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19-0263; Rev 1; 7/95
NUAL KIT MA ATION HEET EVALU DATA S WS FOLLO
12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller
____________________________Features
o 90% Efficiency for 30mA to 2A Load Currents o Up to 24W Output Power o 110A Max Supply Current o 5A Max Shutdown Current o 2V to 16.5V Input Range o Preset 12V or Adjustable Output Voltage o Current-Limited PFM Control Scheme o Up to 300kHz Switching Frequency o Evaluation Kit Available
_______________General Description
The MAX1771 step-up switching controller provides 90% efficiency over a 30mA to 2A load. A unique current-limited pulse-frequency-modulation (PFM) control scheme gives this device the benefits of pulse-widthmodulation (PWM) converters (high efficiency at heavy loads), while using less than 110A of supply current (vs. 2mA to 10mA for PWM converters). This controller uses miniature external components. Its high switching frequency (up to 300kHz) allows surface-mount magnetics of 5mm height and 9mm diameter. It accepts input voltages from 2V to 16.5V. The output voltage is preset at 12V, or can be adjusted using two resistors. The MAX1771 optimizes efficiency at low input voltages and reduces noise by using a single 100mV current-limit threshold under all load conditions. A family of similar devices, the MAX770-MAX773, trades some full-load efficiency for greater current-limit accuracy; they provide a 200mV current limit at full load, and switch to 100mV for light loads. The MAX1771 drives an external N-channel MOSFET switch, allowing it to power loads up to 24W. If less power is required, use the MAX756/MAX757 or MAX761/MAX762 step-up switching regulators with on-board MOSFETs. An evaluation kit is available. Order the MAX1771EVKIT-SO.
MAX1771
______________Ordering Information
PART MAX1771CPA MAX1771CSA MAX1771C/D MAX1771EPA MAX1771ESA MAX1771MJA TEMP. RANGE 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**
________________________Applications
Positive LCD-Bias Generators Flash Memory Programmers High-Power RF Power-Amplifier Supply Palmtops/Hand-Held Terminals Battery-Powered Applications Portable Communicators
* Contact factory for dice specifications. ** Contact factory for availability and processing to MIL-STD-883B.
__________________Pin Configuration
__________Typical Operating Circuit
TOP VIEW
INPUT 2V TO VOUT OUTPUT 12V
EXT V+ 1 2 8 CS GND AGND REF
MAX1771
ON/OFF SHDN
EXT CS
N
MAX1771
7 6 5
FB 3 SHDN 4
REF FB AGND GND V+
DIP/SO
________________________________________________________________ Maxim Integrated Products
1
Call toll free 1-800-998-8800 for free samples or literature.
12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller MAX1771
ABSOLUTE MAXIMUM RATINGS
Supply Voltage V+ to GND ...............................................................-0.3V, 17V EXT, CS, REF, SHDN, FB to GND ...................-0.3V, (V+ + 0.3V) GND to AGND.............................................................0.1V, -0.1V 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 MAX1771C_ A .....................................................0C to +70C MAX1771E_ A ..................................................-40C to +85C MAX1771MJA ................................................-55C to +125C Junction Temperatures MAX1771C_ A/E_ A.......................................................+150C MAX1771MJA ..............................................................+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 Input Voltage Range Minimum Start-Up Voltage Supply Current Standby Current Output Voltage (Note 1) Output Voltage Line Regulation (Note 2) Output Voltage Load Regulation (Note 2) Maximum Switch On-Time Minimum Switch Off-Time Efficiency tON(max) tOFF(min) V+ = 5V, VOUT = 12V, ILOAD = 500mA, Circuit of Figure 2a MAX1771C Reference Voltage VREF IREF = 0A MAX1771E MAX1771M REF Load Regulation REF Line Regulation FB Trip Point Voltage VFB 0A IREF 100A 3V V+ 16.5V MAX1771C MAX1771E MAX1771M 1.4700 1.4625 1.4550 MAX1771C/E MAX1771M 1.4700 1.4625 1.4550 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 12.0V, over full load range, Circuit of Figure 2a V+ = 5V to 7V, VOUT = 12V ILOAD = 700mA, Circuit of Figure 2a V+ = 6V, VOUT = 12V, ILOAD = 0mA to 500mA, Circuit of Figure 2a 12 1.8 11.52 SYMBOL CONDITIONS MAX1771 (internal feedback resistors) MAX1771C/E (external resistors) MAX1771MJA (external resistors) MIN 2.0 3.0 3.1 1.8 85 2 4 12.0 5 12.48 TYP MAX 12.5 16.5 16.5 2.0 110 5 V A A V mV/V V UNITS
