MAX1744EUB/V+ [MAXIM]
High-Voltage, Step-Down DC-DC Controllers in μMAX; 高电压，降压，μMAX封装DC- DC控制器
| 型号： | MAX1744EUB/V+ |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | High-Voltage, Step-Down DC-DC Controllers in μMAX |
| 文件： | 总15页 (文件大小：296K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-1776; Rev 4; 8/09
High-Voltage, Step-Down DC-DC
Controllers in µMAX
/MAX1745
General Description
____________________________Features
♦ High-Voltage Operation (Up to 36V IN)
♦ Efficiency > 90%
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.
♦ Output Power Capability Exceeds 50W
♦ 10-Pin µMax Package
♦ Low-Dropout Voltage
♦ 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.
♦ 90µA Quiescent Current
♦ 4µA Shutdown Current
♦ Up to 330kHz Switching Frequency
The MAX1744/MAX1745 are available in a space-sav-
ing 10-pin μMAX package.
®
♦ Output Voltage
5V or 3.3V (MAX1744)
Adjustable 1.25V to 18V (MAX1745)
________________________Applications
♦ Current-Limited Control Scheme
Automotive Electronics
Telecom Systems
Ordering Information
Wall-Cube-Powered Devices
Industrial Control Systems
PART
TEMP RANGE
-40°C to +85°C
-40°C to +125°C
-40°C to +85°C
-40°C to +85°C
-40°C to +125°C
-40°C to +85°C
PIN-PACKAGE
10 μMAX
10 μMAX
10 μMAX
10 μMAX
10 μMAX
10 μMAX
MAX1744EUB+
MAX1744AUB+
MAX1744EUB/V+
MAX1745EUB+
MAX1745AUB+
MAX1745EUB/V+
®
®
Firewire /IEEE 1394
μMAX is a registered trademark of Maxim Integrated Products, Inc.
Firewire is a registered trademark of Apple, Inc.
IEEE is a registered service mark of the Institute of Electrical
and Electronics Engineers, Inc.
+Denotes a lead(Pb)-free/RoHS-compliant package.
/V Denotes an automotive qualified part.
Typical Operating Circuit
Pin Configuration
IN
TOP VIEW
4.5V TO 36V
+
IN VH
GND
VL
1
2
3
4
5
10 IN
P
EXT
ON
5V
SHDN
3/5
OFF
9
8
7
6
EXT
MAX1744
MAX1745
3.3V
REF
VH
MAX1744
3/5 (FB)
SHDN
CS
VL
OUT
CS
REF
OUT
OUT
3.3V
OR 5V
GND
μMAX
( ) ARE FOR MAX1745 ONLY.
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim's website at www.maxim-ic.com.
High-Voltage, Step-Down DC-DC
Controllers 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 = V
IN
= 5.5V to 36V, 3/5 = GND, I
= 0, T = 0°C to +85°C, unless otherwise noted. Typical values at V = V
=
SHDN
LOAD
A
IN
SHDN
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
V
= V = 5.5V to 36V
90
4
140
12
μA
SHDN
IN
/MAX1745
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
CS
CS
CS
CS
IN
OUT
OUT
OUT
OUT
CS Threshold Voltage
CS Input Current
mV
μA
= V
80
GND
= 2.5V to 18V
= V
0
-25
2.4
0
GND
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
μA
3/5 = GND
1
SHDN Input Current
μA
V
= 36V
12
20
2.5
1.5
SHDN
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
Controllers in µMAX
/MAX1745
ELECTRICAL CHARACTERISTICS
(V = V
IN
= 5.5V to 36V, 3/5 = GND, I = 0, T = -40°C to +85°C, unless otherwise noted.) (Note 1)
LOAD A
SHDN
PARAMETER
CONDITIONS
MIN
MAX
36
UNITS
V
Input Voltage Range
4.5
Supply Current into IN
Shutdown Supply Current
V
= V = 5.5V to 36V
140
12
μA
SHDN
IN
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
CS
CS
CS
CS
IN
OUT
OUT
OUT
OUT
CS Threshold Voltage
CS Input Current
mV
μA
= V
80
GND
= 2.5V to 18V
= V
0
-25
2.4
0
GND
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
μA
3/5 = GND
1
SHDN Input Current
μA
V
= 36V
12
20
2.5
1.5
1.28
10
60
SHDN
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
1.22
V
REF
0 ≤ I
≤ 100μA
mV
μV/V
REF
V
= 4.5V to 36V, I
= 0
IN
REF
_______________________________________________________________________________________
3
High-Voltage, Step-Down DC-DC
Controllers in µMAX
ELECTRICAL CHARACTERISTICS
(V = V
IN
= 5.5V to 36V, 3/5 = GND, I = 0, T = -40°C to +125°C, unless otherwise noted.) (Note 1)
LOAD A
SHDN
PARAMETER
CONDITIONS
MIN
MAX
36
UNITS
V
Input Voltage Range
