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19-1660; Rev 1; 1/01
KIT ATION EVALU BLE AVAILA
0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter
Features
o Up to 94% Efficiency o 0.7V to 5.5V Input Range o Up to 800mA Output o Fixed 3.3V Output (or adj from 2.5V to 5.5V) o PWM Synchronous Rectified Topology o Low-Noise, Constant Frequency Operation (1MHz) o 0.1A Logic-Controlled Shutdown o Synchronizable Switching Frequency o Adjustable Current Limit o Adjustable Soft-Start o 10-Pin MAX Package
General Description
The MAX1760 is a high-efficiency, low-noise, step-up DC-DC converter intended for use in battery-powered wireless applications. It maintains exceptionally low quiescent supply current (100A) despite its high 1MHz operating frequency. Small external components and a tiny 10-pin MAX package make this device an excellent choice for small hand-held applications requiring the longest possible battery life. The MAX1760 uses a synchronous-rectified pulsewidth-modulation (PWM) boost topology to generate 2.5V to 5.5V outputs from a wide range of input sources, such as 1 to 3 alkaline or NiCd/NiMH cells or a single lithium-ion (Li+) cell. Maxim's proprietary IdleModeTM circuitry significantly improves efficiency at light load currents while smoothly transitioning to fixedfrequency PWM operation at higher load currents to maintain excellent full-load efficiency. Low-noise, forced PWM mode is available for applications requiring constant-frequency operation at all load currents. The MAX1760 may also be synchronized to an external clock to protect sensitive frequency bands in communications equipment. Analog soft-start and adjustable current limit permit optimization of efficiency, external component size, and output voltage ripple.
PART
MAX1760
Ordering Information
TEMP. RANGE -40C to +85C PIN-PACKAGE 10 MAX MAX1760EUB
Applications
Digital Cordless Phones Wireless Handsets Palmtop Computers Two-Way Pagers PCS Phones Hand-Held Instruments Personal Communicators
Typical Operating Circuit
TOP VIEW
INPUT = 0.7V TO VOUT
Pin Configuration
ON
MAX1760 LX POUT VOUT = 3.3V, 800mA
ISET 1 REF GND FB 2 3 4 5 10 ON 9 POUT LX PGND CLK/SEL
CLK/SEL
MAX1760
8 7 6
ISET REF OUT FB GND PGND
OUT
MAX
Idle Mode is a trademark of Maxim Integrated Products. ________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter MAX1760
ABSOLUTE MAXIMUM RATINGS
ON, OUT, CLK/SEL to GND .....................................-0.3V to +6V PGND to GND.....................................................................0.3V LX to PGND ............................................-0.3V to (VPOUT + 0.3V) POUT to OUT ......................................................................0.3V REF, FB, ISET, POUT to GND...................-0.3V to (VOUT + 0.3V) Continuous Power Dissipation (TA = +70C) 10-Pin MAX (derate 5.6mW/C above +70C) ...........444mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(CLK/SEL = ON = FB = PGND = GND, ISET = REF, OUT = POUT, VOUT = 3.6V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER DC-DC CONVERTER Input Voltage Range (Note 1) Minimum Startup Voltage (Note 2) Temperature Coefficient of Startup Voltage Frequency in Startup Mode Internal Oscillator Frequency Oscillator Maximum Duty Cycle (Note 3) External Clock Frequency Range Output Voltage FB Regulation Voltage FB Input Leakage Current Load Regulation Output Voltage Adjust Range Output Voltage Lockout Threshold (Note 4) ISET Input Leakage Current Supply Current in Shutdown No-Load Supply Current (Note 5) No-Load Supply Current Forced PWM Mode DC-DC SWITCHES POUT Leakage Current LX Leakage Current Switch On-Resistance N-Channel Current Limit P-Channel Turn-Off Current CLK/SEL = GND VLX = 0, VOUT = 5.5V VLX = V ON = VOUT = 5.5V N-channel P-channel 1.0 20 0.1 0.1 0.15 0.25 1.25 60 10 10 0.28 0.45 1.6 120 A A A mA Rising edge VISET = 1.25V V ON = 3.6V CLK/SEL = GND CLK/SEL = OUT VFB < 0.1V, CLK/SEL = OUT, includes load regulation for 0 < ILX < 0.55A Adjustable output, CLK/SEL = OUT, includes load regulation for 0 < ILX < 0.55A VFB = 1.35V CLK/SEL = OUT, no load to full load (0 < ILX < 1.0A) 2.5 2.00 2.15 0.01 0.1 100 2.5 ILOAD < 1mA, TA = +25C ILOAD < 1mA VOUT = 1.5V CLK/SEL = OUT 125 0.8 80 0.5 3.17 1.215 3.3 1.240 0.01 -1.5 5.5 2.30 50 5 185 0.7 0.9 -2.3 500 1 86 1000 1.2 90 1.2 3.38 1.270 100 5.5 1.1 V V mV/C kHz MHz % MHz V V nA % V V nA A A mA CONDITIONS MIN TYP MAX UNITS
2
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0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(CLK/SEL = ON = FB = PGND = GND, ISET = REF, OUT = POUT, VOUT = 3.6V, TA = 0C to +85C. Typical values are at TA = +25C unless otherwise noted.)