20 16 2.3 92 1.5 1.5 1.5 4 4 40 1.5 1.5 1.5 1.5300 1.5375 1.5450 10 15 100 1.5300 1.5375 1.5450 20 2.8
mV/A s s %
V
mV V/V V
2
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12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETERS SYMBOL MAX1771C FB Input Current IFB MAX1771E MAX1771M SHDN Input High Voltage SHDN Input Low Voltage SHDN Input Current Current-Limit Trip Level CS Input Current EXT Rise Time EXT Fall Time V+ = 5V, 1nF from EXT to ground V+ = 5V, 1nF from EXT to ground VCS VIH VIL V+ = 2.0V to 16.5V V+ = 2.0V to 16.5V V+ = 16.5V, SHDN = 0V or V+ V+ = 5V to 16V MAX1771C/E MAX1771M 85 75 100 100 0.01 55 55 1.6 0.4 1 115 125 1 CONDITIONS MIN TYP MAX 20 40 60 V V A mV A ns ns nA UNITS
MAX1771
Note 1: Output voltage guaranteed using preset voltages. See Figures 4a-4d for output current capability versus input voltage. Note 2: Output voltage line and load regulation depend on external circuit components.
__________________________________________Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (BOOTSTRAPED MODE)
MAX1771-01
EFFICIENCY vs. LOAD CURRENT (NON-BOOTSTRAPED MODE)
MAX1771-02
LOAD CURRENT vs. MINIMUM START-UP INPUT VOLTAGE
VOUT = 12V, CIRCUIT OF FIGURE 2a EXTERNAL FET THRESHOLD LIMITS FULL-LOAD START-UP BELOW 3.5V
MAX1771-TOC3
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 1 10 100 1000 VIN = 5V VOUT = 12V CIRCUIT OF FIGURE 2a VIN = 3V VIN = 10V VIN = 8V
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 VOUT = 12V CIRCUIT OF FIGURE 2b 1 10 100 1000 VIN = 5V VIN = 8V VIN =10V
700 600 LOAD CURRENT (mA) 500 400 300 200 100 0 2.00
10,000
10,000
2.25
2.50
2.75
3.00
3.25
3.50
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MINIMUM START-UP INPUT VOLTAGE (V)
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12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller MAX1771
____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. TEMPERATURE
MAX1771-04
SUPPLY CURRENT vs. SUPPLY VOLTAGE
VOUT = 12V SUPPLY CURRENT (mA) 0.6 BOOTSTRAPPED CIRCUIT OF FIGURE 2a
MAX1771-05
EXT RISE/FALL TIME vs. SUPPLY VOLTAGE
MAX1771-06
4
3 ENTIRE CIRCUIT 2
EXT RISE/FALL TIME (ns)
SUPPLY CURRENT (mA)
VOUT = 12V, VIN = 5V CIRCUIT OF FIGURE 2a BOOTSTRAPPED MODE
0.8
250 200
CEXT = 2200pF CEXT = 1000pF CEXT = 446pF CEXT = 100pF
150
0.4
100
1
SCHOTTKY DIODE LEAKAGE EXCLUDED
0.2 NON-BOOTSTRAPPED CIRCUIT OF FIGURE 2b
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 10 12 SUPPLY VOLTAGE (V) SUPPLY VOLTAGE (V)
REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE
REFERENCE OUTPUT RESISTANCE ()
MAX1771-07
REFERENCE vs. TEMPERATURE
MAX1771-08
MAXIMUM SWITCH ON-TIME vs. TEMPERATURE
MAX1771-09
250
1.506 1.504 1.502 REFERENCE (V) 1.500 1.498 1.496 1.494
16.5
200 10A 150 tON(MAX) (s)
16.0
100
50A 100A
50
0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
1.492 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
15.5 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
SHUTDOWN CURRENT vs. TEMPERATURE
MAX1771-10
MINIMUM SWITCH OFF-TIME vs. TEMPERATURE
MAX1771-11
3.5 SHUTDOWN CURRENT (A) 3.0 2.5 2.0 1.5 V+ = 8V 1.0 0.5 0 -60 -40 -20 V+ = 4V V+ = 15V
tON(MAX)/tOFF(MIN) RATIO
7.5
tOFF(MIN) (s)
2.25
7.0
6.5
2.20 0 20 40 60 80 100 120 140 TEMPERATURE (C) -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
6.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
4
_______________________________________________________________________________________
MAX1771-12
4.0
MAXIMUM SWITCH ON-TIME/ MINIMUM SWITCH OFF-TIME RATIO vs. TEMPERATURE
8.0
2.30
12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2a, TA = +25C, unless otherwise noted.)