4.5
Supply Current into IN
Shutdown Supply Current
V
= V = 5.5V to 36V
140
15
μA
SHDN
IN
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
CS
CS
CS
CS
IN
OUT
OUT
OUT
OUT
/MAX1745
CS Threshold Voltage
CS Input Current
mV
μA
= V
80
GND
= 2.5V to 18V
= V
0
-25
2.4
0
GND
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
μA
3/5 = GND
1
SHDN Input Current
μA
V
= 36V
12
20
2.5
1.5
1.28
10
80
SHDN
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
1.22
V
REF
0 ≤ I
≤ 100μA
mV
μV/V
REF
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
Controllers in µMAX
/MAX1745
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
A: V = +7.2V
B: V = +12.0V
C: V = +24.0V
D: V = +36.0V
IN
IN
IN
IN
B: V = +12.0V
IN
85
C: V = +24.0V
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
95
94
93
92
91
90
89
88
87
86
85
140
120
100
5
4
3
2
1
80
60
40
V
= 3.3V
= 2.0A
20
0
OUT
V
= 3.3V
I
OUT
OUT
0
-50 -25
0
25
50
75 100 125
3.5
4.5
5.5
0
10
20
30
40
TEMPERATURE (°C)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
EXT RISE AND FALL TIMES
vs. TEMPERATURE
EXT RISE AND FALL TIMES
vs. CAPACITANCE
CURRENT-SENSE TRIP LEVEL
vs. TEMPERATURE
120
100
80
60
40
20
0
50
45
40
35
30
25
20
15
10
5
115
110
105
100
95
V
= +5V
V
= +5V
C = 1000pF
IN
IN
L
t
RISE
t
FALL
t
FALL
t
90
RISE
0
85
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
Controllers in µMAX
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T = +25°C, unless otherwise specified.)
A
REFERENCE OUTPUT VOLTAGE CHANGE
MAX1744
ENTERING/EXITING SHUTDOWN
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
/MAX1745
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
= 3.3V, LOAD CURRENT = 0.1A TO 2A
V
= 5V, LOAD CURRENT = 1A
IN
OUT
OUT
A: V , 50mV/div, 3.3V AC-COUPLED
OUT
A: V , 100mV/div, AC-COUPLED
OUT
B: LOAD CURRENT, 1A/div
B: V , 6V TO 12V, 5V/div
IN
6
_______________________________________________________________________________________
High-Voltage, Step-Down DC-DC
Controllers in µMAX
/MAX1745
Pin Description
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
Controllers in µMAX
INPUT
4.5V TO 36V
C3
4.7μF
C2
4.7μF
LOW ESR
0.47μF
D2
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
D1
NIHON
REF
GND
EC2IQ506
Figure 1. Typical Application Circuit
/MAX1745
If the duty cycle is greater than 33%, the off-time sets the
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:
frequency; and the frequency is approximately f ≈ 500kHz
✕
(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
SENSE INDUCTOR
OUT
DS(ON)
⎣
⎦
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
At light output loads, the inductor current is discontinu-
ous, causing the MAX1744/MAX1745 to operate at
✕
approximately f ≈ 1MHz D, where D is the duty cycle.
8
_______________________________________________________________________________________
High-Voltage, Step-Down DC-DC
Controllers in µMAX
/MAX1745
EXT
IN
REF
VH
VH
LINEAR
REGULATOR
SHDN
VL
VL
LINEAR
REGULATOR
1.25
REFERENCE
Q
Q
TRIG
MINIMUM
ON-TIME
OUT
(FB)
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
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)
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
✕
✕
I
= (V
)
t
/ L
RIPPLE
OUT
OFF(MIN)
where t
is the minimum off-time (2μs) for mini-
OFF(MIN)
mum off-time-control.
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
Controllers in µMAX
A
B
C
A
B
C
10μs/div
10μs/div
CIRCUIT OF FIGURE 1, V = 18V, V
IN
= 3.3V, I
= 100mA
CIRCUIT OF FIGURE 1, V = 18V, V
IN
= 3.3V, I
= 1.5A
OUT
LOAD
OUT
LOAD
A: MOSFET DRAIN, 10V/div
A: MOSFET DRAIN, 10V/div
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
/MAX1745
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 shutdown
supply current drops to less than 10μA. SHDN is a logic-
level input. Connect SHDN to IN for normal operation.