PARAMETER REFERENCES Reference Output Voltage Reference Load Regulation Reference Supply Rejection LOGIC INPUTS CLK/SEL Input Low Level CLK/SEL Input High Level ON Input Low Level (Note 6) 2.5V VOUT 5.5V 2.5V VOUT 5.5V 1.1V VOUT 1.8V 1.8V VOUT 5.5V 1.1V VOUT 1.8V 1.8V VOUT 5.5V Input Leakage Current Minimum CLK/SEL Pulse Width Maximum CLK/SEL Rise/Fall Time CLK/SEL, ON VOUT - 0.2 1.6 0.01 200 100 1 A ns ns (0.8)VOUT 0.2 0.4 (0.2)VOUT V V V IREF = 0 -1A < IREF < 50A 2.5V < VOUT < 5V 1.230 1.250 5 0.2 1.270 15 5 V mV mV CONDITIONS MIN TYP MAX UNITS
MAX1760
ON Input High Level (Note 6)
V
ELECTRICAL CHARACTERISTICS
(CLK/SEL = ON = FB = PGND = GND, ISET = REF, OUT = POUT, VOUT = 3.6V, TA = -40C to +85C, unless otherwise noted.) (Note 7)
PARAMETER DC-DC CONVERTER Output Voltage FB Regulation Voltage Internal Oscillator Frequency Oscillator Maximum Duty Cycle (Note 3) Output Voltage Lockout Threshold (Note 4) Supply Current in Shutdown No-Load Supply Current (Note 5) DC-DC SWITCHES Switch On-Resistance N-Channel Current Limit REFERENCE Reference Output Voltage IREF = 0 1.230 1.270 V N-channel P-channel 1.0 0.28 0.45 1.6 A Rising edge V ON = 3.6V CLK/SEL = GND VFB < 0.1V, CLK/SEL = OUT, includes load regulation for 0 < ILX < 0.55A Adjustable output, CLK/SEL = OUT, includes load regulation for 0 < ILX < 0.55A CLK/SEL = OUT 3.17 1.215 0.75 80 2.00 3.38 1.270 1.2 90 2.30 5 185 V V MHz % V A A CONDITIONS MIN MAX UNITS
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3
0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter MAX1760
ELECTRICAL CHARACTERISTICS (continued)
(CLK/SEL = ON = FB = PGND = GND, ISET = REF, OUT = POUT, VOUT = 3.6V, TA = -40C to +85C, unless otherwise noted.) (Note 7)
PARAMETER LOGIC INPUTS CLK/SEL Input Low Level CLK/SEL Input High Level ON Input Low Level (Note 6) ON Input High Level (Note 6) Input Leakage Current 2.5V VOUT 5.5V 2.5V VOUT 5.5V 1.1V VOUT 1.8V 1.8V VOUT 5.5V 1.1V VOUT 1.8V 1.8V VOUT 5.5V CLK/SEL, ON VOUT + 0.2 1.6 1 (0.8)VOUT 0.2 0.4 (0.2)VOUT V V V V A CONDITIONS MIN MAX UNITS
Note 1: Operating voltage--since the regulator is bootstrapped to the output, once started the MAX1760 will operate down to 0.7V input. Note 2: Startup is tested with the circuit shown in Figure 6. Note 3: Defines maximum step-up ratio. Note 4: The regulator is in startup mode until this voltage is reached. Do not apply full load current until the output exceeds 2.3V. Note 5: Supply current into the OUT pin. This current correlates directly to the actual battery-supply current, but is reduced in value according to the step-up ratio and efficiency. Note 6: ON has a hysteresis of approximately 0.15 x VOUT. Note 7: Specifications to -40C are guaranteed by design and not production tested.
