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19-0202; Rev 2; 11/96
ANUAL N KIT M LUATIO ATA SHEET EVA WS D FOLLO
5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers
____________________________Features
o 90% Efficiency for 10mA to 1A Load Currents o Up to 15W Output Power o 110A Max Supply Current o 5A Max Shutdown Current o 2V to 16.5V Input Range (MAX770/MAX771/MAX772) o Internal Shunt Regulator for High Input Voltages (MAX773) o Preset or Adjustable Output Voltages MAX770: 5V or Adjustable MAX771: 12V or Adjustable MAX772: 15V or Adjustable MAX773: 5V, 12V, 15V, or Adjustable o Current-Limited PFM Control Scheme o 300kHz Switching Frequency
_______________General Description
The MAX770-MAX773 step-up switching controllers provide 90% efficiency over a 10mA to 1A load. A unique current-limited pulse-frequency-modulation (PFM) control scheme gives these devices the benefits of pulse-width-modulation (PWM) converters (high efficiency at heavy loads), while using less than 110A of supply current (vs. 2mA to 10mA for PWM converters). These ICs use tiny external components. Their high switching frequencies (up to 300kHz) allow surfacemount magnetics of 5mm height and 9mm diameter. The MAX770/MAX771/MAX772 accept input voltages from 2V to 16.5V. Output voltages are preset at 5V, (MAX770), 12V (MAX771), and 15V (MAX772); they can also be adjusted using two resistors. The MAX773 accepts inputs from 3V to 16.5V. For a wider input range, it features an internal shunt regulator that allows unlimited higher input voltages. The MAX773's output can be set to 5V, 12V, or 15V, or it can be adjusted with two resistors. The MAX770-MAX773 drive external N-channel MOSFET switches, allowing them to power loads up to 15W. If less power is required, use the MAX756/MAX757 or MAX761/MAX762 step-up switching regulators with onboard MOSFETs.
MAX770-MAX773
______________Ordering Information
PART MAX770CPA MAX770CSA MAX770C/D MAX770EPA MAX770ESA MAX770MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP**
________________________Applications
Palmtops/Handy-Terminals High-Efficiency DC-DC Converters Battery-Powered Applications Positive LCD-Bias Generators Portable Communicators Flash Memory Programmers
Ordering Information continued at end of data sheet. *Contact factory for dice specifications. **Contact factory for availability and processing to MIL-STD-883B.
_________________Pin Configurations __________Typical Operating Circuit
INPUT 2V TO VOUT OUTPUT 12V
EXT V+ 1 2 8 CS GND AGND REF 7 6 5
TOP VIEW
MAX771
ON/OFF SHDN
EXT CS
N
FB 3 SHDN 4
MAX770 MAX771 MAX772
DIP/SO
REF FB AGND GND V+
Pin Configurations continued at end of data sheet.
________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 408-737-7600 ext. 3468.
5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
ABSOLUTE MAXIMUM RATINGS
Supply Voltages V+ to GND.............................................................-0.3V to 17V V+ to SGND.............................................................-0.3V to 7V SGND........................................................-0.3V to (V+ + 0.3V) EXT, CS, REF, LBO, LBI, SHDN, FB.............-0.3V to (V+ + 0.3V) EXTH, EXTL ..................................................-0.3V to (V+ + 0.3V) V5, V12, V15 .............................................................-0.3V to 17V GND to AGND .........................................................0.1V to -0.1V ISGND ..................................................................................50mA Continuous Power Dissipation (TA = +70C) 8-Pin Plastic DIP (derate 9.09mW/C above +70C)....727mW 8-Pin SO (derate 5.88mW/C above +70C) ................471mW 8-Pin CERDIP (derate 8.00mW/C above +70C) ........640mW 14-Pin Plastic DIP (derate 10.00mW/C above +70C) .............................800mW 14-Pin SO (derate 8.33mW/C above +70C) ..............667mW 14-Pin CERDIP (derate 9.09mW/C above +70C) ......727mW Operating Temperature Ranges MAX77_C_ _ ........................................................0C to +70C MAX77_E_ _......................................................-40C to +85C MAX77_MJ_ ...................................................-55C to +125C Junction Temperatures MAX77_C_ _/E_ _ ..........................................................+150C MAX77_MJ_..................................................................+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, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER SYMBOL CONDITIONS MAX770-772 (internal feedback resistors) MAX770-772C/E (external resistors) Input Voltage Range MAX770-772MJA (external resistors) MAX773C/E MAX773MJD Minimum Start-Up Voltage Supply Current Standby Current MAX770/MAX771/MAX772 V+ = 16.5V, SHDN = 0V (normal operation) V+ = 10.0V, SHDN 1.6V (shutdown) V+ = 16.5V, SHDN 1.6V (shutdown) V+ = 2.0V to 5.0V, over full load range Output Voltage (Note 1) V+ = 2.0V to 12.0V, over full load range V+ = 2.0V to 15.0V, over full load range Output Voltage Line Regulation (Note 2) Output Voltage Load Regulation (Note 2) Maximum Switch On-Time Minimum Switch Off-Time Efficiency Reference Voltage VREF tON(max) tOFF(min) V+ = 4V, ILOAD = 500mA, VOUT = 5V MAX77_C IREF = 0A MAX77_E MAX77_M 1.4700 1.4625 1.4550 Figure 2a, V+ = 2.7V to 4.5V, ILOAD = 700mA, VOUT = 5V Figure 2a, V+ = 3V, ILOAD = 30mA to 1A, VOUT = 5V 12 1.8 4.80 11.52 14.40 MIN 2.0 3.0 3.1 3.0 3.1 1.8 85 2 4 5.0 12.0 15.0 5 5.20 12.48 15.60 mV/V V TYP MAX 16.5 16.5 16.5 16.5 16.5 2.0 110 5 V A A V UNITS
20 16 2.3 87 1.5 1.5 1.5 1.5300 1.5375 1.5450 20 2.8
mV/A s s % V
2
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5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETERS REF Load Regulation REF Line Regulation SYMBOL CONDITIONS 0A IREF 100A 3V V+ 16.5V MAX77_C FB Trip-Point Voltage VFB MAX77_E MAX77_M MAX77_C FB Input Current IFB MAX77_E MAX77_M SHDN Input High Voltage SHDN Input Low Voltage SHDN Input Current LBI Input Current LBI Hysteresis LBI Delay VIH VIL V+ = 2.0V to 16.5V MAX77_C/E, V+ = 2.0V to 16.5V MAX77_M, V+ = 2.0V to 16.5V V+ = 16.5V, SHDN = 0V or V+ MAX773, V+ = 16.5V, LBI = 1.5V MAX773 5mV overdrive MAX77_C LBI Threshold Voltage MAX773, LBI falling MAX77_E MAX77_M LBO Leakage Current LBO Output Voltage Low Current-Limit Trip Level CS Input Current EXT Rise Time EXT Fall Time Supply Voltage in Shunt Mode VSHUNT V+ = 5V, 1nF from EXT to ground (Note 3) V+ = 5V, 1nF from EXT to ground (Note 3) MAX773, ISHUNT = 1mA to 20mA, SGND = 0V, CSHUNT = 0.1F 5.5 VOL VCS MAX773, V+ = 16.5V, VLBO = 16.5V MAX773, V+ = 5V, LBO sinking 1mA V+ = 5V to 16.5V 170 1.4700 1.4625 1.4550 20 2.5 1.50 1.50 1.50 0.01 0.1 200 0.01 55 55 6.3 1.5300 1.5375 1.5450 1.00 0.4 230 1 A V mV A ns ns V V 1.6 0.4 0.2 1 20 1.4700 1.4625 1.4550 MAX77_C/E MAX77_M MIN TYP 4 4 40 1.50 1.50 1.50 MAX 10 15 100 1.5300 1.5375 1.5450 20 40 60 V V A nA mV s nA V UNITS mV V/V
MAX770-MAX773
3
Note 1: Output voltage guaranteed using preset voltages. See Figures 7a-7d for output current capability versus input voltage. Note 2: Output voltage line and load regulation depend on external circuit components. Note 3: For the MAX773, EXT is EXTH and EXTL shorted together.
