General Description
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.
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.
The MAX1744/MAX1745 are available in a space-sav-
ing 10-pin µMAX package.
________________________Applications
Automotive Electronics
Telecom Systems
Wall-Cube-Powered Devices
Industrial Control Systems
Firewire/IEEE1394
____________________________Features
♦High-Voltage Operation (Up to 36V IN)
♦Efficiency > 90%
♦Output Power Capability Exceeds 50W
♦10-Pin µMax Package
♦Low-Dropout Voltage
♦100% (max) Duty Cycle
♦90µA Quiescent Current
♦4µA Shutdown Current
♦Up to 330kHz Switching Frequency
♦Output Voltage
5V or 3.3V (MAX1744)
Adjustable 1.25V to 18V (MAX1745)
♦Current-Limited Control Scheme
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
________________________________________________________________ Maxim Integrated Products 1
19-1776; Rev 2; 8/06
Ordering Information
PART TEMP RANGE PIN-PACKAGE
MAX1744EUB -40°C to +85°C 10 µMAX
MAX1744AUB
-40°C to +125°C
10 µMAX
MAX1745EUB -40°C to +85°C 10 µMAX
MAX1745AUB
-40°C to +125°C
10 µMAX
GND OUT
REF
VL
CS
MAX1744
3/5
5V
3.3V
ON
OFF SHDN
IN VH EXT P
IN
4.5V TO 36V
OUT
3.3V
OR 5V
Typical Operating Circuit
1
2
3
4
5
10
9
8
7
6
IN
EXT
VH
3/5 (FB)
( ) ARE FOR MAX1745 ONLY.
REF
VL
GND
MAX1744
MAX1745
µMAX
TOP VIEW
CSOUT
SHDN
Pin Configuration
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.
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN = SHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA= 0°C to +85°C, unless otherwise noted. Typical values at VIN = SHDN =
36V, TA= +25°C.)
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.
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
Continuous Power Dissipation (TA= +70°C)
10-Pin µMAX (derate 5.6mW/°C above 70°C) .............444mW
Operating Temperature Range
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
PARAMETER CONDITIONS MIN TYP MAX UNITS
Input Voltage Range 4.5 36 V
Supply Current into IN SHDN = VIN = 5.5V to 36V 90 140 µA
Shutdown Supply Current SHDN = GND 4 12 µA
3/5 = VL 4.85 5.00 5.15
Output Voltage (MAX1744) 3/5 = GND 3.20 3.30 3.40 V
OUT Input Current (MAX1744) 3/5 = VL, VOUT = 5V 28 44 µA
FB Threshold Voltage (MAX1745) Falling edge, hysteresis = 8mV 1.22 1.25 1.28 V
FB Input Current (MAX1745) -50 50 nA
VH Output Voltage with Respect to IN VIN = 5.5V to 36V, IVH = 100µA to 20mA -6.0 -5.3 -4.3 V
VL Output Voltage VIN = 5.5V to 36V, IVL = 100µA to 2mA 4.5 5.0 5.5 V
VL Undervoltage Lockout 2.0 3.0 4.1 V
VCS = VOUT = 2.5V to 18V 85 100 115
CS Threshold Voltage VCS = VOUT = GND 80 110 150 mV
VCS = VOUT = 2.5V to 18V 0 15 25
CS Input Current VCS = VOUT = GND -25 0 µA
SHDN, 3/5 Logic-High Threshold VIN = 4.5V to 36V 2.4 V
SHDN, 3/5 Logic-Low Threshold VIN = 4.5V to 36V 0.4 V
3/5 Input Current SHDN = GND ±1µA
3/5 = GND ±1
SHDN Input Current SHDN = 36V 12 µA
EXT Resistance 820Ω
Minimum EXT Off-Time 1.5 2.0 2.5 µs
Minimum EXT On-Time 0.7 1.0 1.5 µs
