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19-1776; Rev 4; 8/09
KIT ATION EVALU BLE AVAILA
High-Voltage, Step-Down DC-DC Controllers in MAX MAX1744/MAX1745
General Description ____________________________Features
High-Voltage Operation (Up to 36V IN) Efficiency > 90% Output Power Capability Exceeds 50W 10-Pin Max Package Low-Dropout Voltage 100% (max) Duty Cycle 90A Quiescent Current 4A Shutdown Current Up to 330kHz Switching Frequency Output Voltage 5V or 3.3V (MAX1744) Adjustable 1.25V to 18V (MAX1745) Current-Limited Control Scheme
The MAX1744/MAX1745 are step-down DC-DC controllers 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 external components for space-critical applications. Operation to 100% duty cycle permits the lowest possible 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-saving 10-pin MAX(R) package.
________________________Applications
Automotive Electronics Telecom Systems Wall-Cube-Powered Devices Industrial Control Systems Firewire(R)/IEEE(R) 1394
Ordering Information
PART MAX1744EUB+ MAX1744AUB+ MAX1744EUB/V+ MAX1745EUB+ TEMP RANGE -40C to +85C -40C to +125C -40C to +85C -40C to +85C -40C to +125C -40C to +85C PIN-PACKAGE 10 MAX 10 MAX 10 MAX 10 MAX 10 MAX 10 MAX
MAX 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.
MAX1745AUB+ MAX1745EUB/V+
+Denotes a lead(Pb)-free/RoHS-compliant package. /V Denotes an automotive qualified part.
Typical Operating Circuit
IN 4.5V TO 36V
Pin Configuration
TOP VIEW
+
IN ON 5V OFF 3.3V SHDN 3/5 VL REF GND VH EXT P
GND 1 VL REF 2 3 4 5
10 IN 9 EXT VH SHDN CS
MAX1744 MAX1745
8 7 6
MAX1744
CS OUT OUT 3.3V OR 5V
3/5 (FB) OUT
MAX
( ) ARE FOR MAX1745 ONLY.
________________________________________________________________ Maxim Integrated Products
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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 MAX MAX1744/MAX1745
ABSOLUTE MAXIMUM RATINGS
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 = +70C) 10-Pin MAX (derate 5.6mW/C above 70C) .............444mW Operating Temperature Range MAX174_EUB ..................................................-40C to +85C MAX174_AUB ................................................-40C to +125C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) ................................+300C
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
(VIN = VSHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA = 0C to +85C, unless otherwise noted. Typical values at VIN = VSHDN = 36V, TA = +25C.)
PARAMETER Input Voltage Range Supply Current into IN Shutdown Supply Current Output Voltage (MAX1744) OUT Input Current (MAX1744) FB Threshold Voltage (MAX1745) FB Input Current (MAX1745) VH Output Voltage with Respect to IN VL Output Voltage VL Undervoltage Lockout CS Threshold Voltage CS Input Current SHDN, 3/5 Logic-High Threshold SHDN, 3/5 Logic-Low Threshold 3/5 Input Current SHDN Input Current EXT Resistance Minimum EXT Off-Time Minimum EXT On-Time Output Line Regulation Output Load Regulation Reference Voltage REF Load Regulation REF Line Regulation Figure 1, 5.5V < VIN < 36V, ILOAD = 1A Figure 1, VIN = 12V, 30mA < ILOAD < 2A IREF = 0 0 IREF 100A VIN = 4.5V to 36V, IREF = 0 1.22 1.5 0.7 VCS = VOUT = 2.5V to 18V VCS = VOUT = VGND VCS = VOUT = 2.5V to 18V VCS = VOUT = VGND VIN = 4.5V to 36V VIN = 