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5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
__________________________________________Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
MAX770 EFFICIENCY vs. OUTPUT CURRENT (BOOTSTRAPPED)
MAX770-3-01
MAX771 EFFICIENCY vs. OUTPUT CURRENT (BOOTSTRAPPED)
MAX770-3-02
MAX772 EFFICIENCY vs. OUTPUT CURRENT (BOOTSTRAPPED)
VOUT = 15V, CIRCUIT OF FIGURE 2b MAX772 SUBSTITUTED FOR MAX771 90 EFFICIENCY (%)
MAX770-3-03
100 VIN = 4V
100 VIN = 9V 90 EFFICIENCY (%) VIN = 6V
100
90 EFFICIENCY (%)
80
80
80
70 VIN = 3V 60
VIN = 3.5V VOUT = 5V CIRCUIT OF FIGURE 2a 0.001 0.01 0.1 1
70 VIN = 5V 60 50 0.001 VIN = 3V VOUT = 12V CIRCUIT OF FIGURE 2b 0.1 1
70
60
VIN = 12V VIN = 9V VIN = 6V VIN = 5V VIN = 3V 0.001 0.01 0.1 1
50 OUTPUT CURRENT (A)
50 0.01 OUTPUT CURRENT (A) OUTPUT CURRENT (A)
MAX771 EFFICIENCY vs. OUTPUT CURRENT (NON-BOOTSTRAPPED)
MAX770-3-04
MAX770 LOAD CURRENT vs. MINIMUM START-UP INPUT VOLTAGE
MAX770-3-05
MAX771 LOAD CURRENT vs. MINIMUM START-UP INPUT VOLTAGE
VOUT = 12V CIRCUIT OF FIGURE 2b
MAX770-3-06
100
700 600 LOAD CURRENT (mA) 500 400 300 200 100 ABOVE 3.4V, THE CIRCUIT STARTS UP UNDER MAXIMUM LOAD CONDITIONS 1.0 1.5 2.0 2.5 3.0 VOUT = 5V CIRCUIT OF FIGURE 2a
500
VOUT = 12V CIRCUIT OF FIGURE 2c
VIN = 9V
400 LOAD CURRENT (mA)
EFFICIENCY (%)
90 VIN = 6V VIN = 5V 80
300
200
100 0 3.5 2.0 2.5 3.0
70 0.001
0 0.01 0.1 1 10 OUTPUT CURRENT (A)
ABOVE 3.5V THE CIRCUIT STARTS UP UNDER MAXIMUM LOAD CONDITIONS 3.5 4.0
MINIMUM START-UP INPUT VOLTAGE (V)
MINIMUM START-UP INPUT VOLTAGE (V)
SUPPLY CURRENT vs. TEMPERATURE
VOUT = 12V, VIN = 5V CIRCUIT OF FIGURE 2b BOOTSTRAPPED MODE ENTIRE CIRCUIT 2
MAX770-3-07
SUPPLY CURRENT vs. SUPPLY VOLTAGE
VOUT = 12V SUPPLY CURRENT (mA) 0.6 BOOTSTRAPPED CIRCUIT OF FIGURE 2b
MAX770-3-08
EXT RISE/FALL TIME vs. SUPPLY VOLTAGE
MAX770-3-09
4
0.8
250 200
EXT RISE/FALL TIME (ns)
SUPPLY CURRENT (mA)
3
CEXT = 2200pF CEXT = 1000pF CEXT = 446pF CEXT = 100pF
150
0.4
100
1
SCHOTTKY DIODE LEAKAGE EXCLUDED
0.2 NON-BOOTSTRAPPED CIRCUIT OF FIGURE 2c
50 0
0 -75 -50 -25 0 25 50 75 100 125 TEMPERATURE (C)
0 2 4 6 8 10 12 2 4 6 8 V+ (V) 10 12 SUPPLY VOLTAGE (V)
4
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5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers
____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE
REFERENCE OUTPUT RESISTANCE ()
MAX770-3-10
MAX770-MAX773
REFERENCE vs. TEMPERATURE
MAX770-3-11
250
1.506 1.504 1.502 REFERENCE (V) 1.500 1.498 1.496 1.494 1.492
200 10A 150
100
50A 100A
50
0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
-60 -40 -20
0 20 40 60 80 100 120 140 TEMPERATURE (C)
MAXIMUM SWITCH ON-TIME vs. TEMPERATURE
MAX770-3-13
SHUTDOWN CURRENT vs. TEMPERATURE
3.5 3.0
MAX770-3-12
16.5
4.0
tON(MAX) (s)
16.0
ICC (A)
2.5 2.0 1.5 V+ = 8V 1.0 0.5 V+ = 4V -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) V+ = 15V
15.5 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
0
MINIMUM SWITCH OFF-TIME vs. TEMPERATURE
MAX770-3-14
MAXIMUM SWITCH ON-TIME/ MINIMUM SWITCH OFF-TIME RATIO vs. TEMPERATURE
MAX770-3-15
2.30
8.0 tON(MAX)/tOFF(MIN) RATIO
7.5
tOFF(MIN) (s)
2.25
7.0
6.5
2.20 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
6.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
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5
5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2a, TA = +25C, unless otherwise noted.) MAX770 HEAVY-LOAD SWITCHNG WAVEFORMS
VOUT 0 ILIM ILIM 2 0 C A ILIM B 2 0 B C A
MAX770 LIGHT-LOAD SWITCHING WAVEFORMS
20s/div VIN = 2.9V, IOUT = 0.9A A: EXT VOLTAGE, 5V/div B: INDUCTOR CURRENT 1A/div C: VOUT RIPPLE 100mV/div, AC-COUPLED
20s/div V+ = 3V, IOUT = 165mA A: EXT VOLTAGE, 5V/div B: INDUCTOR CURRENT, 1A/div C: VOUT RIPPLE 100mV/div, AC-COUPLED
MAX770 LINE-TRANSIENT RESPONSE
MAX770 LOAD-TRANSIENT RESPONSE
4.5V 2.7V 0
A 0
A
B B
2ms/div IOUT = 0.7A A: VIN, 2.7V TO 4.5V, 2V/div B: VOUT RIPPLE, 100mV/div, AC-COUPLED
2ms/div VIN = 3V A: LOAD CURRENT 0.5A/div (0A to 1A) B: VOUT RIPPLE, 100mV/div, AC-COUPLED
6
_______________________________________________________________________________________
5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2a, TA = +25C, unless otherwise noted.)
MAX770 EXITING SHUTDOWN
A 0
B
0
200s/div VIN = 3V, IOUT = 0.5A A: SHDN, 2V/div B: VOUT, 2V/div
______________________________________________________________Pin Description
PIN MAX770 MAX771 MAX772 1 2 3 4 5 6 7 8 -- -- -- -- -- MAX773 -- 3 6 7 8 -- 9 11 1 2 4 5 10 NAME FUNCTION
EXT V+ FB SHDN REF AGND GND CS V12 V5 LBO LBI SGND
