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19-0201; Rev 0; 11/93
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters
____________________________Features
o o o o o o o o o o High Efficiency for a Wide Range of Load Currents 12V/150mA Flash Memory Programming Supply 110A Max Supply Current 5A Max Shutdown Supply Current 2V to 16.5V Input Voltage Range 12V (MAX761), 15V (MAX762) or Adjustable Output Current-Limited PFM Control Scheme 300kHz Switching Frequency Internal, 1A, N-Channel Power FET LBI/LBO Low-Battery Comparator
_______________General Description
The MAX761/MAX762 step-up switching regulators provide high efficiency over a wide range of load currents, delivering up to 150mA. A unique, current-limited pulse-frequency-modulated (PFM) control scheme gives the devices the benefits of pulse-width-modulated (PWM) converters (high efficiency with heavy loads), while using less than 110A of supply current (vs. 2mA to 10mA for PWM converters). The result is high efficiency over a wide range of loads. The MAX761/MAX762 input voltage range is 2V to 16.5V. Output voltages are preset to 12V (MAX761) and 15V (MAX762), or they can be set with two external resistors. With a 5V input, the MAX761 guarantees a 12V, 150mA output. Its high efficiency, low supply current, fast start-up time, SHDN controlling capability, and small size make the MAX761 ideal for powering flash memory. The MAX761/MAX762 have an internal 1A power MOSFET, making them ideal for minimum-component, low- and medium-power applications. These devices use tiny external components, and their high switching frequencies (up to 300kHz) allow for small surface-mount magnetics. For increased output drive capability or higher output voltages, use the MAX770-MAX773, which are similar in design to the MAX761/MAX762, but drive external power MOSFETs. For stepping up to 5V, see the MAX756/ MAX757 and MAX856-MAX859 data sheets.
MAX761/MAX762
______________Ordering Information
PART MAX761CPA MAX761CSA MAX761C/D MAX761EPA MAX761ESA MAX761MJA MAX762CPA MAX762CSA MAX762C/D MAX762EPA MAX762ESA MAX762MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP** 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP**
_________________________Applications
Flash Memory Programming PCMCIA Cards Battery-Powered Applications High-Efficiency DC-DC Converters
* Contact factory for dice specifications. ** Contact factory for availability and processing to MIL-STD-883.
__________Typical Operating Circuit
INPUT 4.75V TO 12V
__________________Pin Configuration
TOP VIEW
33F 18H
LX
OUTPUT 12V 150mA
33F
LBO LBI FB
1 2 3
8
V+ LX GND REF
ON/OFF
MAX761 SHDN
LBI
V+
MAX761 MAX762
7 6 5
LOW-BATTERY DETECTOR INPUT
LBO REF FB GND
LOW-BATTERY DETECTOR OUTPUT
SHDN 4
DIP/SO
________________________________________________________________ Maxim Integrated Products
1
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters MAX761/MAX762
ABSOLUTE MAXIMUM RATINGS