Typical Operating Characteristics
(Circuit of Figure 2, VIN = 2.4V, VOUT = 3.3V, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT VOUT = 3.3V
A
MAX1760 toc01
EFFICIENCY vs. OUTPUT CURRENT VOUT = 5V
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 1 0 0.0001 0.001 = AUTO MODE = FPWM MODE A: VIN = 3.6V B: VIN = 2.4V C: VIN = 1.2V 0.01 0.1 1 C B A
MAX1760-02
MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE
0.8 OUTPUT CURRENT (A) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.5 1.0 VOUT = 5V VOUT = 3.3V
MAX1760-03
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.0001 0.001 C
100
0.9
B
= AUTO MODE = FPWM MODE A: VIN = 2.4V B: VIN = 1.2V C: VIN = 0.9V 0.01 0.1
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
1.5 2.0 2.5 3.0 INPUT VOLTAGE (V)
3.5
4.0
4
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0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 2.4V, VOUT = 3.3V, TA = +25C, unless otherwise noted.)
NO-LOAD BATTERY CURRENT vs. INPUT VOLTAGE
MAX1760-04
MAX1760
TOTAL SHUTDOWN CURRENT (ILX + IOUT) vs. INPUT VOLTAGE
MAX1760-05
INTERNAL OSCILLATOR FREQUENCY vs. TEMPERATURE
1.15 1.10 FREQUENCY (MHz) 1.05 1.00 0.95 0.90 0.85 0.80
MAX1760-06
2.5
10
1.20
1.5
SHUTDOWN CURRENT (nA)
2.0 INPUT CURRENT (mA)
1
1.0 VOUT = 5V VOUT = 3.3V 0.0 0 1 2 3 4 5 INPUT VOLTAGE (V)
0.5
0.1 0 1 2 3 4 5 6 INPUT VOLTAGE (V)
0.75 -40 -15 10 35 60 85 TEMPERATURE (C)
STARTUP VOLTAGE vs. OUTPUT CURRENT
MAX1760-07
PEAK INDUCTOR CURRENT vs. VISET
MAX1760-08
HEAVY-LOAD SWITCHING WAVEFORMS
MAX1760-09
3.0 2.5 STARTUP VOLTAGE (V) 2.0
PEAK INDUCTOR CURRENT (A)
A = +85C B = +25C C = -40C
1.4 1.2 1.0 0.8
A
A 1.5 1.0 0.5
B
C
B 0.6 0.4 0.2 0.0 C 0V 0.1 0.3 0.5 0.7 VISET (V) 0.9 1.1 1.3 A = LX PIN, 5V/div B = INDUCTOR CURRENT, 200mA/div C = OUTPUT RIPPLE, 50mV/div AC-COUPLED t = 400ns/div
CIRCUIT OF FIGURE 6 0.0 0.001 0.01
0.1
1
OUTPUT CURRENT (A)
LIGHT-LOAD SWITCHING WAVEFORMS
MAX1760-10
LOAD-TRANSIENT RESPONSE
MAX1760-11
LINE-TRANSIENT RESPONSE
MAX1760-12
A
A
A
B
C
B
B
t = 400ns/div A = LX PIN, 5V/div B = INDUCTOR CURRENT, 200mA/div C = OUTPUT RIPPLE, 50mV/div, AC-COUPLED
t = 200ms/div VIN = 1.1V, VOUT = 3.3V, IOUT = 0 AND 0.2A A = IOUT, 100mA/div B = VOUT, 50mV/div, AC-COUPLED
400s/div VIN = 2.4V TO 1.4V, IOUT = 70mA A = VIN, 1V/div B = VOUT, 5mA/div, AC-COUPLED
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5
0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter MAX1760
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 2.4V, VOUT = 3.3V, TA = +25C, unless otherwise noted.)