Gate drive for external N-channel power transistor Power-supply input. Also acts as a voltage-sense point when in bootstrapped mode for the MAX770/MAX771/MAX772, or as a shunt regulator when SGND is connected to ground for the MAX773. Bypass to SGND with 0.1F when using the shunt regulator. Feedback input for adjustable-output operation. Connect to ground for fixed-output operation. Use a resistor divider network to adjust the output voltage. See Setting the Output Voltage section. Active-high TTL/CMOS logic-level shutdown input. In shutdown mode, VOUT is a diode drop below V+ (due to the DC path from V+ to the output) and the supply current drops to 5A maximum. Connect to ground for normal operation. 1.5V reference output that can source 100A for external loads. Bypass to GND with 0.1F. The reference is disabled in shutdown. Analog ground High-current ground return for the output driver Positive input to the current-sense amplifier. Connect the current-sense resistor between CS and GND. Input sense point for 12V-output operation. Connect VOUT to V12 for 12V-output operation. Leave unconnected for adjustable-output operation. Input sense point for 5V-output operation. Connect VOUT to V5 for 5V-output operation. Leave unconnected for adjustable-output operation. 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 floating if not used. LBO is high impedance in shutdown mode. Input to the internal low-battery comparator. Tie to GND or V+ if not used. Shunt regulator ground. Leave unconnected if the shunt regulator is not used.
_______________________________________________________________________________________
7
5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
_________________________________________________Pin Description (continued)
PIN MAX770 MAX771 MAX772 -- -- -- MAX773 12 13 14 NAME FUNCTION Low-level gate/base drive for external power transistor. Connect to the gate of an external N-channel MOSFET or to the base of an external NPN transistor. High-level gate/base drive for external power transistor. Connect to EXTL when using an external N-channel MOSFET. When using an external NPN transistor, connect a resistor RBASE from EXTH to the base of the NPN to set the maximum base-drive current. Input sense point for 15V-output operation. Connect VOUT to V15 for 15V-output operation. Leave unconnected for adjustable-output operation
EXTL EXTH V15
_______________Detailed Description
The MAX770-MAX773 are BiCMOS, step-up, switchmode power-supply controllers that provide preset 5V, 12V, and 15V output voltages, in addition to adjustableoutput operation. Their unique control scheme combines the advantages of pulse-frequency modulation (low supply current) and pulse-width modulation (high efficiency with heavy loads), providing high efficiency over a wide output current range, as well as increased output current capability over previous PFM devices. In addition, the external sense resistor and power transistor allow the user to tailor the output current capability for each application. Figure 1 shows the MAX770-MAX773 block diagram. The MAX770-MAX773 offer three main improvements over prior pulse-skipping control solutions: 1) the converters operate with tiny (5mm height and less than 9mm diameter) surface-mount inductors due to their 300kHz switching frequency; 2) the current-limited PFM control scheme allows 87% efficiencies over a wide range of load currents; and 3) the maximum supply current is only 110A. The MAX773 can be configured to operate from an internal 6V shunt regulator, allowing very high input/output voltages. Its output can be configured for an adjustable voltage or for one of three fixed voltages (5V, 12V, or 15V), and it has a power-fail comparator for low-battery detection. All devices have shutdown capability, reducing the supply current to 5A max.
range is 2V to VOUT. The voltage applied to the gate of the external power transistor is switched from VOUT to ground, providing more switch gate drive and thus reducing the transistor's on resistance. In non-bootstrapped mode, the IC is powered from the input voltage (V+) and operates with minimum supply current. In this mode, FB is the output voltage sense point. Since the voltage swing applied to the gate of the external power transistor is reduced (the gate swings from V+ to ground), the power transistor's on resistance increases at low input voltages. However, the supply current is also reduced because V+ is at a lower voltage, and because less energy is consumed while charging and discharging the external MOSFET's gate capacitance. The minimum input voltage for the MAX770-MAX773 is 3V when using external feedback resistors. With supply voltages below 5V, bootstrapped mode is recommended. Note: When using the MAX770/MAX771/MAX772 in non-bootstrapped mode, there is no preset output operation because V+ is also the output voltage sense point for fixed-output operation. External resistors must be used to set the output voltage. Use 1% external feedback resistors when operating in adjustable-output mode (Figures 2c, 2d, 3b, 3d, 3e) to achieve an overall output voltage accuracy of 5%. The MAX773 can be operated in non-bootstrapped mode without using external feedback resistors because V+ does not act as the output voltage sense point with preset-output operation. To achieve highest efficiency, operate in bootstrapped mode whenever possible.