Output Line Regulation Figure 1, 5.5V < VIN < 36V, ILOAD = 1A 5 mV/V
Output Load Regulation Figure 1, VIN = 12V, 30mA < ILOAD < 2A 15 mV/A
Reference Voltage IREF = 0 1.22 1.25 1.28 V
REF Load Regulation 0 ≤ IREF ≤ 100µA 4 10 mV
REF Line Regulation VIN = 4.5V to 36V, IREF = 0 30 60 µV/V
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS
(VIN = SHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)
PARAMETER CONDITIONS MIN MAX UNITS
Input Voltage Range 4.5 36 V
Supply Current into IN SHDN = VIN = 5.5V to 36V 140 µA
Shutdown Supply Current SHDN = GND 12 µA
3/5 = VL 4.85 5.15
Output Voltage (MAX1744) 3/5 = GND 3.20 3.40 V
OUT Input Current (MAX1744) 3/5 = VL, VOUT = 5V 44 µA
FB Threshold Voltage (MAX1745) Falling edge, hysteresis = 8mV 1.22 1.28 V
FB Input Current (MAX1745) -50 50 nA
VH Output Voltage with Respect to IN VIN = 5.5V to 36V, IVH = 100µA to 20mA -6.0V -4.3V V
VL Output Voltage VIN = 5.5V to 36V, IVL = 100µA to 2mA 4.5 5.5 V
VL Undervoltage Lockout 2.0 4.1 V
VCS = VOUT = 2.5V to 18V 85 115
CS Threshold Voltage VCS = VOUT = GND 80 150 mV
VCS = VOUT = 2.5V to 18V 0 25
CS Input Current VCS = VOUT = GND -25 0 µA
SHDN, 3/5 Logic-High Threshold VIN = 4.5V to 36V 2.4 V
SHDN, 3/5 Logic-Low Threshold VIN = 4.5V to 36V 0.4 V
3/5 Input Current SHDN = GND ±1µA
3/5 = GND ±1
SHDN Input Current SHDN = 36V 12 µA
EXT Resistance 20 Ω
Minimum EXT Off-Time 1.5 2.5 µs
Minimum EXT On-Time 0.7 1.5 µs
Reference Voltage IREF = 0 1.22 1.28 V
REF Load Regulation 0 ≤ IREF ≤ 100µA 10 mV
REF Line Regulation VIN = 4.5V to 36V, IREF = 0 60 µV/V
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS
(VIN = SHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA= -40°C to +125°C, unless otherwise noted.) (Note 1)
PARAMETER CONDITIONS MIN MAX UNITS
Input Voltage Range 4.5 36 V
Supply Current into IN SHDN = VIN = 5.5V to 36V 140 µA
Shutdown Supply Current SHDN = GND 15 µA
3/5 = VL 4.85 5.15
Output Voltage (MAX1744) 3/5 = GND 3.20 3.40 V
OUT Input Current (MAX1744) 3/5 = VL, VOUT = 5V 44 µA
FB Threshold Voltage (MAX1745) Falling edge, hysteresis = 8mV 1.22 1.28 V
FB Input Current (MAX1745) -50 50 nA
VH Output Voltage with Respect to IN VIN = 5.5V to 36V, IVH = 100µA to 20mA -6.0V -4.3V V
VL Output Voltage VIN = 5.5V to 36V, IVL = 100µA to 2mA 4.5 5.5 V
VL Undervoltage Lockout 1.6 4.1 V
VCS = VOUT = 2.5V to 18V 85 115
CS Threshold Voltage VCS = VOUT = GND 80 150 mV
VCS = VOUT = 2.5V to 18V 0 25
CS Input Current VCS = VOUT = GND -25 0 µA
SHDN, 3/5 Logic-High Threshold VIN = 4.5V to 36V 2.4 V
SHDN, 3/5 Logic-Low Threshold VIN = 4.5V to 36V 0.4 V
3/5 Input Current SHDN = GND ±1µA
3/5 = GND ±1
SHDN Input Current SHDN = 36V 12 µA
EXT Resistance 20 Ω
Minimum EXT Off-Time 1.5 2.5 µs
Minimum EXT On-Time 0.7 1.5 µs
Reference Voltage IREF = 0 1.22 1.28 V
REF Load Regulation 0 ≤ IREF ≤ 100µA 10 mV
REF Line Regulation VIN = 4.5V to 36V, IREF = 0 80 µV/V
Note 1: Specifications to -40°C are guaranteed by design, not production tested.