MAX1771
HEAVY-LOAD SWITCHING WAVEFORMS
VOUT A 0V B ILIM B 0A C C
MEDIUM-LOAD SWITCHING WAVEFORMS
VOUT 0V ILIM
A
0A
2s/div VIN = 5V, IOUT = 900mA, VOUT = 12V A: EXT VOLTAGE, 10V/div B: INDUCTOR CURRENT, 1A/div C: VOUT RIPPLE, 50mV/div, AC-COUPLED
10s/div VIN = 5V, IOUT = 500mA, VOUT = 12V A: EXT VOLTAGE, 10V/div B: INDUCTOR CURRENT, 1A/div C: VOUT RIPPLE, 50mV/div, AC-COUPLED
LINE-TRANSIENT RESPONSE
LOAD-TRANSIENT RESPONSE
A
7V 500mA 5V A 0A
0V B B
5ms/div IOUT = 700mA, VOUT = 12V A: VIN, 5V to 7V, 2V/div B: VOUT RIPPLE, 100mV/div, AC-COUPLED
5ms/div VIN = 6V, VOUT = 12V A: LOAD CURRENT, 0mA to 500mA, 500mA/div B: VOUT RIPPLE, 100mV/div, AC-COUPLED
_______________________________________________________________________________________
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12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller MAX1771
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2a, TA = +25C, unless otherwise noted.)
ENTERING/EXITING SHUTDOWN
A 0V
B 5V 0V 2ms/div IOUT = 500mA, VIN = 5V A: SHDN, 5V/div B: VOUT, 5V/div
______________________________________________________________Pin Description
PIN 1 2 3 NAME EXT V+ FB FUNCTION Gate Drive for External N-Channel Power Transistor Power-Supply Input. Also acts as a voltage-sense point when in bootstrapped mode. 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.
4
SHDN
5 6 7 8
REF AGND GND CS
6
_______________________________________________________________________________________
12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller
_______________Detailed Description
The MAX1771 is a BiCMOS, step-up, switch-mode power-supply controller that provides a preset 12V output, in addition to adjustable-output operation. Its unique control scheme combines the advantages of pulse-frequency modulation (low supply current) and pulsewidth 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 MAX1771 functional diagram. The MAX1771 offers three main improvements over prior pulse-skipping control solutions: 1) the converter operates with miniature (5mm height and less than 9mm diameter) surface-mount inductors due to its 300kHz switching frequency; 2) the current-limited PFM control scheme allows 90% efficiencies over a wide
REF FB DUAL-MODE COMPARATOR
range of load currents; and 3) the maximum supply current is only 110A. The device has a shutdown mode that reduces the supply current to 5A max.
MAX1771
Bootstrapped/Non-Bootstrapped Modes
Figure 2 shows the 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 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
SHDN 50mV 1.5V REFERENCE MIN OFF-TIME ONE-SHOT Q TRIG 2.3s F/F S R LOW-VOLTAGE OSCILLATOR 2.5V Q N V+
MAX1771
BIAS CIRCUITRY
ERROR COMPARATOR
MAX ON-TIME ONE-SHOT TRIG Q 16s
CURRENT-SENSE AMPLIFIER
EXT 0.1V CS
Figure 1. Functional Diagram
_______________________________________________________________________________________
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12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller MAX1771
VIN = 5V
C2 0.1F 5 REF C3 0.1F 4 SHDN 3 FB 6 AGND GND 7 V+ L1 22H
VIN = 5V
C1 68F C2 0.1F 2 V+ C1 68F 5 REF C3 0.1F L1 22H D1 1N5817-22 C4 300F N MTD20N03HDL RSENSE 40m R1 18k
2
VOUT = 12V @ 0.5A
MAX1771
D1 1N5817-22 N Si9410DY/ MTD20N03HDL RSENSE 40m
VOUT = 12V @ 0.5A
4 SHDN MAX1771 6 AGND
EXT
1
EXT
1
CS
8 R2 127k
CS
8
FB C4 300F R2 = (R1) GND 7
3
( VOUT -1) V
REF
C5 100pF
VREF = 1.5V
Figure 2a. 12V Preset Output, Bootstrapped
Figure 2b. 12V Output, Non-Bootstrapped
VIN = 4V
C2 0.1F 2 5 REF C3 0.1F V+ L1 22H
C1 47F D1 1N5817-22 C4 N 200F Si9410DY/ MTD20N03HDL RSENSE 40m R1 28k R2 140k
VOUT = 9V
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 2b, 2c) to achieve an overall output voltage accuracy of 5%. To achieve highest efficiency, operate in bootstrapped mode whenever possible.