Bootstrapped/Non-Bootstrapped Modes
Figures 2 and 3 show standard application circuits for bootstrapped and non-bootstrapped modes. In bootstrapped mode, the IC is powered from the output (VOUT, which is connected to V+) and the input voltage
8
MAX773 Shunt-Regulator Operation
The MAX773 has an internal 6V shunt regulator that allows the device to step up from very high input voltages (Figure 4).
_______________________________________________________________________________________
5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
LBO V15 V12 V5 FB
V+
LBI N
MAX773 ONLY
N
DUAL-MODE COMPARATOR
MAX770-MAX773
SHDN
200mV REF 1.5V REFERENCE BIAS CIRCUITRY ERROR COMPARATOR MAX770 MAX771 MAX772 6V F/F S R LOW-VOLTAGE OSCILLATOR ONE-SHOT TRIG Q CURRENT-SENSE AMPLIFIER 0.2V 0.1V EXT MAX770 MAX771 MAX772 CS Q EXT CONTROL 2.5V EXTH MAX773 ONLY EXTL SGND
ONE-SHOT Q TRIG N
V+
Figure 1. Block Diagram
Floating the shunt-regulator ground (SGND) disables the shunt regulator. To enable it, connect SGND to GND. The shunt regulator requires 1mA minimum current for proper operation; the maximum current must not exceed 20mA. The MAX773 operates in non-bootstrapped mode when the shunt regulator is used, and EXT swings between the 6V shunt-regulator voltage and GND. When using the shunt regulator, use an N-channel power FET instead of an NPN power transistor as the power switch. Otherwise, excessive base drive will collapse the shunt regulator.
External Power-Transistor Control Circuitry
PFM Control Scheme The MAX770-MAX773 use a proprietary current-limited PFM control scheme to provide high efficiency over a wide range of load currents. This control scheme combines the ultra-low supply current of PFM converters (or pulse skippers) with the high full-load efficiency of PWM converters.
Unlike traditional PFM converters, the MAX770- MAX773 use a sense resistor to control the peak inductor current. They also operate with high switching
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9
5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
VIN = 3V
C2 0.1F 5 REF C3 0.1F 4 SHDN 3 FB 6 AGND GND 7 V+ L1 22H C2 0.1F C1 100F D1 1N5817 N MTP3055EL CS 8 RSENSE 75m C4 300F 6 AGND GND 7 CS 8 RSENSE 100m C4 200F 5 REF V+ L1 22H
VIN = 5V
2
2 C1 68F D1 1N5817 N Si9410DY
MAX770
VOUT = 5V @ 1A
C3 0.1F
4 SHDN 3 FB
MAX771
VOUT = 12V @ 0.5A
EXT
1
EXT
1
Figure 2a. 5V Preset Output, Bootstrapped
Figure 2b. 12V Preset Output, Bootstrapped
VIN = 5V
C1 68F C2 0.1F 2 V+ 5 REF C3 0.1F 4 SHDN MAX770 1 L1 22H D1 1N5817 N C2 0.1F 2
VIN = 4V
VOUT = 12V @ 0.5A
C4 200F
5 REF C3 0.1F
V+
L1 20H
C1 47F D1 1N5817 N Si9410DY
VOUT = 9V
C4 100F
MAX771 MAX772
EXT
4 SHDN MAX770
MAX771 MAX772
EXT
1
6 AGND
CS
8 RSENSE 100m R1 18k R2 = (R1) R2 127k
6
AGND
CS
8 RSENSE R2 140k
FB GND 7 R2 = (R1)
3
FB GND 7
3 R1 28k
( VOUT -1) V
REF
( VOUT -1) V
REF
VREF = 1.5V
VREF = 1.5V
Figure 2c. 12V Output, Non-Bootstrapped
Figure 2d. 9V Output, Bootstrapped
frequencies (up to 300kHz), allowing the use of tiny external components. As with traditional PFM converters, the power transistor is not turned on until the voltage comparator senses that the output is out of regulation. However, unlike traditional PFM converters, the MAX770-MAX773 switch using the combination of a peak current limit and a pair of one-shots that set the maximum on-time (16s) and
minimum off-time (2.3s); there is no oscillator. Once off, the minimum off-time one-shot holds the switch off for 2.3s. 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 control circuitry allows the ICs to operate in continuous-conduction mode (CCM) while maintaining high efficiency with heavy loads. When the power switch is
10
______________________________________________________________________________________
5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
VIN = 5V
3 V+ C2 0.1F
VIN
1 V12 14 V15 R4 63.4k (1%) 2 V5
C1 47F
C2 0.1F 3 V+ EXTH 13 EXTL 12
L1 150H 910
D1 VOUT = 24V 1N5818 @ 30mA ZTX694B C4 150F
SGND
10 100k
C1
8 REF MAX773 C3 0.1F 5 LBI 7 SHDN 6
L1 22H D1 1N5817 N C4
LBO 4 Si9410DY EXTH 13 EXTL 12 CS 11
VOUT = 12V
10 SGND 4 LBO 8 REF MAX773 5
C3 0.1F
CS 11 V15 V12 14 1 RSENSE 0.4
R3 10k (1%)
LBI
7
FB GND 9 RSENSE
SHDN
V5 2 FB GND 9 6 R1 34k R2 510k
VTRIP (V) MIN 10.6 NOMINAL MAX 11.0 11.4 R4 = R3
( VTRIP -1) REF
V
R2 = (R1)
( VOUT -1) V
REF
VREF = 1.5V
VREF = 1.5V
Figure 3a. 12V Preset Output, Bootstrapped, N-Channel Power MOSFET
Figure 3b. 24V Output, Non-Bootstrapped, NPN Power Transistor
VIN VIN = 5V
C1 C2 0.1F 3 V+ EXTH EXTL 13 12 C1 L1 22H D1 1N5817 VOUT = 15V N Si9410DY 4 LBO C4 RSENSE V15 14 V12 V5 GND 9 1 2 8 REF C3 0.1F 5 7 10 3 V+ EXTH EXTL 13 12 L1 20H C2 0.1F D1 1N5817 N Si9410DY
VOUT = 16V
10 SGND 4 LBO 8 REF C3 0.1F 7 6 5
C4
CS 11
CS 11 RSENSE
MAX773
SHDN FB LBI
MAX773
LBI SHDN SGND GND 9 V15 14 V12 1 V5 2 FB 6 R1 13.7k R2 = (R1) R2 133k
( VOUT -1) V
REF
VREF = 1.5V
Figure 3c. 15V Preset Output, Non-Bootstrapped N-Channel Power MOSFET
Figure 3d. 16V Output, Bootstrapped, N-Channel Power MOSFET
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11
5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
VIN = 24V TO 28V
C1 47F C2 0.1F RSHUNT 3k 3 10 SGND v+ EXTH EXTL 13 12 N Si9420DY C4 100F RSENSE 1.0 R2 732k (1%) R1 11.3k (1%) R2 = (R1) L1 250H D1 MUR115
VIN
VOUT = 100V @ 10mA
RSHUNT
MAX773
3 V+
4 LBO 8 REF C3 0.1F 5 LBI 7
CS 11
MAX773 V15 14