Supply Voltage V+ to GND .......................................-0.3V to 17V REF, LBO, LBI, SHDN, FB ............................-0.3V to (V+ + 0.3V) LX..............................................................................-0.3V to 17V LX Peak Current ....................................................................1.5A LBO Current ..........................................................................5mA Continuous Power Dissipation (TA = +70C) Plastic DIP (derate 9.09mW/C above +70C) ............727mW SO (derate 5.88mW/C above +70C) .........................471mW CERDIP (derate 8.00mW/C above +70C) .................640mW Operating Temperature Ranges: MAX76_C_A ........................................................0C to +70C MAX76_E_A .....................................................-40C to +85C MAX76_MJA ..................................................-55C to +125C Junction Temperatures: MAX76_C_A/E_A..........................................................+150C MAX76_MJA.................................................................+175C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+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
(V+ = 5V, ILOAD = 0mA, CREF = 0.1F, TA = TMIN to TMAX, typical values are at TA = +25C, unless otherwise noted.) PARAMETER Supply Voltage Minimum Operating Voltage Minimum Start-Up Voltage SYMBOL V+ CONDITIONS Figure 2, bootstrapped Figure 3 or 5 with external resistors. Figure 2, bootstrapped Figure 2, bootstrapped V+ = 16.5V, normal operation, SHDN = 0V, non-bootstrapped Supply Current Figure 2, MAX761, VIN = 5V, SHDN = 0V, normal operation Shutdown Current V+ = 10.0V, shutdown mode, SHDN = V+ Figure 2, MAX761, bootstrapped VOUT Figure 2, MAX762, bootstrapped IPEAK tON tOFF Figure 2, 0mA ILOAD 200mA, bootstrapped Figure 2, 4V VIN 6V, bootstrapped Figure 2, bootstrapped, VOUT = 12V, 60mA ILOAD 120mA MAX76_C Reference Voltage VREF MAX76_E MAX76_M 1.4700 1.4625 1.4550 See Figure 4b 0mA ILOAD 75mA, 3V V+ 12V 0mA ILOAD 150mA, 4.75V V+ 12V 0mA ILOAD 50mA, 3V V+ 15V 0mA ILOAD 100mA, 4.75V V+ 15V 11.52 11.52 14.4 14.4 0.75 6 1.0 300 1 12.0 12.0 15.0 15.0 1.0 8 1.3 0.0042 0.08 86 1.50 1.50 1.50 1.5300 1.5375 1.5450 V 5 12.48 12.48 V 15.6 15.6 1.25 10 1.6 A s s %/mA %/V % A MAX76_C/E MAX76_M MIN 2 3 3.1 1.7 1.7 88 2.0 110 A TYP MAX 16.5 16.5 16.5 V V UNITS V
Output Voltage (Note 1)
Peak Current at LX Maximum Switch-On Time Minimum Switch-Off Time Load Regulation Line Regulation Efficiency
2
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, ILOAD = 0mA, CREF = 0.1F, TA = TMIN to TMAX, typical values are at TA = +25C, unless otherwise noted.) PARAMETER Reference Load Regulation Reference Line Regulation LX Leakage Current SYMBOL CONDITIONS 0A ILOAD 100A 3.0V V+ 16.5V MAX76_C V+ = 16.5V, LX = 17V MAX76_C FB Leakage Current IFB MAX76_E MAX76_M MAX76_C Voltage Trip Point LX On Resistance SHDN Input High Voltage SHDN Input Low Voltage SHDN Leakage Current LBI Threshold Voltage LBI Hysteresis LBI Leakage Current LBO Leakage Current LBO Voltage LBI to LBO Delay Note 1: VOL V+ = 16.5V, VLBI = 1.5V V+ = 16.5V, VLBO = 16.5V V+ = 5.0V, ISINK = 1mA Overdrive = 5mV 2.5 -20 -1 VIH VIL VFB MAX76_E MAX76_M V+ > 5.0V 2.0V V+ 16.5V 2.0V V+ 16.5V V+ = 16.5V, SHDN = 0V or V+ MAX76_C LBI falling MAX76_E MAX76_M -1 1.4700 1.4625 1.4550 1.50 1.50 1.50 20 20 1 0.4 1.6 0.4 1 1.5300 1.5375 1.5450 mV nA A V s V MAX76_E MAX76_M -5 -10 -30 -20 -40 -60 1.4700 1.4625 1.4550 1.50 1.50 1.50 1.0 MAX76_C/E MAX76_M 30 MIN TYP MAX 10 15 100 5 10 30 20 40 60 1.5300 1.5375 1.5450 2.2 V V A V nA A UNITS mV V/V
MAX761/MAX762
See Typical Operating Characteristics for output current capability versus input voltage. Guarantees based on correlation to switching on and off times, on-resistance, and peak-current ratings.