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
_______________________________________________________________________________________ 5
100
0
0.0001 0.001 0.01 0.1 1 10
EFFICIENCY vs. LOAD CURRENT
(VOUT = +3.3V)
20
MAX1744/5toc01
LOAD CURRENT (A)
EFFICIENCY (%)
40
60
80
A: VIN = +5.5V
B: VIN = +12.0V
C: VIN = +24.0V
D: VIN = +36.0V
CD
B
A
100
0
0.0001 0.001 0.01 0.1 1 10
EFFICIENCY vs. LOAD CURRENT
(VOUT = +5.0V)
20
MAX1744/5toc02
LOAD CURRENT (A)
EFFICIENCY
(%)
40
60
80
A: VIN = +7.2V
B: VIN = +12.0V
C: VIN = +24.0V
D: VIN = +36.0V
B
A
D
C
80
85
90
95
100
105
110
0 10203040
IN PIN QUIESCENT CURRENT
vs. INPUT VOLTAGE (5.5V TO 36V)
MAX1744/5toc03
INPUT VOLTAGE (V)
QUIESCENT CURRENT (µA)
0
1
4
5
6
3.5 4.5 5.5
IN PIN QUIESCENT CURRENT
vs. INPUT VOLTAGE (3.5V TO 5.5V)
MAX1744/5toc04
INPUT VOLTAGE (V)
QUIESCENT CURRENT (mA)
2
3
VOUT = 3.3V
0
60
40
20
80
120
100
140
0 10203040
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
MAX1744/5toc05
INPUT VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
VOUT = 3.3V
IOUT = 2.0A
IN PIN QUIESCENT CURRENT
vs. TEMPERATURE
MAX1744/5toc06
TEMPERATURE (°C)
QUIESCENT CURRENT (µA)
1007525 500-25
86
87
88
89
90
91
92
93
94
95
85
-50 125
0
40
20
60
100
80
120
0 1000 2000 3000 4000 5000
EXT RISE AND FALL TIMES
vs. CAPACITANCE
MAX1744/5toc07
CAPACITANCE (pF)
tRISE AND tFALL (ns)
VIN = +5V
tFALL
tRISE
EXT RISE AND FALL TIMES
vs. TEMPERATURE
MAX1744/5toc08
TEMPERATURE (°C)
tRISE AND tFALL (ns)
1007525 500-25
5
10
15
20
25
30
35
40
45
50
0
-50 125
tRISE
tFALL
VIN = +5V
CL = 1000pF
CURRENT-SENSE TRIP LEVEL
vs. TEMPERATURE
MAX1744/5toc09
TEMPERATURE (°C)
CURRENT-SENSE TRIP LEVEL (mV)
1007550250-25
90
95
100
105
110
115
85
-50 125
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25°C, unless otherwise specified.)
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise specified.)
REFERENCE OUTPUT VOLTAGE CHANGE
vs. TEMPERATURE
MAX1744/5toc10
TEMPERATURE (°C)
REFERENCE OUTPUT VOLTAGE CHANGE (%)
1007525 500-25
-4
-3
-2
-1
0
1
2
3
4
5
-5
-50 125
VOUT
2V/div
SHUTDOWN
PULSE
5V/div
MAX1744
ENTERING/EXITING SHUTDOWN
MAX1744/5toc11
2ms/div
RL = 3.3Ω
VIN = 7.2V, VOUT = 3.3V, LOAD CURRENT = 0.1A TO 2A
A: VOUT, 50mV/div, 3.3V AC-COUPLED
B: LOAD CURRENT, 1A/div
LOAD-TRANSIENT RESPONSE
MAX1744/5toc12
A
B
50µs/div
VOUT = 5V, LOAD CURRENT = 1A
A: VOUT, 100mV/div, AC-COUPLED
B: VIN, 6V TO 12V, 5V/div
LINE-TRANSIENT RESPONSE
MAX1744/5toc13
A
B
4ms/div
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
_______________________________________________________________________________________ 7
Detailed Description
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.
Operating Modes
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.
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.
Pin Description
NAME
PIN
MAX1744 MAX1745
FUNCTION
1 GND GND Ground
2VL VL
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.
3 REF 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.
4 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.
4— FB
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.
5 OUT OUT
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.
6CS CS
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).
7SHDN SHDN
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.
8VH 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 EXT Gate Drive for External P-Channel MOSFET. EXT swings between IN and VH.
10 IN IN Positive Supply Input. Bypass IN to GND with a 0.47µF or greater ceramic capacitor.
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
8 _______________________________________________________________________________________
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:
Regulation Control Scheme
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
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
✕(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.
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 drop in voltage is approximately VDROP ≈VRIPPLE / 2.