V12 1 V5 2 6 FB GND 9
C2 0.1F
6V (typ)
10 SGND
SHDN
RSHUNT =
VIN (MIN) - VSHUNT (MAX)
(
VREF = 1.5V
VOUT -1 VREF
)
I SHUNT * * SEE TEXT FOR ISHUNT CALCULATION
Figure 3e. 100V Output, Shunt Regulator, N-Channel Power MOSFET
Figure 4. MAX773 Shunt Regulator
turned on, it stays on until either 1) the maximum ontime one-shot turns it off (typically 16s later), or 2) the switch current reaches the peak current limit set by the current-sense resistor. To increase light-load efficiency, the current limit for the first two pulses is set to one-half the peak current limit. If those pulses bring the output voltage into regulation, the error comparator holds the MOSFET off and the current limit remains at one-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 peak current limit set by the external sense resistor (see inductor current waveforms in the Typical Operating Characteristics). The MAX770-MAX773 switching frequency is variable (depending on load current and input voltage), causing variable switching noise. However, the subharmonic noise generated does not exceed the peak current limit times the filter capacitor equivalent series resistance (ESR). For example, when generating a 12V output at 500mA from a 5V input, only 180mV of output ripple occurs using the circuit of Figure 2b.
Low-Voltage Start-Up Oscillator The MAX770/MAX771/MAX772 feature a low input voltage start-up oscillator that guarantees start-up with no load down to 2V when operating in bootstrapped mode and using internal feedback resistors. At these low voltages, the supply voltage is not large enough for proper error-comparator operation and internal biasing. The start-up oscillator has a fixed 50% duty cycle and the MAX770/MAX771/MAX772 disregard the error-comparator output when the supply voltage is less than 2.5V. Above 2.5V, the error-comparator and normal oneshot timing circuitry are used. The low voltage start-up circuitry is disabled if non-bootstrapped mode is selected (FB is not tied to ground).
The MAX773 does not provide the low-voltage 50% duty-cycle oscillator. Its minimum start-up voltage is 3V for all modes.
External Transistor
An N-FET power switch is recommended for the MAX770/MAX771/MAX772. The MAX773 can drive either an N-channel MOSFET (N-FET) or an NPN because it provides two separate
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5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers
drive outputs (EXTH and EXTL) that operate 180 out of phase (Figures 3a and 3b). In Figure 3b, the resistor in series with EXTH limits the base current, and EXTL (which is connected directly to the base) turns the transistor off.
MAX770-MAX773
R2 FB VOUT R1
Shutdown Mode
When SHDN is high, the MAX770-MAX773 enter shutdown mode. In this mode, the internal biasing circuitry is turned off (including the reference) and V OUT falls to a diode drop below VIN (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. The MAX773's shunt regulator is not disabled in shutdown mode.
MAX770 MAX771 MAX772 MAX773
R1 = 10k TO 500k GND R2 = R1
( VOUT -1) REF
V
VREF = 1.5V
Figure 5. Adjustable Output Circuit
Low-Battery Detector
The MAX773 provides a low-battery comparator that compares the voltage on LBI to the reference voltage. When the LBI voltage is below VREF, LBO (an opendrain 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. LBO is high impedance in shutdown mode.
See Table 1 for a summary of operating characteristics and requirements for the ICs in bootstrapped and nonbootstrapped modes. The MAX770-MAX773's output voltage can be adjusted from very high voltages down to 3V, using external resistors R1 and R2 configured as shown in Figure 5. For adjustable-output operation, select feedback resistor R1 in the range of 10k to 500k. R2 is given by: VOUT R2 = (R1) ----- -1 VREF
__________________Design Procedure
Setting the Output Voltage
To set the output voltage, first determine the mode of operation, either bootstrapped or non-bootstrapped. Bootstrapped mode provides more output current capability, while non-bootstrapped mode reduces the supply current (see Typical Operating Characteristics). If a decaying voltage source (such as a battery) is used, see the additional notes in the Low Input Voltage Operation section. Use the MAX770/MAX771/MAX772 unless one or more of the following conditions applies. If one or more of the following is true, use the MAX773: 1) An NPN power transistor will be used as the power switch 2) The LBI/LBO function is required 3) The shunt regulator must accommodate a high input voltage 4) Preset-output non-bootstrapped operation is desired--for example, to reduce the no-load supply current in a 5V to 12V application.
(
)
where VREF equals 1.5V. For preset-output operation, tie FB to GND (this forces bootstrapped-mode operation for the MAX770/MAX771/MAX772). Configure the MAX773 for a preset voltage of 5V, 12V, or 15V by connecting the output to the corresponding sense input pin (i.e., V5, V12, or V15). FB must be tied to ground for preset-output operation. Leave all unused sense input pins unconnected. Failure to do so will cause an incorrect output voltage. The MAX773 can provide a preset output voltage in both bootstrapped and nonbootstrapped modes. Figures 2 and 3 show various circuit configurations for bootstrapped/non-bootstrapped, preset/adjustable operation.