TURN-ON WAVEFORMS NO SOFT-START COMPONENTS
MAX1760-13 MAX1760-14
NOISE SPECTRUM
16 A
SOFT-START WAVEFORMS RSS = 500k, CSS = 0.1F
A
MAX1760-15
12 NOISE (mVRMS)
8 B 4 C C OV 0.1 1 FREQUENCY (MHz) 10 t = 2ms/div A = ON, 5V/div B = INPUT CURRENT, 500mA/div C = VOUT, 2V/div 2.00ms/div A = ON, 5V/div B = INPUT CURRENT, 100mA/div C = VOUT, 2V/div 0V B
0
Pin Description
PIN NAME FUNCTION N-Channel Current-Limit Control. For maximum current limit, connect to REF. To reduce current, supply a voltage between REF and GND by means of a resistive voltage-divider. If soft-start is desired, connect a capacitor from ISET to GND. When ON = high, or VREF <80% of nominal value, an on-chip 100k switched resistor discharges this pin to GND. 1.250V Voltage Reference Bypass. Connect a 0.22F ceramic bypass capacitor to GND. Up to 50A of external load current is allowed. Ground. Connect to PGND with short trace. DC-DC Converter Feedback Input. To set fixed output voltage of +3.3V, connect FB to ground. For adjustable output of 2.5V to 5.5V, connect to a resistive divider from OUT to GND. FB set point = 1.24V. IC Power, Supplied from the Output. Bypass to GND with a 0.68F ceramic capacitor, and connect to POUT with a series 4.7 resistor (Figure 2). Clock Input for the DC-DC Converter. Also serves to program operating mode of switcher as follows: CLK/SEL = LO: Normal operation--operates at a fixed frequency, automatically switching to low-power mode if load is minimized. CLK/SEL = HI: Forced PWM mode--operates in low-noise, constant-frequency mode at all loads. CLK/SEL = Clocked: Forced PWM mode with the internal oscillator synchronized to CLK in 500kHz to 1200kHz range. Source of N-Channel Power MOSFET Switch Inductor Connection Power Output. P-channel synchronous-rectifier source. Shutdown Control Input. When ON = high, the IC is in shutdown. Connect ON to GND for normal operation.
1
ISET
2 3 4
REF GND FB
5
OUT
6
CLK/SEL
7 8 9 10 6
PGND LX POUT ON
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0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter MAX1760
OUT IC POWER CONTROLLER 2.15V EN ON STARTUP OSCILLATOR Q D PCH POUT UNDERVOLTAGE LOCKOUT
ON 1.25V
RDY REFERENCE EN OSCILLATOR 1MHz CLK/SEL MODE OSC
LX EN OSC NCH MODE PGND
REF GND CLK/SEL FB
REF
GND
FB
ISET
ISET
MAX1760
Figure 1. Functional Diagram
Detailed Description
The MAX1760 is a highly efficient, low-noise power supply for portable RF and hand-held instruments. It combines a boost switching regulator, N-channel power MOSFET, P-channel synchronous rectifier, precision reference, and shutdown control (Figure 1). The DC-DC converter boosts a 1-cell to 3-cell battery voltage input to a fixed 3.3V or adjustable voltage between 2.5V and 5.5V. An external Schottky diode is required for output voltages greater than 4V. The MAX1760 guarantees startup with an input voltage as low as 1.1V and remains operational down to an input of just 0.7V. It is optimized for use in cellular phones and other applications requiring low noise and low quiescent current for maximum battery life. It features fixed-frequency operation at medium and heavy loads, but at light loads, switches only as needed for optimum efficiency. This device is also capable of constant-frequency (1MHz), low-noise PWM operation at all load currents, or frequency-synchronized PWM operation when connected to an external clock. Table 1 lists some typical outputs. Shutdown reduces quiescent current to just 1A. Figure 2 shows the standard application circuit for the MAX1760.