Figure 1. Typical Application Circuit
GND
REF
VL
D2
IN
MAX1744
VH
3/5
5V
3.3V
ON
OFF SHDN
EXT
INPUT
4.5V TO 36V
P
D1
NIHON
EC2IQ506
M1
FAIRCHILD
NDS9407
OUT
CS
OUT
3.3V OR 5V
2A
C1
220µF
L1
22µH
RSENSE
40mΩ
C3
4.7µF
C2
4.7µF
LOW ESR
0.47µF
Dropout voltage=
I x R + R + R
OUT DS(ON) SENSE INDUCTOR
[]
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
_______________________________________________________________________________________ 9
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.
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
error comparator’s (VOUT / 1.25V) ✕8mV hysteresis,
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:
ESRCRITICAL = (VOUT / 1.25V) ✕8mV / IRIPPLE
where IRIPPLE is the inductor ripple current, and can be
determined by:
IRIPPLE = (VIN - VOUT) ✕tON(MIN) / L
where tON(MIN) is the minimum on-time (1µs) for mini-
mum on-time-control, or:
IRIPPLE = (VOUT) ✕tOFF(MIN) / L
where tOFF(MIN) is the minimum off-time (2µs) for mini-
mum off-time-control.
REF
MINIMUM
OFF-TIME
ONE SHOT
Q TRIG
VL
LINEAR
REGULATOR
VH
LINEAR
REGULATOR
1.25
REFERENCE
(FB)
ERROR
COMPARATOR
OUT
3/5
SHDN
CS
100mV
+-
Q
RS
VL
SHDN
VH
EXT IN
MINIMUM
ON-TIME
ONE SHOT
Q TRIG
( ) MAX1745 ONLY
- - - MAX1744 ONLY
Figure 2. Simplified Functional Diagram
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
10 ______________________________________________________________________________________
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.
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 EC05Q04 is sufficient
for most applications.
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-
cuit of Figure 1, with IN at 30V and VOUT at 5V, typical
no-load supply current for the entire circuit is 100µA.
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.
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 VLto ensure a 5V output.
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:
where VREF = 1.25V. Since the input bias current at FB
has a maximum value of 50nA, large values (10kΩto
200kΩ) can be used for R3 with no significant accuracy
R2= R3 x V
V
OUT
REF
−
⎛
⎝
⎜⎞
⎠
⎟
1
CIRCUIT OF FIGURE 1, VIN = 18V, VOUT = 3.3V, ILOAD = 100mA
A: MOSFET DRAIN, 10V/div
B: OUT, 50mV/div, 3.3V DC OFFSET
C: INDUCTOR CURRENT, 1A/div
A
C
B
10µs/div
Figure 3. Discontinuous-Conduction Mode, Light-Load-Current
Waveform
CIRCUIT OF FIGURE 1, VIN = 18V, VOUT = 3.3V, ILOAD = 1.5A
A: MOSFET DRAIN, 10V/div
B: OUT, 50mV/div, 3.3V DC OFFSET
C: INDUCTOR CURRENT, 1A/div
A
C
B
10µs/div
Figure 4. Continuous-Conduction Mode, Heavy-Load-Current
Waveform
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
______________________________________________________________________________________ 11
loss. For 1% error, the current through R2 should be at
least 100 times FB’s input bias current.
Current-Sense-Resistor Selection
The current-sense comparator limits the peak switching
current to VCS/RSENSE, where RSENSE is the value of
the current-sense resistor and VCS is the current-sense
threshold. VCS is typically 100mV. Minimizing the peak
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.
Set the peak current limit to 1.3 times the maximum
load current by setting the current-sense resistor to:
Inductor Selection
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.
Calculate the minimum inductance value as follows:
where 1µs is the minimum on-time. Inductor values
between 2 and 10 times L(MIN) 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).
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.
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.
Four important parameters for selecting a P-channel
MOSFET are drain-to-source breakdown voltage, cur-
rent rating, total gate charge (Qg), and RDS(ON). The
drain-to-source breakdown voltage rating should be at
least a few volts higher than VIN. Choose a MOSFET
with a maximum continuous drain-current rating higher
than the peak current limit:
The Qg specification should be 80nC or less to ensure
fast drain voltage rise and fall times, and reduce power
losses during transition through the linear region. Qg
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. RDS(ON) should be as low as practical to
reduce power losses while the MOSFET is on. It should
be equal to or less than the current-sense resistor.