Shunt-Regulator Operation
When using the shunt regulator, connect SGND to ground and place a 0.1F capacitor between V+ and SGND, as close to the IC as possible. Increase C2 to 1.0F to improve shunt regulators performance with heavy loads. Select RSHUNT such that 1mA ISHUNT 20mA.
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13
5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
Table 1. Bootstrapped vs. Non-Bootstrapped Operation
PARAMETER Gate Drive FET On Resistance Gate-Drive Capacitive Losses No-Load Supply Current GND to VOUT Lower Higher Higher 2V to 16.5V (MAX770/MAX771/MAX772), (internal feedback resistors) 3V to 16.5V (MAX770/MAX771/MAX772), (external feedback resistors) 3V to 16.5V (MAX773) 2V to 5V (MAX770/MAX771/MAX772), 3V to 5V (MAX773) MAX770-MAX773(N) MAX770-MAX773(N) BOOTSTRAPPED* NON-BOOTSTRAPPED GND to V+ Higher Lower Lower 3V to 16.5V (MAX770/MAX771/MAX772), 3V and up (MAX773) 5V to 16.5V (MAX770/MAX771/MAX772), 5V and up (MAX773) MAX773(N)/MAX773(S) MAX770/MAX771/MAX772/ MAX773(N)/MAX773(S)
Possible Input Voltage Range
Normally Recommended Input Voltage Range Fixed Output Available Adjustable Output Available
*MAX773(S) indicates shunt mode; MAX773(N) indicates NOT in shunt mode.
Use an N-channel FET as the power switch when using the shunt regulator (see MAX773 Shunt-Regulator Operation in the Detailed Description). The shunt-regulator current powers the MAX773 and also provides the FET gate-drive current, which depends largely on the FET's total gate charge at VGS = 5V. To determine the shunt-resistor value, first determine the maximum shunt current required. ISHUNT = ISUPP + IGATE See N-Channel MOSFETs in the Power-Transistor Selection section to determine IGATE. Determine the shunt-resistor value using the following equation: VIN(min) - VSHUNT(max) RSHUNT(max) = ------------------------ ISHUNT where VSHUNT(max) is 6.3V. The shunt regulator is not disabled in shutdown mode, and continues to draw the calculated shunt current. If the calculated shunt regulator current exceeds 20mA, or if the shunt current exceeds 5mA and less shunt regulator current is desired, use the circuit of Figure 6 to provide increased drive and reduced shunt current when driving N-FETs with large gate capacitances. Select ISHUNT = 3mA. This provides adequate biasing current for this circuit, although higher shunt currents can be used.
14
VIN C2 0.1F 3 V+ 10 13 12 100 PNP 2N2907A CS 11 RSENSE FB 6 R1 NPN 2N2222A N R2 C4 C1
RSHUNT
SGND EXTH EXTL
L1 20H D1 VOUT
MAX773
Figure 6. Increased N-FET Gate Drive when Using the Shunt Regulator
To prevent the shunt regulator from drawing current in shutdown mode, place a switch in series with the shunt resistor.
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5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
3.5 MAXIMUM OUTPUT CURRENT (A) 3.0 2.5 2.0 1.5 1.0 0.5 RSENSE = 200m 0 2 3 4 INPUT VOLTAGE (V) 5 0 2 4 RSENSE = 100m VOUT = 5V L = 22H MAXIMUM OUTPUT CURRENT (A) RSENSE = 40m RSENSE = 50m RSENSE = 75m 3.5 3.0 2.5 2.0 1.5 1.0 0.5 RSENSE = 200m 6 8 INPUT VOLTAGE (V) 10 12 RSENSE = 100m VOUT = 12V L = 22H RSENSE = 40m RSENSE = 50m RSENSE = 75m
Figure 7a. Maximum Output Current vs. Input Voltage (VOUT = 5V)
3.5 MAXIMUM OUTPUT CURRENT (A) 3.0 2.5 2.0 1.5 1.0 0.5 0 2 4 6 RSENSE = 200m RSENSE = 100m VOUT = 15V L = 22H
Figure 7b. Maximum Output Current vs. Input Voltage (VOUT = 12V)
0.8 VOUT = 24V L =150H MAXIMUM OUTPUT CURRENT (A) 0.6 RSENSE = 100m 0.4 RSENSE = 200m
RSENSE = 40m RSENSE = 50m RSENSE = 75m
0.2 RSENSE = 400m 0 2 6 10 INPUT VOLTAGE (V) 14
8 10 12 INPUT VOLTAGE (V)
14
16
Figure 7c. Maximum Output Current vs. Input Voltage (VOUT = 15V)
Figure 7d. Maximum Output Current vs. Input Voltage (VOUT = 24V)
Determining RSENSE The Typical Operating Characteristics graphs show the output current capability for various modes, sense resistors, and input/output voltages. Use these graphs, along with the theoretical output current curves shown in Figures 7a-7d, to select RSENSE. These theoretical curves assume that an external N-FET power switch is used. They were derived using the minimum (worstcase) current-limit comparator threshold value, and the inductance value. No tolerance was included for R SENSE . The voltage drop across the diode was assumed to be 0.5V, and the drop across the power switch rDS(ON) and coil resistance was assumed to be 0.3V. To use the graphs, locate the graph with the appropriate output voltage or the graph having the nearest output voltage higher than the desired output voltage. On this graph, find the curve for the largest
sense-resistor value with an output current that is adequate at the lowest input voltage.
Determining the Inductor (L)
Practical inductor values range from 10H to 300H. 20H is a good choice for most applications. In applications with large input/output differentials, the IC's output current capability will be much less when the inductance value is too low, because the IC will always operate in discontinuous mode. If the inductor value is too low, the current 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 2s; select an inductor that allows the current to ramp up to I LIM/2 in no less than 2s. Choosing a value of ILIM/2 allows the half-size current pulses to occur, increasing light-load efficiency and minimizing output ripple.
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5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
IC(PEAK) L
MAX770 MAX771 MAX772
EXT N
MAX773
IB EXTH EXTL CS RSENSE CS RSENSE RBASE NPN
Figure 8a. Use an N-Channel MOSFET with the MAX770/MAX771/MAX772
Figure 8c. Using an NPN Transistor with the MAX773
L
MAX773
preferably under 20m. To minimize radiated noise, use a toroid, a pot core, or a shielded coil. Table 2 lists inductor suppliers and specific recommended inductors.