4 SHDN MAX1771 6
EXT
1
External Power-Transistor Control Circuitry
PFM Control Scheme The MAX1771 uses 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 MAX1771 uses a sense resistor to control the peak inductor current. The device also operates with high switching frequencies (up to 300kHz), allowing the use of miniature external components. As with traditional PFM converters, the power transistor is not turned on until the voltage comparator senses the output is out of regulation. However, unlike traditional PFM converters, the MAX1771 switch uses the combination of a peak current limit and a pair of one-shots that set the maximum on-time (16s) and minimum offtime (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.
AGND
CS
8
FB GND 7 R2 = (R1)
3
( VOUT -1) V
REF
C5 100pF
VREF = 1.5V
Figure 2c. 9V Output, Bootstrapped
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 is 3V when using external feedback resistors. With supply voltages below 5V, bootstrapped mode is recommended. Note: When using the MAX1771 in non-bootstrapped mode, there is no preset output operation because V+ is also the output voltage sense point
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_______________________________________________________________________________________
12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller
The control circuitry allows the IC to operate in continuous-conduction mode (CCM) while maintaining high efficiency with heavy loads. When the power switch is 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. The MAX1771 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 100mV of output ripple occurs using the circuit of Figure 2a.
MAX1771
R2 FB VOUT R1
MAX1771
C5*
R1 = 10k TO 500k GND R2 = R1
( VOUT -1) REF
V
VREF = 1.5V * SEE TEXT FOR VALUE
Low-Voltage Start-Up Oscillator The MAX1771 features 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 MAX1771 disregards the error-comparator output when the supply voltage is less than 2.5V. Above 2.5V, the error-comparator and normal one-shot timing circuitry are used. The lowvoltage start-up circuitry is disabled if non-bootstrapped mode is selected (FB is not tied to ground).
Figure 3. Adjustable Output Circuit
R1 and R2 configured as shown in Figure 3. For adjustable-output operation, select feedback resistor R1 in the 10k to 500k range. R2 is given by: VOUT R2 = (R1) ----- -1 VREF
(
)
Shutdown Mode
When SHDN is high, the MAX1771 enters shutdown mode. In this mode, the internal biasing circuitry is turned off (including the reference) and VOUT falls to a diode drop below V IN (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.
where VREF equals 1.5V. For preset-output operation, tie FB to GND (this forces bootstrapped-mode operation. Figure 2 shows various circuit configurations for bootstrapped/non-bootstrapped, preset/adjustable operation.