4.5V to 36V SHDN = GND 3/5 = GND VSHDN = 36V 8 2.0 1.0 5 15 1.25 4 30 1.28 10 60 VIN = 5.5V to 36V, IVH = 100A to 20mA VIN = 5.5V to 36V, IVL = 100A to 2mA VSHDN = VIN = 5.5V to 36V SHDN = GND 3/5 = VL 3/5 = GND 3/5 = VL, VOUT = 5V Falling edge, hysteresis = 8mV 1.22 -50 -6.0 4.5 2.0 85 80 0 -25 2.4 0.4 1 1 12 20 2.5 1.5 -5.3 5.0 3.0 100 110 15 4.85 3.20 CONDITIONS MIN 4.5 90 4 5.00 3.30 28 1.25 TYP MAX 36 140 12 5.15 3.40 44 1.28 50 -4.3 5.5 4.1 115 150 25 0 UNITS V A A V A V nA V V V mV A V V A A s s mV/V mV/A V mV V/V
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High-Voltage, Step-Down DC-DC Controllers in MAX MAX1744/MAX1745
ELECTRICAL CHARACTERISTICS
(VIN = VSHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Range Supply Current into IN Shutdown Supply Current Output Voltage (MAX1744) OUT Input Current (MAX1744) FB Threshold Voltage (MAX1745) FB Input Current (MAX1745) VH Output Voltage with Respect to IN VL Output Voltage VL Undervoltage Lockout CS Threshold Voltage CS Input Current SHDN, 3/5 Logic-High Threshold SHDN, 3/5 Logic-Low Threshold 3/5 Input Current SHDN Input Current EXT Resistance Minimum EXT Off-Time Minimum EXT On-Time Reference Voltage REF Load Regulation REF Line Regulation IREF = 0 0 IREF 100A VIN = 4.5V to 36V, IREF = 0 1.5 0.7 1.22 VCS = VOUT = 2.5V to 18V VCS = VOUT = VGND VCS = VOUT = 2.5V to 18V VCS = VOUT = VGND VIN = 4.5V to 36V VIN = 4.5V to 36V SHDN = GND 3/5 = GND VSHDN = 36V VIN = 5.5V to 36V, IVH = 100A to 20mA VIN = 5.5V to 36V, IVL = 100A to 2mA VSHDN = VIN = 5.5V to 36V SHDN = GND 3/5 = VL 3/5 = GND 3/5 = VL, VOUT = 5V Falling edge, hysteresis = 8mV 1.22 -50 -6.0V 4.5 2.0 85 80 0 -25 2.4 0.4 1 1 12 20 2.5 1.5 1.28 10 60 4.85 3.20 CONDITIONS MIN 4.5 MAX 36 140 12 5.15 3.40 44 1.28 50 -4.3V 5.5 4.1 115 150 25 0 UNITS V A A V A V nA V V V mV A V V A A s s V mV V/V
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High-Voltage, Step-Down DC-DC Controllers in MAX MAX1744/MAX1745
ELECTRICAL CHARACTERISTICS
(VIN = VSHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA = -40C to +125C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Range Supply Current into IN Shutdown Supply Current Output Voltage (MAX1744) OUT Input Current (MAX1744) FB Threshold Voltage (MAX1745) FB Input Current (MAX1745) VH Output Voltage with Respect to IN VL Output Voltage VL Undervoltage Lockout CS Threshold Voltage CS Input Current SHDN, 3/5 Logic-High Threshold SHDN, 3/5 Logic-Low Threshold 3/5 Input Current SHDN Input Current EXT Resistance Minimum EXT Off-Time Minimum EXT On-Time Reference Voltage REF Load Regulation REF Line Regulation IREF = 0 0 IREF 100A VIN = 4.5V to 36V, IREF = 0 1.5 0.7 1.22 VCS = VOUT = 2.5V to 18V VCS = VOUT = VGND VCS = VOUT = 2.5V to 18V VCS = VOUT = VGND VIN = 4.5V to 36V VIN = 4.5V to 36V SHDN = GND 3/5 = GND VSHDN = 36V VIN = 5.5V to 36V, IVH = 100A to 20mA VIN = 5.5V to 36V, IVL = 100A to 2mA VSHDN = VIN = 5.5V to 36V SHDN = GND 3/5 = VL 3/5 = GND 3/5 = VL, VOUT = 5V Falling edge, hysteresis = 8mV 1.22 -50 -6.0V 4.5 1.6 85 80 0 -25 2.4 0.4 1 1 12 20 2.5 1.5 1.28 10 80 4.85 3.20 CONDITIONS MIN 4.5 MAX 36 140 15 5.15 3.40 44 1.28 50 -4.3V 5.5 4.1 115 150 25 0 UNITS V A A V A V nA V V V mV A V V A A s s V mV V/V
Note 1: Specifications to -40C are guaranteed by design, not production tested.