Power Transistor Selection
EXTH EXTL CS RSENSE N
Use an N-channel MOSFET power transistor with the MAX770/MAX771/MAX772 (Figure 8a). Use an N-FET whenever possible with the MAX773. An NPN transistor can be used, but be extremely careful when determining the base current (see NPN Transistors section). An NPN transistor is not recommended when using the shunt regulator.
Figure 8b. Using an N-Channel MOSFET with the MAX773
The standard operating circuits use a 22H inductor. If a different inductance value is desired, select L such that: VIN(max) x tON(min) L -------------------- ILIM / 2 Larger inductance values tend to increase the start-up time slightly, while smaller inductance values allow the coil current to ramp up to higher levels before the switch turns off, increasing the ripple at light loads. Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for use with high switching frequencies. Make sure the inductor's saturation current rating (the current at which the core begins to saturate and the inductance starts to fall) exceeds the peak current rating set by RSENSE. However, 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,
16
N-Channel MOSFETs To ensure the external N-channel MOSFET (N-FET) is turned on hard, use logic-level or low-threshold N-FETs when the input drive voltage is less than 8V. This applies even in bootstrapped mode, to ensure start-up. N-FETs provide the highest efficiency because they do not draw any DC gate-drive current, but they are typically more expensive than NPN transistors. When using an N-FET with the MAX773, connect EXTH and EXTL to the N-FET's gate (Figure 8b).
When selecting an N-FET, three important parameters are the total gate charge (Qg), on resistance (rDS(ON)), and reverse transfer capacitance (CRSS). Qg takes into account all capacitances associated with charging the gate. Use the typical Qg value for best results; the maximum value is usually grossly overspecified since it is a guaranteed limit and not the measured value. The typical total gate charge should be 50nC or less. With larger numbers, the EXT pins may not be able to adequately drive the gate. The EXT rise/fall time with various capacitive loads as shown in the Typical Operating Characteristics.
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5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers
The two most significant losses contributing to the N-FET's power dissipation are I2R losses and switching losses. Select a transistor with low r DS(ON) and low CRSS to minimize these losses. Determine the maximum required gate-drive current from the Qg specification in the N-FET data sheet. The MAX773's maximum allowed switching frequency during normal operation is 300kHz; but at start-up the maximum frequency can be 500kHz, so the maximum current required to charge the N-FET's gate is f(max) x Qg(typ). Use the typical Qg number from the transistor data sheet. For example, the Si9410DY has a Qg(typ) of 17nC (at VGS = 5V), therefore the current required to charge the gate is: IGATE (max) = (500kHz) (17nC) = 8.5mA. The bypass capacitor on V+ (C2) must instantaneously furnish the gate charge without excessive droop (e.g., less than 200mV): Qg V+ = ---- C2 Continuing with the example, V+ = 17nC/0.1F = 170mV. Use I GATE when calculating the appropriate shunt resistor. See the Shunt Regulator Operation section. Figure 2a's application circuit uses an MTD3055EL logic-level N-FET with a guaranteed threshold voltage (V TH) of 2V. Figure 2b's application circuit uses an 8-pin Si9410DY surface-mount N-FET that has 50m on resistance with 4.5V VGS, and a guaranteed VTH of less than 3V. rent IC(PEAK). Calculate IB as follows: IB = ILIM/ Use the worst-case (lowest) value for given in the transistor's electrical specification, where the collector current used for the test is approximately equal to ILIM. It may be necessary to use even higher base currents (e.g., IB = ILIM /10), although excessive IB may impair operation by extending the transistor's turn-off time. RBASE is determined by:
MAX770-MAX773
(VEXTH - VBE - VCS(min )) RBASE = ------------------------- IB
Where V EXTH is the voltage at V+ (in bootstrapped mode VEXTH is the output voltage), VBE is the 0.7V transistor base-emitter voltage, VCS(min) is the voltage drop across the current-sense resistor, and I B is the minimum base current that forces the transistor into saturation. This equation reduces to (V+ - 700mV 170mV) / IB. For maximum efficiency, make RBASE as large as possible, but small enough to ensure the transistor is always driven near saturation. Highest efficiency is obtained with a fast-switching NPN transistor (fT 150MHz) with a low collector-emitter saturation voltage and a high current gain. A good transistor to use is the Zetex ZTX694B.
Diode Selection
The MAX770-MAX773's high switching frequency demands a high-speed rectifier. Schottky diodes such as the 1N5817-1N5822 are recommended. Make sure that the Schottky diode's average current rating exceeds the peak current limit set by RSENSE, and that its breakdown voltage exceeds VOUT. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; high-speed silicon diodes may be used instead. At heavy loads and high temperatures, the benefits of a Schottky diode's low forward voltage may outweigh the disadvantages of its high leakage current.
NPN Transistors The MAX773 can drive NPN transistors, but be extremely careful when determining the base-current requirements. Too little base current can cause excessive power dissipation in the transistor; too much base current can cause the base to oversaturate, so the transistor remains on continually. Both conditions can damage the transistor. When using the MAX773 with an NPN transistor, connect EXTL to the transistor's base, and connect RBASE between EXTH and the base (Figure 8c). To determine the required peak inductor current, IC(PEAK), observe the Typical Operating Characteristics efficiency graphs and the theoretical output current capability vs. input voltage graphs to determine a sense resistor that will allow the desired output current. Divide the 170mV worst-case (smallest) voltage across the current-sense amplifier VCS(max) by the senseresistor value. To determine IB, set the peak inductor current (ILIM) equal to the peak transistor collector cur-
Capacitor Selection
Output Filter Capacitor The primary criterion for selecting the output filter capacitor (C2) is low effective series resistance (ESR). The product of the peak inductor current and the output filter capacitor's ESR determines the amplitude of the ripple seen on the output voltage. An OS-CON 300F, 6.3V output filter capacitor has approximately 50m of ESR and typically provides 180mV ripple when stepping up from 3V to 5V at 1A (Figure 2a).
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5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
Smaller capacitors are acceptable for light loads or in applications that can tolerate higher output ripple. Since the output filter capacitor's ESR affects efficiency, use low-ESR capacitors for best performance. The smallest low-ESR surface-mount tantalum capacitors currently available are the Sprague 595D series. Sanyo OS-CON organic semiconductor through-hole capacitors and the Nichicon PL series also exhibit low ESR. See Table 2.