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters MAX761/MAX762
__________________________________________Typical Operating Characteristics
(Circuit of Figure 2, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT BOOTSTRAPPED
MAX761-01
EFFICIENCY vs. OUTPUT CURRENT NON-BOOTSTRAPPED
MAX761-02
QUIESCENT CURRENT vs. INPUT VOLTAGE
1.75 QUIESCENT CURRENT (mA) 1.50 1.25 1.00 0.75 0.50 0.25 0 NON-BOOTSTRAPPED 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 INPUT VOLTAGE (V) BOOTSTRAPPED (EXTERNAL RESISTORS) VOUT = 12V BOOTSTRAPPED (INTERNAL RESISTORS)
MAX761-03 MAX761-09 MAX761-06
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 10 100 VOUT = 12V VIN = 2V VIN = 10V VIN = 5V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 10 VOUT = 12V VIN = 10V VIN = 5V
2.00
1000
100
1000
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE
MAX761-04
REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE
REFERENCE OUTPUT RESISTANCE ()
MAX761-05
REFERENCE vs.TEMPERATURE COEFFICIENT
1.506 1.504 REFERENCE OUTPUT (V) 1.502 1.500 1.498 1.496 1.494 1.492
400 MAXIMUM OUTPUT CURRENT (mA) 350 BOOTSTRAPPED 300 250 200 150 NON-BOOTSTRAPPED 100 50 0 3.0 3.5 4.0 4.5 5.0 5.5 VOUT = 12V
250
200 10A 150
100
50A 100A
50
0 6.0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) SUPPLY VOLTAGE (V)
-60 -40 -20
0 20 40 60 80 100 120 140 TEMPERATURE (C)
NO-LOAD START-UP VOLTAGE
MAX761-07
MAX761 START-UP VOLTAGE vs. RLOAD
2.1 START-UP VOLTAGE (V) 2.0 1.9 1.8 1.7 1.6 1.5 0.6 1.4 1.3 0.4 0.1 1 10 RLOAD (k) 100 1000 VOUT = 12V BOOTSTRAPPED INTERNAL RESISTORS
MAX761-08
LX ON-RESISTANCE vs. TEMPERATURE
1.6 1.4 LX ON-RESISTANCE () V+ = 5V 1.2 1.0 0.8
3.5 VOUT = 12V NO-LOAD START-UP VOLTAGE (V) 3.0 2.5 NON-BOOTSTRAPPED (EXTERNAL RESISTORS) 2.0 1.5 1.0 0.5 -60 -40 -20 BOOTSTRAPPED (INTERNAL RESISTORS) BOOTSTRAPPED (EXTERNAL RESISTORS)
2.2
V+ = 12V
0 20 40 60 80 100 120 140 TEMPERATURE (C)
-60 -40 -20
0 20 40 60 80 100 120 140 TEMPERATURE (C)
4
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, TA = +25C, unless otherwise noted.)
MAX761/MAX762
LX LEAKAGE vs. TEMPERATURE
MAX761-10
PEAK CURRENT AT LX vs. TEMPERATURE
MAX761-11
SHUTDOWN CURRENT vs. TEMPERATURE
3.5 3.0 ICC (A) 2.5 2.0 1.5 V+ = 8V 1.0 0.5 0 V+ = 4V -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) V+ = 15V
MAX761-12
1000 V+ = 15V 100 LX LEAKAGE (nA)
1.5 1.4 1.3 1.2 V+ = 12V
4.0
10
IPEAK (A)
1.1 1.0 0.9 0.8
V+ = 5V
1
0.1
VLX = 16.5V
0.7 0.6
0.01 20 40 60 80 100 120 TEMPERATURE (C) 140
0.5 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
SWITCH-ON TIME vs. TEMPERATURE
MAX761-13
SWITCH-OFF TIME vs. TEMPERATURE
MAX761-14
POWER-SUPPLY CURRENT vs. TEMPERATURE
MAX761-15
8.5
2.0
100 V+ = 16.5V V+ = 3V
V+ = 5V toff (s) ton (s)
V+ = 5V ICC (A) -60 0 60 120
8.0
1.5
90
7.5 -60 0 60 120 TEMPERATURE (C)
1.0 TEMPERATURE (C)
80 -60 0 60 120 TEMPERATURE (C)
SWITCH-ON/SWITCH-OFF TIME RATIO vs.TEMPERATURE
MAX761-16
SHDN RESPONSE TIME
12V
7
ton/toff RATIO (s/s)
V+ = 5V 6 5V 4V
0V 5 -60 0 60 120 ILOAD = 100mA, VIN = 5V A: VOUT, 2V/div B: SHDN (0V to 4V) TEMPERATURE (C) 2ms/div
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters MAX761/MAX762
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, TA = +25C, unless otherwise noted.) LOAD-TRANSIENT RESPONSE
200mA A 0mA