3.3H VIN = 2.4V 33F ON CLK/SEL MAX1760 ISET REF 0.22F FB OUT GND PGND 0.68F LX POUT 4.7 VOUT = 3.3V, 800mA 100F
Figure 2. Standard Application Circuit
Step-Up Converter
During DC-DC converter operation, the internal N-channel MOSFET switch turns on for the first part of each cycle, allowing current to ramp up in the inductor and store energy in a magnetic field. During the second part of each cycle, the MOSFET turns off and inductor current flows through the synchronous rectifier to the
7
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0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter MAX1760
Table 1. Typical Available Output Current
NUMBER OF NiCd/NiMH CELLS 1 2 2.4 3 3.6 5.0 5.0 500 750 INPUT VOLTAGE (V) 1.2 2.4 OUTPUT VOLTAGE (V) 3.3 3.3 OUTPUT CURRENT (mA) 350 800
Table 2. Selecting the Operating Mode
CLK/SEL 0 1 External Clock 500kHz to 1.2MHz MODE Normal Operation Forced PWM Synchronized PWM FEATURES High efficiency at all loads. Fixed frequency at all but light loads. Low noise, fixed frequency at all loads. Low noise, fixed frequency at all loads.
output filter capacitor and the load. As the energy stored in the inductor is depleted, the current ramps down and the synchronous rectifier turns off. At light loads, the device operates at fixed frequency or only as needed to maintain regulation, depending on the CLK/SEL setting (Table 2).
Synchronous Rectifier
The MAX1760 features an internal, P-channel synchronous rectifier to enhance efficiency. Synchronous rectification provides 5% improved efficiency over similar nonsynchronous boost regulators. In PWM mode, the synchronous rectifier is turned on during the second half of each switching cycle. In low-power mode, an internal comparator turns on the synchronous rectifier when the voltage at LX exceeds the boost regulator output, and turns it off when the inductor current drops below 60mA. When setting output voltages greater than 4V, an external 0.5A Schottky diode must be connected in parallel with the on-chip synchronous rectifier.
Normal Operation
Pulling CLK/SEL low selects the MAX1760's normal operating mode. In this mode, the device operates in PWM when driving medium-to-heavy loads, and automatically switches to PFM if the load requires less power. PFM operation allows higher efficiency than PWM under light load conditions.
Forced PWM Operation
When CLK/SEL is high, the MAX1760 operates in a lownoise PWM-only mode. During forced PWM operation, the MAX1760 switches at a constant frequency (1MHz), and modulates the MOSFET switch pulse width to control the power transferred per cycle to regulate the output voltage. Switching harmonics generated by fixed-frequency operation are consistent and easily filtered. See the Noise Spectrum plot in the Typical Operating Characteristics.
Low-Voltage Startup Oscillator
The MAX1760 uses a CMOS, low-voltage startup oscillator for a 1.1V guaranteed minimum startup input voltage. At startup, the low-voltage oscillator switches the N-channel MOSFET until the output voltage reaches 2.15V. Above this level, the normal boost-converter feedback and control circuitry take over. Once the device is in regulation, it can operate down to 0.7V input since internal power for the IC is bootstrapped from the output through OUT. Do not apply full load until the output exceeds 2.3V.
Synchronized PWM Operation
The MAX1760 can be synchronized in PWM mode to an external frequency of 500kHz to 1.2MHz by applying an external clock signal to CLK/SEL. This allows interference to be minimized in wireless applications. The synchronous rectifier is active during synchronized PWM operation.
8
Shutdown
The MAX1760 has a shutdown mode that reduces quiescent current to 0.1A. During shutdown (ON = high), the reference and all feedback and control circuitry are off. During shutdown, the output voltage is one diode drop below the input voltage.
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0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter
3.3H VIN = 2.4V 33F CLK/SEL ON MAX1760 ISET REF 0.22F PGND GND LX POUT 4.7 OUT FB R1 0.68F VOUT 100F
MAX1760
R2
R1 = R2
(
VOUT -1 VFB
)
VFB = 1.24V
Figure 3. Connecting Resistors for External Feedback
REF 0.22F RSS ILIM = 1.25A 0.22F RSS1
REF ILIM = 1.25A
(
RSS2
RSS1 + RSS2
)
MAX1760
ISET
tSS = RSSCSS
MAX1760
ISET
tSS = (RSS1 II RSS2) CSS
CSS RSS 470k
RSS2
CSS RSS1 470k
Figure 4. Soft-Start with Maximum Switch Limit Current
Figure 5. Soft-Start with Reduced Switch Current Limit
Reference
The MAX1760 has an internal 1.250V 1% reference. Connect a 0.22F ceramic bypass capacitor from REF to GND within 0.2in (5mm) of the REF pin. REF can source up to 50A of external load current.
where VFB, the boost-regulator feedback set point, is 1.24V.