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High-Voltage, Step-Down DC-DC Controllers in MAX
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25C, unless otherwise specified.)
MAX1744/MAX1745
EFFICIENCY vs. LOAD CURRENT (VOUT = +3.3V)
MAX1744/5toc01
EFFICIENCY vs. LOAD CURRENT (VOUT = +5.0V)
B A 80 EFFICIENCY (%) C 60 D
MAX1744/5toc02
IN PIN QUIESCENT CURRENT vs. INPUT VOLTAGE (5.5V TO 36V)
MAX1744/5toc03
100 B A 80 EFFICIENCY (%) C D
100
110 105 QUIESCENT CURRENT (A) 100 95 90 85 80
60
40 A: VIN = +5.5V B: VIN = +12.0V C: VIN = +24.0V D: VIN = +36.0V 0.0001 0.001 0.01 0.1 1 10
40 A: VIN = +7.2V B: VIN = +12.0V C: VIN = +24.0V D: VIN = +36.0V 0.0001 0.001 0.01 0.1 1 10
20
20
0 LOAD CURRENT (A)
0 LOAD CURRENT (A)
0
10
20 INPUT VOLTAGE (V)
30
40
IN PIN QUIESCENT CURRENT vs. INPUT VOLTAGE (3.5V TO 5.5V)
MAX1744/5toc04
SWITCHING FREQUENCY vs. INPUT VOLTAGE
MAX1744/5toc05
IN PIN QUIESCENT CURRENT vs. TEMPERATURE
94 QUIESCENT CURRENT (A) 93 92 91 90 89 88 87
MAX1744/5toc06
6 5 QUIESCENT CURRENT (mA) 4 3 2 1
140 120 SWITCHING FREQUENCY (kHz) 100 80 60 40 20 VOUT = 3.3V IOUT = 2.0A 0 10 20 INPUT VOLTAGE (V) 30
95
VOUT = 3.3V 0 3.5 4.5 INPUT VOLTAGE (V) 5.5 0
86 85 40 -50 -25 0 25 50 75 100 125 TEMPERATURE (C)
EXT RISE AND FALL TIMES vs. CAPACITANCE
MAX1744/5toc07
EXT RISE AND FALL TIMES vs. TEMPERATURE
MAX1744/5toc08
CURRENT-SENSE TRIP LEVEL vs. TEMPERATURE
MAX1744/5toc09
120 VIN = +5V 100 tRISE AND tFALL (ns) 80 60 tFALL 40 20 0 0 1000 2000 3000 4000 tRISE
50 45 40 tRISE AND tFALL (ns) 35 30 25 20 15 10 5 0 tFALL VIN = +5V CL = 1000pF tRISE
115 CURRENT-SENSE TRIP LEVEL (mV) 110 105 100 95 90 85
5000
-50
-25
0
25
50
75
100
125
-50
-25
0
25
50
75
100
125
CAPACITANCE (pF)
TEMPERATURE (C)
TEMPERATURE (C)
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High-Voltage, Step-Down DC-DC Controllers in MAX MAX1744/MAX1745
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise specified.)
REFERENCE OUTPUT VOLTAGE CHANGE vs. TEMPERATURE
MAX1744/5toc10
MAX1744 ENTERING/EXITING SHUTDOWN
MAX1744/5toc11
5 REFERENCE OUTPUT VOLTAGE CHANGE (%) 4 3 2 1 0 -1 -2 -3 -4 -5 -50 -25 0 25 50 75 100
RL = 3.3
VOUT 2V/div
SHUTDOWN PULSE 5V/div
125
2ms/div
TEMPERATURE (C)
LOAD-TRANSIENT RESPONSE
MAX1744/5toc12
LINE-TRANSIENT RESPONSE
MAX1744/5toc13
A
A
B
B
50s/div 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
4ms/div VOUT = 5V, LOAD CURRENT = 1A A: VOUT, 100mV/div, AC-COUPLED B: VIN, 6V TO 12V, 5V/div
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High-Voltage, Step-Down DC-DC Controllers in MAX
Pin Description
NAME PIN MAX1744 1 2 3 4 GND VL REF 3/5 MAX1745 GND VL REF -- 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 4.7F or greater capacitor. 1.25V Reference Output. REF can supply up to 100A to an external load. Bypass REF to GND with a 0.1F or greater ceramic capacitor. 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. 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. 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 4.7F or greater capacitor (see the Capacitor Selection section). Gate Drive for External P-Channel MOSFET. EXT swings between IN and VH. Positive Supply Input. Bypass IN to GND with a 0.47F or greater ceramic capacitor. FUNCTION
MAX1744/MAX1745
4
--
FB
5
OUT
OUT
6
CS
CS
7
SHDN
SHDN
8 9 10
VH EXT IN
VH EXT IN
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 applications such as telecom, automotive, and industrial control. Using an external P-channel MOSFET and current-sense resistor allows design flexibility and improved efficiency. The MAX1744/MAX1745 automatically switch from PWM operation at medium and heavy loads to pulse-skipping operation at light loads to improve light-load efficiency. The low 90A 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 component cost. Figure 1 shows the MAX1744 typical application 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 during 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-conduction 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 switching current set by the current-sense resistor.