LINE-TRANSIENT RESPONSE
6V A 4V
B
B
5s/div A: ILOAD, (0mA to 200mA) B: VOUT , AC COUPLED, 100mV/div VIN = 5V, VOUT = 12V
5ms/div A: VIN (4V to 6V) B: VOUT, AC COUPLED, 20mV/div IOUT = 50mA, VOUT = 12V
______________________________________________________________Pin Description
PIN NAME FUNCTION Low-battery output is an open-drain output that goes low when LBI is less than 1.5V. Connect to V+ through a pull-up resistor. Leave LBO floating if not used. Input to the internal low-battery comparator. Tie to GND or V+ if not used. Feedback input. For fixed-output bootstrapped operation, connect FB to GND. For adjustable-output bootstrapped operation, connect a resistor divider between V+, FB and GND. For non-bootstrapped operation, there is no fixed-output option. Connect a resistor divider network between VOUT, FB and GND. See Bootstrapped/Non-Bootstrapped Modes section. Active-high TTL/CMOS logic-level input. In shutdown mode (SHDN = V+), the internal switch is turned off and the output voltage equals V+ minus a diode drop (due to the DC path from the input to the output). Tie to GND for normal operation. 1.5V reference output that can source 100A for external loads. Bypass with 0.1F or larger capacitor. Ground Drain of the internal N-channel FET. LX has an output resistance of 1 and a peak current limit of 1A. Power-supply input. In bootstrapped mode, V+ is also the output voltage sense input.
1
2
LBO LBI
3
FB
4
SHDN
5 6 7 8
REF GND LX V+
6
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters MAX761/MAX762
LBO V+ FB
LBI N LBI 100mV REF
DUAL-MODETM COMPARATOR
MAX761 MAX762
ERROR COMPARATOR V+
1.5V REFERENCE
Q TRIG ONE-SHOT
N
S R
Q LOW INPUT VOLTAGE OSCILLATOR UNDER VOLTAGE COMPARATOR 2.5V
Q TRIG ONE-SHOT
LX CURRENT COMPARATOR N
0.2V CURRENT CONTROL CIRCUITRY
0.1V
GND
Figure 1. Simple Block Diagram
________________Detailed Description
Operating Principle
The MAX761/MAX762 BiCMOS step-up switch-mode power supplies provide fixed outputs of 12V and 15V, respectively. They have a unique control scheme that combines the advantages of pulse-frequency modulation (low supply current) and pulse-width modulation (high efficiency at high loads). The internal N-channel power MOSFET allows 1A peak currents, increasing the output current capability over previous pulse-frequency-modulation (PFM) devices. Figure 1 shows the MAX761/ MAX762 block diagram. The MAX761/MAX762 offer three main improvements over prior solutions: (1) the converters operate with tiny surface-mount inductors (less than 5mm diameter)
because of their 300kHz switching frequency, (2) the current-limited PFM control scheme allows 86% efficiencies over a wide range of load currents, and (3) the maximum supply current is only 110A.
Bootstrapped/Non-Bootstrapped Modes
Figures 2 and 3 show the standard application circuits for bootstrapped and non-bootstrapped modes. In bootstrapped mode, the IC is powered from the output (VOUT). In other words, the current needed to power the bootstrapped circuit is different from the V+ current the chip consumes. The voltage applied to the gate of the internal N-channel FET is switched from VOUT to ground, providing more switch-gate drive and increasing the efficiency of the DC-DC converter compared with non-bootstrapped operation.