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 regu-
lator (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. Fast line
transients may drive the voltage on VH negative. The
clamp diode (D2) prevents damage to the IC during
such a condition. A Schottky diode with a minimum 40V
reverse rating such as the Nihon EP05Q04 is sufficient
for most applications.
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
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:
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-
⎛
⎞
V
V
OUT
R2= R3 x
−1
⎟
⎜
⎝
⎠
REF
where V
= 1.25V. Since the input bias current at FB
REF
cuit of Figure 1, with IN at 30V and V
at 5V, typical
OUT
has a maximum value of 50nA, large values (10kΩ to
no-load supply current for the entire circuit is 100μA.
200kΩ) can be used for R3 with no significant accuracy
10 ______________________________________________________________________________________
High-Voltage, Step-Down DC-DC
Controllers in µMAX
/MAX1745
loss. For 1% error, the current through R2 should be at
least 100 times FB’s input bias current.
FROM
OUTPUT
Current-Sense-Resistor Selection
The current-sense comparator limits the peak switching
current to V /R , where R is the value of
R2
CS SENSE
the current-sense resistor and V
SENSE
is the current-sense
TO FB
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.
R3
Figure 5. Adjustable-Output Operation Using the MAX1745
Set the peak current limit to 1.3 times the maximum
load current by setting the current-sense resistor to:
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.
V
CS(MIN)
R
=
CS
1.3 x I
OUT(MAX)
Inductor Selection
Four important parameters for selecting a P-channel
MOSFET are drain-to-source breakdown voltage, cur-
The essential parameters for inductor selection are induc-
tance and current rating. The MAX1744/MAX1745 ope-
rate 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 frequency.
rent rating, total gate charge (Q ), and R
. The
DS(ON)
g
drain-to-source breakdown voltage rating should be at
least a few volts higher than V . Choose a MOSFET
IN
with a maximum continuous drain-current rating higher
than the peak current limit:
Calculate the minimum inductance value as follows:
V
- V
V
x1μs
(
)
IN
OUT
V
L
=
CS(MAX)
(MIN)
I
≥I
=
D(MAX)
LIM(MAX)
CS(MIN)
R
SENSE
R
CS
The Qg specification should be 80nC or less to ensure
fast drain voltage rise and fall times, and reduce power
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)
losses during transition through the linear region. Q
g
specifies all of the capacitances associated with charging
the MOSFET gate. EXT pin rise and fall times vary with dif-
ferent capacitive loads, as shown in the Typical Operating
Characteristics. R
should be as low as practical to
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.
DS(ON)
reduce power losses while the MOSFET is on. It should
be equal to or less than the current-sense resistor.
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
Controllers 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
803-946-0690
or
FAX
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
847-843-7500
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
512-992-3377
602-994-6430
847-843-2798
Motorola
USA
Japan
Nichicon
Nihon
/MAX1745
V
≈ (R
)Δ I
ESR p−p
RIPPLE,ESR
USA
Japan
The ripple due to the capacitance is approximately:
USA
Japan
Sanyo
2
LI
2CV
81-7-2070-6306 81-7-2070-1174
PEAK
V
≈
RIPPLE,C
O
408-988-8000
Siliconix
USA
USA
or
408-970-3950
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
1
2
2
LI
USA
Japan
847-956-0666
81-3-3607-5111 81-3-3607-5144
ΔL
C
C
=
IN
V
V
RIPPLE,CIN IN
United
Chemi-Con
1
2
2
USA
714-255-9500 714-255-9400
LI
⎛
⎞
V
ΔL
IN
=
OUT
⎜
⎟
V
V
V
− V
⎝
⎠
RIPPLE,COUT OUT
IN 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.
ΔL
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
Controllers in µMAX
/MAX1745
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.
PROCESS: BiCMOS
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
Controllers in µMAX
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
PACKAGE TYPE
PACKAGE CODE
DOCUMENT NO.
21-0061
10 μMAX
U10CN+1
/MAX1745
α
α
Note: MAX1744/MAX1745 do not feature exposed pads.
14 ______________________________________________________________________________________
High-Voltage, Step-Down DC-DC
Controllers in µMAX
/MAX1745
Revision History
REVISION REVISION
PAGES
DESCRIPTION
CHANGED
NUMBER
DATE
7/00
8/06
4/09
8/09
0
2
3
4
Initial release.
—
—
—
Added lead-free and automotive qualified packages to Ordering Information.
Added MAX1744 automotive package to Ordering Information.
1–4, 10, 13
1
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15
© 2009 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.
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