__________________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. The MAX1771's output voltage can be adjusted from very high voltages down to 3V, using external resistors
Determining RSENSE Use the theoretical output current curves shown in Figures 4a-4d to select RSENSE. They were derived using the minimum (worst-case) current-limit comparator threshold value over the extended temperature range (-40C to +85C). 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. Determining the Inductor (L)
Practical inductor values range from 10H to 300H. 22H 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 (t ON (min)) is
_______________________________________________________________________________________
9
12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller MAX1771
3.5 MAXIMUM OUTPUT CURRENT (A) 3.0 2.5 2.0 1.5 1.0 0.5 RSENSE = 100m 0 2 3 4 INPUT VOLTAGE (V) 5 0 2 4 RSENSE = 50m VOUT = 5V L = 22H MAXIMUM OUTPUT CURRENT (A) RSENSE = 20m RSENSE = 25m RSENSE = 35m 3.5 3.0 2.5 2.0 1.5 1.0 0.5 RSENSE = 100m 6 8 INPUT VOLTAGE (V) 10 12 RSENSE = 50m VOUT = 12V L = 22H RSENSE = 20m RSENSE = 25m RSENSE = 35m
Figure 4a. Maximum Output Current vs. Input Voltage (VOUT = 5V)
Figure 4b. Maximum Output Current vs. Input Voltage (VOUT = 12V)
3.5 MAXIMUM OUTPUT CURRENT (A) 3.0 2.5 2.0 1.5 1.0 0.5 0 2
VOUT = 15V L = 22H MAXIMUM OUTPUT CURRENT (A) RSENSE = 20m RSENSE = 25m RSENSE = 35m
0.8 VOUT = 24V L =150H 0.6 RSENSE = 50m 0.4 RSENSE = 100m
RSENSE = 50m RSENSE = 100m
0.2 RSENSE = 200m 0
4
6
8 10 12 INPUT VOLTAGE (V)
14
16
2
6
10 INPUT VOLTAGE (V)
14
Figure 4c. Maximum Output Current vs. Input Voltage (VOUT = 15V)
Figure 4d. Maximum Output Current vs. Input Voltage (VOUT = 24V)
approximately 2s; select an inductor that allows the current to ramp up to I LIM. The standard operating circuits use a 22H inductor. If a different inductance value is desired, select L such that: VIN(max) x 2s L -----------------ILIM 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, preferably under 20m. To minimize radiated noise, use a toroid, a pot core, or a shielded coil. Table 1 lists inductor suppliers and specific recommended inductors.
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12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller
Power Transistor Selection
Use an N-channel MOSFET power transistor with the MAX1771. 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. 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 varies with different capacitive loads as shown in the Typical Operating Characteristics. 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 MAX1771'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. Figure 2a'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. Figure 2b's application circuit uses an MTD20N03HDL logiclevel N-FET with a guaranteed threshold voltage (VTH) of 2V.
Diode Selection
The MAX1771's high switching frequency demands a high-speed rectifier. Schottky diodes such as the 1N5817-1N5822 are recommended. Make sure the Schottky diode's average current rating exceeds the peak current limit set by RSENSE, and that its breakdown voltage exceeds V OUT. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; high-speed silicon diodes such as the MUR105 or EC11FS1 can 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.
MAX1771
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 peak inductor current and the output filter capacitor's ESR determines the amplitude of the ripple seen on the output voltage. Two OS-CON 150F, 16V output filter capacitors in parallel with 35m of ESR each typically provide 75mV ripple when stepping up from 5V to 12V at 500mA (Figure 2a). Smaller-value and/or higher-ESR 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. See Table 1 for component selection. 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 MAX1771. 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 1A, 68F (C1) is adequate, although smaller bypass capacitors may also be acceptable. Bypass the IC with a 0.1F ceramic capacitor (C2) placed as close to the V+ and GND pins as possible. Reference Capacitor Bypass REF with a 0.1F capacitor (C3). REF can source up to 100A of current for external loads. Feed-Forward Capacitor In adjustable output voltage and non-bootstrapped modes, parallel a 47pF to 220pF capacitor across R2, as shown in Figures 2 and 3. Choose the lowest capacitor value that insures stability; high capacitance values may degrade line regulation.