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters MAX761/MAX762
VIN = +5V
C1 33F 7 5 C3 0.1F R4 2 LX REF C1 R4 V+ 8 C2 0.1F LBI 1 100k R3 R3 3 FB GND 6 LBO C3 4 SHDN GND LBO 1 2 5 LBI REF R1 C2 8 LX V+ R2 L1 18H D1 1N5817
+12V at 150mA
C4 33F
VIN
L1 18H
D1 1N5817 C4
ADJUSTABLE OUTPUT (VOUT)
R2 = R1
( VOUT-1) VREF
7
4
MAX761
SHDN
MAX761 MAX762
FB
3
100k
LOW-BATTERY OUTPUT
Figure 2. Bootstrapped Operating Circuit
In non-bootstrapped mode, the IC is powered from the supply voltage, VIN, and operates with minimum supply current. Since the voltage applied to the gate of the internal FET is reduced, efficiency declines with low input voltages. Note: In non-bootstrapped mode, there is no fixed-output operation; external resistors must be used to set the output voltage. Use 1% external feedback resistors when operating in non-bootstrapped mode (Figure 3). Use bootstrapped mode when VIN is below approximately 4V. For VIN between 4V and 6V, the trade-off is lower supply current in non-bootstrapped mode versus higher output current in bootstrapped mode (see Typical Operating Characteristics).
LOW-BATTERY DETECT VTRIP - VREF R4 = R3 VREF VREF = 1.5V NOMINAL C1 = 33F C2 = 0.1F C3 = 0.1F C4 = 33F
(
)
6
LOW-BATTERY DETECT OUTPUT
Figure 3. Non-Bootstrapped Operating Circuit
Pulse-Frequency Modulation (PFM) Control Scheme
The MAX761/MAX762 use a proprietary current-limited PFM control scheme. This control scheme combines the ultra-low supply current of pulse-skipping PFM converters with the high full-load efficiency characteristic of current-mode pulse-width-modulation (PWM) converters. It allows the devices to achieve high efficiency over a wide range of loads, while the current-sense function and high operating frequency allow the use of tiny external components. As with traditional PFM converters, the internal power MOSFET is turned on when the voltage comparator senses the output is out of regulation (Figure 1). However, unlike traditional PFM converters, switching is accomplished through the combination of a peak cur8
rent limit and a pair of one-shots that set the maximum on-time (8s) and minimum off-time (1.3s) for the switch. Once off, the minimum off-time one-shot holds the switch off for 1.3s. After this minimum time, the switch either (1) stays off if the output is in regulation, or (2) turns on again if the output is out of regulation. The MAX761/MAX762 also limit the peak inductor current, allowing the devices to run in continuous-conduction mode (CCM) and maintain high efficiency with heavy loads (Figure 4a). This current-limiting feature is a key component of the control circuitry. Once turned on, the switch stays on until either (1) the maximum ontime one-shot turns it off (8s later), or (2) the current limit is reached. To increase light-load efficiency, the current limit for the first two pulses is set to half the peak current limit. If those pulses bring the output voltage into regulation, the voltage comparator holds the MOSFET off, and the current limit remains at half the peak current limit. If the output voltage is still out of regulation after two pulses, the current limit for the next pulse is raised to the full current limit of 1A (Figure 4b).
Internal vs. External Resistors
When external feedback resistors are used, an internal undervoltage lockout system prevents start-up until V+ rises to about 2.7V. When external feedback resistors are
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters MAX761/MAX762
1A 500mA
1A 500mA 0A
Figure 4a. CCM, Heavy Load Current Waveform (500mA/div)
Figure 4b. Light/Medium Load Current Waveform (500mA/div)
used in a bootstrapped circuit (Figure 5), undervoltage lockout prevents start-up at low input voltages; but once started, operation can continue down to a lower voltage that depends on the load. There is no undervoltage lockout when the internal feedback resistors are used (Figure 2), and special circuitry guarantees start-up at 2.0V. The start-up circuitry fixes the duty cycle at 50% until V+ is driven to 2.5V, above which the normal control system takes over.
ringing (the inductor's self-resonant frequency). This ringing is normal and poses no operational problems.
Low-Battery Detector
The MAX761/MAX762 provide a low-battery comparator that compares the voltage on LBI to the 1.5V reference voltage. When the LBI voltage is below VREF, LBO (an open-drain output) goes low. The low-battery comparator's 20mV of hysteresis adds noise immunity, preventing repeated triggering of LBO. Use a resistor-divider network between V+, LBI, and GND to set the desired trip voltage VTRIP (Figure 3). When SHDN is high, LBI is ignored and LBO is high impedance. The value of resistor R3 should be no larger than 500k to ensure the LBI leakage current does not cause inaccuracies in VTRIP.