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High-Voltage, Step-Down DC-DC Controllers in MAX MAX1744/MAX1745
INPUT 4.5V TO 36V C2 4.7F LOW ESR ON 5V OFF 3.3V 3/5 0.47F C3 4.7F D2 IN SHDN EXT VH
P M1 FAIRCHILD NDS9407 L1 22H
RSENSE 40m
MAX1744
VL REF GND
CS OUT D1 NIHON EC2IQ506
OUT 3.3V OR 5V 2A C1 220F
Figure 1. Typical Application Circuit
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 proportional to the load current: Dropout voltage= IOUT x R DS(ON) + R SENSE + RINDUCTOR N
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 1s minimum on-time or the 2s minimum offtime, 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.
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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-ontime operation. During the MOSFET on-time, the output voltage rises. Once the MOSFET is turned off, the voltage drops to the regulation threshold (set by the internal 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 regulation threshold. Then the MOSFET turns off for the minimum 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 voltage 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 voltage, a decrease in line regulation can be expected. The drop in voltage is approximately VDROP VRIPPLE / 2. At light output loads, the inductor current is discontinuous, causing the MAX1744/MAX1745 to operate at
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High-Voltage, Step-Down DC-DC Controllers in MAX MAX1744/MAX1745
EXT IN REF VH
SHDN
VH LINEAR REGULATOR VL LINEAR REGULATOR Q TRIG MINIMUM ON-TIME ONE SHOT
VL
1.25 REFERENCE OUT
ERROR COMPARATOR Q TRIG MINIMUM OFF-TIME ONE SHOT
(FB)
3/5
Q S R
SHDN
( ) MAX1745 ONLY - - - MAX1744 ONLY
-+
CS
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 minimum on-time of 1s. 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 ontime, 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 (1s) for minimum on-time-control, or: IRIPPLE = (VOUT) tOFF(MIN) / L where tOFF(MIN) is the minimum off-time (2s) for minimum off-time-control.
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High-Voltage, Step-Down DC-DC Controllers in MAX MAX1744/MAX1745
A A
B
B C
C
10s/div 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
10s/div 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
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 regulator (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 frequency. When the input voltage goes below 5.5V, this regulator goes into dropout and the IN pin quiescent current will rise. See the Typical Operating Characteristics. Bypass VL with a 4.7F 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 10A. SHDN is a logiclevel 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 regulation while sourcing load currents up to 100A. The reference is turned off during shutdown. Bypass the reference with 0.1F 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 regulator (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 4.7F 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 VL to ensure a 5V output. The MAX1745's output voltage is set using two resistors, R2 and R3 (Figure 5), which form a voltage-divider between the output and FB. R2 is given by: V R2= R3 x OUT - 1 VREF where VREF = 1.25V. Since the input bias current at FB has a maximum value of 50nA, large values (10k to 200k) can be used for R3 with no significant accuracy
Quiescent Current
The devices' typical quiescent current is 90A. 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 circuit of Figure 1, with IN at 30V and VOUT at 5V, typical no-load supply current for the entire circuit is 100A.
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High-Voltage, Step-Down DC-DC Controllers in MAX
loss. For 1% error, the current through R2 should be at least 100 times FB's input bias current.