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12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller MAX1771
Table 1. Component Suppliers
PRODUCTION INDUCTORS Sumida CD54 series CDR125 series Coiltronics CTX20 series Coilcraft DO3316 series DO3340 series CAPACITORS TRANSISTORS Siliconix Si9410DY Si9420DY (high voltage) Motorola MTP3055EL MTD20N03HDL MMFT3055ELT1 MTD6N1O MMBT8099LT1 MMBT8599LT1 DIODES Central Semiconductor CMPSH-3 CMPZ5240 Nihon EC11 FS1 series (highspeed silicon) Motorola MBRS1100T3 MMBZ5240BL Motorola 1N5817-1N5822 MUR115 (high voltage) MUR105 (high-speed silicon)
Surface Mount
Matsuo 267 series Sprague 595D series AVX TPS series
Through Hole
Sumida RCH855 series RCH110 series
Sanyo OS-CON series Nichicon PL series
SUPPLIER AVX Central Semiconductor Coilcraft Coiltronics Matsuo Motorola Nichicon Nihon Sanyo Siliconix Sprague Sumida
PHONE USA: (803) 448-9411 USA: (516) 435-1110 USA: (708) 639-6400 USA: (407) 241-7876 USA: (714) 969-2491 Japan: 81-6-337-6450 USA: (800) 521-6274 USA: (708) 843-7500 USA: (805) 867-2555 USA: (619) 661-6835 Japan: 81-7-2070-1005 USA: (800) 554-5565 USA: (603) 224-1961 USA: (708) 956-0666 Japan: 81-3-3607-5111
FAX (803) 448-1943 (516) 435-1824 (708) 639-1469 (407) 241-9339 (714) 960-6492 81-6-337-6456 (602) 952-4190 (708) 843-2798 (805) 867-2556 (619) 661-1055 81-7-2070-1174 (408) 970-3950 (603) 224-1430 (708) 956-0702 81-3-3607-5144
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12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller
Layout Considerations
C1 2.2F 2 4 3V = OFF C4 5 REF 0.1F V+ SHDN VIN* 3V TO 11V D2 1N5817 L1 20H 1 CTX20-4
MAX1771
D1 1N5817
VOUT 5V 500mA
MAX1771
EXT CS
1 8
C2 Q1** 47F 16V R1 0.1 R3
L2
C3 220F 10V
GND AGND 6 7 R2
FB 3
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 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+.
C5 47pF SEE TEXT FOR FURTHER COMPONENT INFO **VIN MAY BE LOWER THAN INDICATED IF THE SUPPLY IS NOT **REQUIRED TO START UNDER FULL LOAD **MOTOROLA MMFT3055ELT1 FOR 5V: R2 = 200k, R3 = 470k 3.3V: R2 = 100k, R3 = 20k
Other Application Circuits
4 Cells to 5V (or 3 Cells to 3.3V), 500mA Step-Up/Down Converter The circuit shown in Figure 5 generates 5V (or 3.3V) at 500mA with 85% efficiency, from an input voltage that varies above and below the output. The output couples to the switching circuitry via a capacitor. This configuration offers two advantages over flyback-transformer and step-up linear-regulator circuits: smooth regulation as the input passes through the output, and no output current in shutdown. This circuit requires two inductors, which can be wound on one core with no regard to coupling since they do not work as a transformer. L1 and L2 can either be wound together (as with the Coiltronics CTX20-4) or kept as two separate inductors; both methods provide equal performance. Capacitors C2 and C3 should be low-ESR types for best efficiency. A 1F ceramic capacitor will work at C2, but with about 3% efficiency loss. C2's voltage rating must be greater than the maximum input voltage. Also note that the LX switch must withstand a voltage equal to the sum of the input and output voltage; for example, when converting 11V to 5V, the switch must withstand 16V. LX switch pulses are captured by Schottky diode D2 to boost V+ to (VOUT + VIN). This improves efficiency with a low input voltage, but also limits the maximum input supply to 11V. If the input voltage does not fall below 4V and if a 3V logic threshold FET is used for Q1, you may omit D2 and connect V+ directly to the input supply. 12V Output Buck/Boost The circuit in Figure 6 generates 12V from a 4.5V to 16V input. Higher input voltages are possible if you
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Figure 5. Step-Up/Down for a 5V/3.3V Output
__________Applications Information
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.
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 MAX1771 is not designed to start up under full load in bootstrapped mode with low input voltages.
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12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller MAX1771
VIN 4.5V TO 15V C1 33F 16V 2 OFF ON 4 V+ SHDN
L1 20H C2* 1F 1 8 6 7 R1 0.1 D1 1N5819
VOUT 12V 250mA
tries to use internal feedback and looks to V+ for its feedback signal. However, since V+ may be greater than the internally set feedback (12V for the MAX1771), the IC may think the output is sufficiently high and not start. D2 ensures start-up by pulling FB above ground and forcing the external feedback mode. In a normal (not AC-coupled) boost circuit, D2 isn't needed, since the output and FB rise as soon as input power is applied.