Shutdown Mode
The MAX761/MAX762 enter shutdown mode when SHDN is high. In this mode, the internal biasing circuitry is turned off (including the reference) and VOUT equals V+ minus a diode drop (due to the DC path from the input to the output). In shutdown mode, the supply current drops to less than 5A. SHDN is a TTL/CMOS logic level input. Connect SHDN to GND for normal operation. LBO is high impedance during shutdown.
__________________Design Procedure
Setting the Output Voltage
The MAX761/MAX762's output voltage can be adjusted from 5V to 16.5V using external resistors R1 and R2 configured as shown in Figures 3 and 5. For adjustableoutput operation, select feedback resistor R1 in the 10k to 250k range. Higher R1 values within this range give lowest supply current and best light-load efficiency. R2 is given by: R2 = (R1)( VOUT - 1) VREF where VREF = 1.5V. Note: Tie FB to GND for fixed-output operation (bootstrapped mode only).
9
Modes of Operation
When delivering high output currents, the MAX761/ MAX762 operate in CCM. In this mode, current always flows in the inductor, and the control circuit adjusts the switch's duty cycle on a cycle-by-cycle basis to maintain regulation without exceeding the switch-current capability. This provides excellent load-transient response and high efficiency. In discontinuous-conduction mode (DCM), current through the inductor starts at zero, rises to a peak value, then ramps down to zero on each cycle. Although efficiency is still excellent, the switch waveforms contain
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters MAX761/MAX762
VIN
C1 LX 5 C3 2 LBI FB SHDN GND 6 VREF = 1.5V NOMINAL R2 = R1 R1 3 R2 L1 18H D1 1N5817
VOUT
C4
7
REF
MAX761 MAX762
V+
8 C2
Inductors with a ferrite core or equivalent are recommended. The inductor's incremental saturation-current rating should be greater than the 1A peak current limit. It is generally acceptable to bias the inductor into saturation by approximately 20% (the point where the inductance is 20% below the nominal value). For highest efficiency, use a coil with low DC resistance, preferably under 100m. To minimize radiated noise, use a toroid, a pot core, or a shielded coil. Table 1 lists inductor types and suppliers for various applications. The listed surface-mount inductors' efficiencies are nearly equivalent to those of the larger throughhole inductors.
4
Diode Selection
The MAX761/MAX762's high switching frequency demands a high-speed rectifier. Use a Schottky diode with a 1A average current rating, such as a 1N5817. For high-temperature applications, use a high-speed silicon diode, such as the MUR105 or the EC11FS1. These diodes have lower high-temperature leakage than Schottky diodes (Table 1).
C1 = 33F C2 = 0.1F C3 = 0.1F C4 = 33F
( VOUT -1) VREF
Figure 5. Bootstrapped Operation with Adjustable Output
Selecting the Inductor (L)
In both CCM and DCM, practical inductor values range from 10H to 50H. If the inductor value is too low, the current in the coil will ramp up to a high level before the current-limit comparator can turn off the switch. The minimum on-time for the switch (tON(min)) is approximately 2.5s, so select an inductance that allows the current to ramp up to ILIM/2 in no less than 2.5s. Choosing a value of ILIM/2 allows the half-size pulses to occur, giving higher light-load efficiency and minimizing ripple. Hence, calculate the minimum inductance value as: L (VIN(max))(tON(min)) ILIM/2
OR
Capacitor Selection
Output Filter Capacitor The primary criterion for selecting the output filter capacitor (C4) is low effective series resistance (ESR). The product of the inductor current variation and the output filter capacitor's ESR determines the amplitude of the high-frequency ripple seen on the output voltage. A 33F, 16V Sanyo OS-CON capacitor with 100m ESR typically provides 100mV ripple when stepping up from 5V to 12V at 150mA. Because the output filter capacitor's ESR affects efficiency, use low-ESR capacitors for best performance. The smallest low-ESR SMT tantalum capacitors currently available are the Sprague 595D series. Sanyo OS-CON organic semiconductor through-hole capacitors and Nichicon PL series also exhibit very low ESR. Table 1 lists some suppliers of low-ESR capacitors. Input Bypass Capacitors The input bypass capacitor, C1, reduces peak currents drawn from the voltage source, and also reduces noise at the voltage source caused by the MAX761/MAX762's switching action. The input voltage source impedance determines the size of the capacitor required at the V+ input. As with the output filter capacitor, a low-ESR capacitor is recommended. For output currents up to 250mA, 33F (C1) is adequate, although smaller bypass capacitors may also be acceptable. Bypass the IC separately with a 0.1F ceramic capacitor, C2, placed close to the V+ and GND pins.
