE,COUT OUT
Diode Selection
The MAX1744/MAX1745’s high switching frequency
demands a high-speed rectifier. Schottky diodes, such
as the 1N5817–1N5822 family or surface-mount equiva-
lents, are recommended. Ultra-high-speed rectifiers
with reverse recovery times around 50ns or faster
should be used for high output voltages, where the
increased forward drop causes less efficiency degra-
dation. Make sure that the diode’s peak current rating
where I is the change in inductor current.
∆L
These equations are suitable for initial capacitor selec-
tion; final values should be set by testing a prototype or
evaluation kit. When using tantalum capacitors, use
good soldering practices to prevent excessive heat
from damaging the devices and increasing their ESR.
Also, ensure that the tantalum capacitors’ surge-current
ratings exceed the startup inrush and peak switching
currents.
exceeds the peak current limit set by R
, and that
SENSE
its breakdown voltage exceeds V . Schottky diodes
IN
Pursuing output ripple lower than the error compara-
tor’s hysteresis (0.6% of the output voltage) is not prac-
tical, since the MAX1744/MAX1745 will switch at slower
frequencies, increasing inductor ripple current thresh-
old. Choose an output capacitor with a working voltage
rating higher than the output voltage.
are preferred for heavy loads due to their low forward
voltage, especially in low-voltage applications. For
high-temperature applications, some Schottky diodes
may be inadequate due to their high leakage currents.
In such cases, ultra-high-speed rectifiers are recom-
mended, although a Schottky diode with a higher
reverse voltage rating can often provide acceptable
performance.
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
12 ______________________________________________________________________________________
High-Voltage, Step-Down DC-DC
Controller in µMAX
ripple at IN, caused by the circuit’s switching action.
degrade performance. The current-sense resistor must
be placed within 0.2 inches (5mm) of the controller IC,
directly between OUT and CS. Place voltage feedback
resistors (MAX1745) next to the FB pin (no more than
0.2in) rather than near the output. Place the 0.47µF input
bypass capacitor within 0.2in (5mm) of IN.
Use a low-ESR capacitor. Two smaller-value low-ESR
capacitors can be connected in parallel if necessary.
Choose input capacitors with working voltage ratings
higher than the maximum input voltage.
Place a surface-mount ceramic capacitor very close to
IN and GND. This capacitor bypasses the MAX1744/
MAX1745, minimizing the effects of spikes and ringing
on the power source (IN).
Refer to the MAX1744 Evaluation Kit manual for a two-
layer PC board example.
Chip Information
Bypass REF with 0.1µF. This capacitor should be
placed within 0.2 inches (5mm) of the IC, next to REF,
with a direct trace to GND.
TRANSISTOR COUNT: 645
Layout Considerations
High-frequency switching regulators are sensitive to PC
board layout. Poor layout introduces switching noise into
the current and voltage feedback signals and may
______________________________________________________________________________________ 13
High-Voltage, Step-Down DC-DC
Controller in µMAX
Package Information
Note: MAX1744/MAX1745 do not feature exposed pads
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2001 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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