MAX1744/MAX1745
FROM OUTPUT R2
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: R CS = VCS(MIN) 1.3 x IOUT(MAX)
TO FB R3
Figure 5. Adjustable-Output Operation Using the MAX1745
External Switching Transistor
The MAX1744/MAX1745 drive a P-channel enhancement-mode MOSFET. The EXT output swings from VH to IN. Be sure that the MOSFET's on-resistance is specified 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, current 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: VCS(MAX) ID(MAX) ILIM(MAX) = R SENSE 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 different 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.
Inductor Selection
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 4.7H and 100H take best advantage of the controller's high switching frequency. Calculate the minimum inductance value as follows: L (MIN) =
( VIN - VOUT ) x 1s
VCS(MIN) R CS
where 1s 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 prevent 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' resistance 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 inductor types and suppliers for various applications.
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High-Voltage, Step-Down DC-DC Controllers in MAX MAX1744/MAX1745
Table 1. Component Suppliers
COMPANY AVX Coilcraft Coiltronics Dale/Vishay Kemet International Rectifier IRC Motorola Nichicon Nihon Sanyo COUNTRY USA USA USA USA USA USA USA USA USA Japan USA Japan USA Japan USA USA USA Japan USA PHONE 803-946-0690 or 800-282-4975 847-639-6400 516-241-7876 402-564-3131 408-986-0424 310-322-3331 512-992-7900 602-303-5454 847-843-7500 81-7-5231-8461 805-867-2555 81-3-3494-7411 619-661-6835 81-7-2070-6306 408-988-8000 Siliconix Sprague Sumida United Chemi-Con or 800-554-5565 603-224-1961 847-956-0666 81-3-3607-5111 714-255-9500 603-224-1430 847-956-0702 81-3-3607-5144 714-255-9400 408-970-3950 847-639-1469 516-241-9339 402-563-6418 408-986-1442 310-322-3332 512-992-3377 602-994-6430 847-843-2798 81-7-5256-4158 805-867-2698 81-3-3494-7414 619-661-1055 81-7-2070-1174 803-626-3123 FAX
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 ripple, so low-ESR capacitors are recommended. LowESR tantalum, polymer, or ceramic capacitors are best. Low-ESR aluminum electrolytic capacitors are tolerable, but standard aluminum electrolytic capacitors are not recommended. Voltage ripple is the sum of contributions from ESR and the capacitor value: VRIPPLE VRIPPLE,ESR + VRIPPLE,C 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 consequence of ESR is approximated by: VRIPPLE,ESR (R ESR ) Ip -p The ripple due to the capacitance is approximately: VRIPPLE,C LI 2 PEAK 2CVO
Estimate input and output capacitor values for given voltage ripple as follows: C IN =
1 LI 2 2 L
VRIPPLE,CINVIN VIN V -V VRIPPLE,COUT VOUT IN OUT
1 LI 2 2 L
C 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 equivalents, 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 degradation. 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 recommended, although a Schottky diode with a higher reverse voltage rating can often provide acceptable performance.
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where IL is the change in inductor current. These equations are suitable for initial capacitor selection; 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 comparator's hysteresis (0.6% of the output voltage) is not practical, since the MAX1744/MAX1745 will switch at slower frequencies, increasing inductor ripple current threshold. 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
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High-Voltage, Step-Down DC-DC Controllers in MAX
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 0.1F. This capacitor should be placed within 0.2 inches (5mm) of the IC, next to REF, with a direct trace to GND. 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 0.47F input bypass capacitor within 0.2in (5mm) of IN. Refer to the MAX1744 Evaluation Kit manual for a twolayer PC board example.
MAX1744/MAX1745
Chip Information
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
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High-Voltage, Step-Down DC-DC Controllers in MAX MAX1744/MAX1745
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE 10 MAX PACKAGE CODE U10CN+1 DOCUMENT NO. 21-0061
10LUMAX.EPS
Note: MAX1744/MAX1745 do not feature exposed pads.
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High-Voltage, Step-Down DC-DC Controllers in MAX
Revision History
REVISION NUMBER 0 2 3 4 REVISION DATE 7/00 8/06 4/09 8/09 Initial release. -- Added lead-free and automotive qualified packages to Ordering Information. Added MAX1744 automotive package to Ordering Information. DESCRIPTION PAGES CHANGED -- -- 1-4, 10, 13 1
MAX1744/MAX1745
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
(c) 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.

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