MAX1771
EXT CS AGND 5 C5 0.1F REF R3 28k 1% GND FB 3
Q1** L2* 20H C4 C3 100F 100F 16V 16V NOTE: HIGHCURRENT GND R2 200k 1% *SEE TEXT FOR FURTHER COMPONENT INFORMATION **Q1 = MOTOROLA MMFT3055ELT1 L1 + L2 = ONE COILTRONICS CTX20-4
D2* 1N4148 NOTE: KEEP ALL TRACES CONNECTED TO PIN 3 AS SHORT AS POSSIBLE
Figure 6. 12V Buck/Boost from a 4.5V to 15V Input
carefully observe the component voltage ratings, since some components must withstand the sum of the input and output voltage (27V in this case). The circuit operates as an AC-coupled boost converter, and does not change operating modes when crossing from buck to boost. There is no instability around a 12V input. Efficiency ranges from 85% at medium loads to about 82% at full load. Also, when shutdown is activated (SHDN high) the output goes to 0V and sources no current. A 1F ceramic capacitor is used for C2. A larger capacitor value improves efficiency by about 1% to 3%. D2 ensures start-up for this AC-coupled configuration by overriding the MAX1771's Dual-Mode feature, which allows the use of preset internal or user-set external feedback. When operating in Dual-Mode, the IC first
Transformerless -48V to +5V at 300mA The circuit in Figure 7 uses a transformerless design to supply 5V at 300mA from a -30V to -75V input supply. The MAX1771 is biased such that its ground connections are made to the -48V input. The IC's supply voltage (at V+) is set to about 9.4V (with respect to -48V) by a zener-biased emitter follower (Q2). An N-channel FET (Q1) is driven in a boost configuration. Output regulation is achieved by a transistor (Q3), which level shifts a feedback signal from the 5V output to the IC's FB input. Conversion efficiency is typically 82%. When selecting components, be sure that D1, Q1, Q2, Q3, and C6 are rated for the full input voltage plus a reasonable safety margin. Also, if D1 is substituted, it should be a fast-recovery type with a trr less than 30ns. R7, R9, C8, and D3 are optional and may be used to soft start the circuit to prevent excessive current surges at power-up. Battery-Powered LCD Bias Supply The circuit in Figure 8 boosts two cells (2V min) to 24V for LCD bias or other positive output applications. Output power is boosted from the battery input, while V+ voltage for the MAX1771 is supplied by a 5V or 3.3V logic supply. 5V, 1A Boost Converter The circuit in Figure 9 boosts a 2.7V to 5.5V input to a regulated 5V, 1A output for logic, RF power, or PCMCIA applications. Efficiency vs. load current is shown in the adjacent graph.
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12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller MAX1771
L1 D03340 220H-680H
D1 MBRS1100T3 3 Q1 MTD6N10 1 R2 47k 1%
+5V 300mA
4 C5 0.1F SHDN 5
EXT
1
2
REF
CS
8
R9 5.1k
C8 1F
R7 200
C7 220pF
C1 220F 10V
C2 220F 10V
C3 0.33F
3
FB
MAX1771
V+
2 -48V Q2 MMBT8099LT1 R5 1k
D3 CMPSH-3
Q3 MMBT8599LT1
7
GND
AGND
6
C6 10F 100V
R1 0.15
R6 200k
C4 2.2F 20V D2 CMPZ5240/ MMBZ5240BL
R4 100k R3 16k 1%
Figure 7. -48V Input to 5V Output at 300mA, Without a Transformer
BATTERY INPUT 2V TO 12V 3.3V OR 5V LOGIC SUPPLY
L1 22H 0.1F 2 V+ 1 8 RSENSE 0.2 R2 150k
1N5817 N MMFT3055ELT1
OUTPUT Adj. = 12V TO 24V (AS SHOWN)
47F
EXTL CS
OFF ON
4
SHDN
MAX1771
REF 5 FB 3 GND 6, 7
0.1F
R3 10k 10k
Figure 8. 2V Input to 24V Output LCD Bias
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12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller MAX1771
INPUT 2.7V TO 5.5V 150F 1N5820 EXT OFF ON 0.04 4 SHDN CS 1 8 330F 22H
EFFICIENCY vs. LOAD CURRENT
OUTPUT 5V 1A 100
90 EFFICIENCY (%) MTD20N03HDL VIN = 4V VIN = 3V 70
80
MAX1771
5 REF V+ 2 232k 100pF
60
0.1F GND 7 0.1F AGND FB 6 3
50 1m 100k 10m 100m 1 LOAD CURRENT (A)
Figure 9. 5V/1A Boost Converter
___________________Chip Topography
EXT
V+
CS
0.126" (3.200mm)
GND FB AGND
SHDN
0.080" (2.032mm)
REF
TRANSISTOR COUNT: 501 SUBSTRATE CONNECTED TO V+
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