L (VIN(max))(5) where VIN(max) is in volts and L is in microhenries. The coil's inductance need not satisfy this criterion exactly, as the circuit can tolerate a wide range of values. Larger inductance values tend to produce physically larger coils and increase the start-up time, but are otherwise acceptable. Smaller inductance values allow the coil current to ramp up to higher levels before the switch can turn off, producing higher ripple at light loads. In general, an 18H inductor is sufficient for most applications (VIN 5V). An 18H inductor is appropriate for input voltages up to 3.6V, as calculated above. However, the same 18H coil can be used with input voltages up to 5V with only small increases in peak current, as shown in Figures 4a and 4b.
10
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12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters
Reference Capacitor Bypass REF with a 0.1F capacitor. REF can source up to 100A.
Connect a pull-up resistor (e.g., 100k) between LBO and VOUT. Tie LBI to GND or V+ and leave LBO floating if the low-battery detector is not used.
MAX761/MAX762
Setting the Low-Battery Detector Voltage
To set the low-battery detector's falling trip voltage (VTRIP), select R3 between 10k and 500k (Figures 2 and 3), and calculate R4 as follows: (VTRIP - VREF) R4 = R3 [ ] VREF where VREF = 1.5V. The rising trip voltage is higher because of the comparator's hysteresis of approximately 20mV, and can be calculated by: VTRIP(rising) = (VREF + 20mV)(1 + R4/R3). Connect a high-value resistor (larger than R3 + R4) between LBI and LBO if additional hysteresis is required.
___________Applications Information
Layout Considerations
Proper PC board layout is essential because of high current levels and fast switching waveforms that radiate noise. Minimize ground noise by connecting GND, the input bypass-capacitor ground lead, and the output filtercapacitor ground lead to a single point (star ground configuration). Also minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. The traces connected to FB and LX, in particular, must be short. Place bypass capacitor C2 as close as possible to V+ and GND.
Table 1. Component Suppliers
PRODUCTION METHOD INDUCTORS Sumida CD54-180 (22H) Coiltronics CTX 100-series Sumida RCH855-180M Sanyo OS-CON series Low-ESR organic semiconductor Nichicon PL series Low-ESR electrolytics United Chemi-Con LXF series Coiltronics Matsuo Matsuo Nichicon Nihon Renco Sanyo Sanyo Sumida Sumida United Chem-Con (USA) (USA) (Japan) (USA) (USA) (USA) (USA) (Japan) (USA) (Japan) (USA) (407) 241-7876 (714) 969-2491 81-6-337-6450 (708) 843-7500 (805) 867-2555 (516) 586-5566 (619) 661-6835 (0720) 70-1005 (708) 956-0666 81-3-607-5111 (714) 255-9500 FAX (407) 241-9339 FAX (714) 960-6492 FAX 81-6-337-6456 FAX (708) 843-2798 FAX (805) 867-2556 FAX (516) 586-5562 FAX (619) 661-1055 FAX (0720) 70-1174 FAX 81-3-607-5144 FAX (714) 255-9400 CAPACITORS Matsuo 267 series DIODES Nihon EC10 series
Surface Mount
Miniature Through-Hole
Motorola 1N5817, MUR105
Low-Cost Through-Hole
Renco RL 1284-18
______________________________________________________________________________________
11
12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters MAX761/MAX762
___________________Chip Topography
LBO V+
LBI
LX
FB
0.142" (3.607mm)
GND
REF SHDN 0.080" (2.030mm)
TRANSISTOR COUNT: 492; SUBSTRATE CONNECTED TO V+.
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.
12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 (c) 1993 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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