I
D(MAX LIM MAX CS MAX
SENSE
IV
R
)() ()
≥=
L = V - V
(MIN) IN OUT
()
()
xs
V
R
CS MIN
CS
1µ
R = V
1.3 x I
CS CS(MIN)
OUT(MAX)
R3
R2
FROM
OUTPUT
TO FB
Figure 5. Adjustable-Output Operation Using the MAX1745
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
12 ______________________________________________________________________________________
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
exceeds the peak current limit set by RSENSE, and that
its breakdown voltage exceeds VIN. Schottky diodes
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.
Capacitor Selection
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.
Voltage ripple is the sum of contributions from ESR and
the capacitor value:
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:
The ripple due to the capacitance is approximately:
Estimate input and output capacitor values for given
voltage ripple as follows:
where I∆Lis the change in inductor current.
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.
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.
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
CLI
VV
CLI
VV
V
VV
IN L
RIPPLE CIN IN
OUT L
RIPPLE COUT OUT
IN
IN OUT
=
=−
⎛
⎝
⎜⎞
⎠
⎟
1
2
2
1
2
2
∆
∆
,
,
V
LI
2CV
RIPPLE,C PEAK
O
≈2
V
RIPPLE,ESR ≈−
()RESR Ipp
∆
V
RIPPLE ≈+
,,
VV
RIPPLEESR RIPPLE C
Table 1. Component Suppliers
COMPANY COUNTRY PHONE FAX
803-946-0690
or
AVX USA
800-282-4975
803-626-3123
Coilcraft USA 847-639-6400 847-639-1469
Coiltronics USA 516-241-7876 516-241-9339
Dale/Vishay USA 402-564-3131 402-563-6418
Kemet USA 408-986-0424 408-986-1442
Inter nati onal
Recti fi er USA 310-322-3331 310-322-3332
IRC USA 512-992-7900 512-992-3377
Motorola USA 602-303-5454 602-994-6430
Nichicon USA
Japan
847- 843- 7500
81- 7- 5231- 8461
847-843-2798
81-7-5256-4158
Nihon USA
Japan
805- 867- 2555
81- 3- 3494- 7411
805-867-2698
81-3-3494-7414
Sanyo USA
Japan
619- 661- 6835
81- 7- 2070- 6306
619-661-1055
81-7-2070-1174
408-988-8000
or
Siliconix USA
800-554-5565
408-970-3950
Sprague USA 603-224-1961 603-224-1430
Sumida USA
Japan
847- 956- 0666
81- 3- 3607- 5111
847-956-0702
81-3-3607-5144
United
Chemi-Con USA 714-255-9500 714-255-9400
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
______________________________________________________________________________________ 13
ripple at IN, caused by the circuit’s switching action.
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).
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.
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
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.
Refer to the MAX1744 Evaluation Kit manual for a two-
layer PC board example.
Chip Information
TRANSISTOR COUNT: 645
MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
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
© 2006 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.
10LUMAX.EPS
PACKAGE OUTLINE, 10L uMAX/uSOP
1
1
21-0061
REV.DOCUMENT CONTROL NO.APPROVAL
PROPRIETARY INFORMATION
TITLE:
TOP VIEW
FRONT VIEW
1
0.498 REF
0.0196 REF
S
6°
SIDE VIEW
α
BOTTOM VIEW
0° 0° 6°
0.037 REF
0.0078
MAX
0.006
0.043
0.118
0.120
0.199
0.0275
0.118
0.0106
0.120
0.0197 BSC
INCHES
1
10
L1
0.0035
0.007
e
c
b
0.187
0.0157
0.114
H
L
E2
DIM
0.116
0.114
0.116
0.002
D2
E1
A1
D1
MIN
-A
0.940 REF
0.500 BSC
0.090
0.177
4.75
2.89
0.40
0.200
0.270
5.05
0.70
3.00
MILLIMETERS
0.05
2.89
2.95
2.95
-
MIN
3.00
3.05
0.15
3.05
MAX
1.10
10
0.6±0.1
0.6±0.1
Ø0.50±0.1
H
4X S
e
D2
D1
b
A2 A
E2
E1 L
L1
c
α
GAGE PLANE
A2 0.030 0.037 0.75 0.95
A1
Package Information
Note: MAX1744/MAX1745 do not feature exposed pads