VIN
R4
V+
R5 100k LBO LOW-BATTERY OUTPUT
LBI R3
MAX773
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 switching action of the MAX770-MAX773. The input voltage source impedance determines the size of the capacitor required at the V+ input. As with the output filter capacitor, a lowESR capacitor is recommended. For output currents up to 1A, 150F (C1) is adequate, although smaller bypass capacitors may also be acceptable. Bypass the IC with a 0.1F ceramic capacitor (C2) placed close to the V+ and GND pins. Reference Capacitor Bypass REF with a 0.1F capacitor (C3). REF can source up to 100A of current.
GND
R4 = R3
( VTRIP -1)
REF
V
VREF = 1.5V
Figure 9. Input Voltage Monitor Circuit
__________Applications Information
MAX773 Operation with High Input/Output Voltages
The MAX773's shunt regulator input allows high voltages to be converted to very high voltages. Since the MAX773 runs off the 6V shunt (bootstrapped operation is not allowed), the IC will not see the high input voltage. Use an external logic-level N-FET as the power switch, since only 6V of VGS are available. Also, make sure all external components are rated for very high output voltage. Figure 3e shows a circuit that converts 28V to 100V.
Setting the Low-Battery-Detector Voltage
To set the low-battery detector's falling trip voltage (VTRIP(falling)), select R3 between 10k and 500k (Figure 9), and calculate R4 as follows: R4 = (R3) VTRIP - VREF (--------------) V
REF
Low Input Voltage Operation
When using a power supply that decays with time (such as a battery), the N-FET transistor will operate in its linear region when the voltage at EXT approaches the threshold voltage of the FET, dissipating excessive power. Prolonged operation in this mode may damage the FET. This effect is much more significant in nonbootstrapped mode than in bootstrapped mode, since bootstrapped mode typically provides much higher VGS voltages. To avoid this condition, make sure VEXT is above the VTH of the FET, or use a voltage detector (such as the MAX8211) to put the IC in shutdown mode once the input supply voltage falls below a predetermined minimum value. Excessive loads with low input voltages can also cause this condition.
where VREF = 1.5V. The rising trip voltage is higher because of the comparator's approximately 20mV of hysteresis, and is determined by: R4 VTRIP (rising) = (VREF + 20mV) (1 + ----) R3 Connect a high value resistor (larger than R3 + R4) between LBI and LBO if additional hysteresis is required. Connect a pull-up resistor (e.g., 100k) between LBO and V+. Tie LBI to GND and leave LBO floating if the low-battery detector is not used.
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5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers
Starting Up under Load
The Typical Operating Characteristics show the StartUp Voltage vs. Load Current graph for bootstrappedmode operation. This graph depends on the type of power switch used. The MAX770-MAX773 are not designed to start up under full load in bootstrapped mode with low input voltages. lead, and the output filter capacitor ground lead to a single point (star ground configuration). Also, minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. Place input bypass capacitor C2 as close as possible to V+ and GND. Excessive noise at the V+ input may falsely trigger the timing circuitry, resulting in short pulses at EXT. If this occurs it will have a negligible effect on circuit efficiency. If desired, place a 4.7F directly across the V+ and GND pins (in parallel with the 0.1F C2 bypass capacitor) to reduce the noise at V+.
MAX770-MAX773
Layout Considerations
Due to high current levels and fast switching waveforms, which radiate noise, proper PC board layout is essential. Protect sensitive analog grounds by using a star ground configuration. Minimize ground noise by connecting GND, the input bypass capacitor ground
Table 2. Component Suppliers
PRODUCTION INDUCTORS Sumida CD54 series CDR125 series Coiltronics CTX20 series CAPACITORS TRANSISTORS N-FET Siliconix Si9410DY Si9420DY (high voltage) Motorola MTP3055EL MTD20N03HDL DIODES
Surface Mount
Matsuo 267 series Sprague 595D series
Nihon EC10 series
Through Hole
Sumida RCH855 series RCH110 series Renco RL1284-18
Sanyo OS-CON series Nichicon PL series United Chemi-Con LXF series
NPN Zetex ZTX694B
Motorola 1N5817-1N5822 MUR115 (high voltage)
SUPPLIER Coiltronics Matsuo Nichicon Nihon Renco Sanyo Sumida United Chemi-Con Zetex
PHONE USA: (561) 241-7876 USA: (714) 969-2491 Japan: 81-6-337-6450 USA: (847) 843-7500 USA: (805) 867-2555 USA: (516) 586-5566 USA: (619) 661-6835 Japan: 81-7-2070-6306 USA: (847) 956-0666 Japan: 81-3-3607-5111 USA: (714) 255-9500 USA: (516) 543-7100 UK: 44-61-627-4963
FAX (561) 241-9339 (714) 960-6492 81-6-337-6456 (847) 843-2798 (805) 867-2698 (516) 586-5562 (619) 661-1055 81-7-2070-1174 81-3-3607-5144 (714) 255-9400 (516) 864-7630 44-61-627-5467
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5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770-MAX773
___Ordering Information (continued)
PART MAX771CPA MAX771CSA MAX771C/D MAX771EPA MAX771ESA MAX771MJA MAX772CPA MAX772CSA MAX772C/D MAX772EPA MAX772ESA MAX772MJA MAX773CPD MAX773CSD MAX773C/D MAX773EPD MAX773ESD MAX773MJD 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 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 14 Plastic DIP 14 SO Dice* 14 Plastic DIP 14 Narrow SO 14 CERDIP
_________________Chip Topographies
MAX770/MAX771/MAX772
EXT
V+
CS
0.126" (3.200mm)
GND FB AGND
SHDN
0.080" (2.032mm)
REF
*Contact factory for dice specifications.
____Pin Configurations (continued)
TOP VIEW
TRANSISTOR COUNT: 501; SUBSTRATE CONNECTED TO V+.
MAX773
V5 V12 V15
EXTH
V12 1 V5 2 V+ 3 LBO 4 LBI 5 FB 6 SHDN 7
14 V15 13 EXTH 12 EXTL
V+ LBO
EXTL CS
SGND LBI 0.126" (3.200mm) GND FB GND
MAX773
11 CS 10 SGND 9 8 GND REF
DIP/SO
SHDN
0.080" (2.032mm)
REF
TRANSISTOR COUNT: 501; SUBSTRATE CONNECTED TO V+.
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