Setting the Switch Current Limit and Soft-Start
The ISET pin adjusts the inductor current limit and implements soft-start. With ISET connected to REF, the inductor current limits at 1.25A. With ISET connected to a resistive divider set from REF to GND, the current limit is reduced according to: V ILIM = 1.25A ISET 1.25V Implement soft-start by placing a resistor from ISET to REF and a capacitor from ISET to GND. In shutdown, ISET is discharged to GND through an on-chip 100k resistor. At power-up, ISET is 0V and the LX current is zero. As the capacitor voltage rises, the current increases and the output voltage rises. The soft-start
9
Design Procedure
Setting the Output Voltages
For a fixed 3.3V output, connect FB to GND. To set other output voltages between 2.5V and 5.5V, connect a resistor voltage-divider to FB from OUT to GND (Figure 3). The input bias current into FB is <20nA, allowing large-value divider resistors without sacrificing accuracy. Connect the resistor voltage-divider as close to the IC as possible, within 0.2in (5mm) of FB. Choose R2 of 270k or less, then calculate R1 using: V R1 = R2 OUT - 1 VFB
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0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter MAX1760
Table 3. Component Selection Guide
PRODUCTION METHOD INDUCTORS Coilcraft DS3316P Coilcraft LPT3305 Surface Mount Sumida CDRH5D18 Sumida CD43 Sanyo POSCAP series -- -- -- CAPACITORS AVX TPS series Kemet T510 series DIODES EIC SB series Motorola MBR0520L
Table 4. Component Suppliers
SUPPLIER AVX Coilcraft EIC Kemet Motorola Sumida PHONE USA: 843-448-9411 USA: 847-639-6400 USA: 916-941-0712 USA: 810-287-2536 USA: 408-629-4789 Japan: 81-45-474-7030 USA: 847-956-0666 Japan: 011-81-3-3667-3302
External Diode
For output voltages greater than 4V, an external Schottky diode must be connected from LX to POUT, in parallel with the on-chip synchronous rectifier (Figure 6). The diode should be rated for 0.5A. Representative devices are Motorola MBR0520L, Nihon EP05Q03L, or generic 1N5817. This external diode is also recommended for applications that must start with input voltages at or below 1.8V. The Schottky diode carries current during startup and after the synchronous rectifier turns off; thus, its current rating only needs to be 500mA. Connect the diode as close to the IC as possible. Do not use ordinary rectifier diodes; their slow switching speeds and long reverse-recovery times render them unacceptable. For circuits that do not require startup with inputs below 1.8V, and have an output of 4V or less, no external diode is needed.
Note: Please indicate that you are using the MAX1760 when contacting these component suppliers.
time constant is: t SS = RSS CSS where RSS 470k. Placing a capacitor across the lower resistor of the current-limiting resistive divider provides both current-limit and soft-start features simultaneously (Figures 4 and 5).
Input and Output Filter Capacitors
Choose input and output filter capacitors that will service the input and output peak currents with acceptable voltage ripple. Choose input capacitors with working voltage ratings over the maximum input volt-
Inductor Selection
The MAX1760's high switching frequency allows the use of a small 3.3H surface-mount inductor. The chosen inductor should generally have a saturation current rating exceeding the N-channel switch current limit; however, it is acceptable to bias the inductor current into saturation by as much as 20% if a slight reduction in efficiency is acceptable. Lower current-rated inductors may be used if ISET is employed to reduce the peak inductor current (see Setting the Switch Current Limit and Soft-Start). For high efficiency, choose an inductor with a high-frequency ferrite core material to reduce core losses. To minimize radiated noise, use a toroid or shielded inductor. See Table 3 for suggested components and Table 4 for a list of component suppliers. Connect the inductor from the battery to the LX pin as close to the IC as possible.
10 ___________________________________________________
VIN = 0.7V TO VOUT 33F CLK/SEL ON MAX1760 ISET REF 0.22F PGND GND OUT FB 0.68F LX POUT 4.7 MRB0520L VOUT 100F 3.3H
Figure 6. Connection with External Schottky Diode for Output Voltages Greater than 4V, or to Assist Low-Voltage Startup
0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter
age, and output capacitors with working voltage ratings higher than the output. The input filter capacitor reduces peak currents drawn from the input source and also reduces input switching noise. The input voltage source impedance determines the required value of the input capacitor. When operating directly from one or two NiMH cells placed close to the MAX1760, use a single 33F low-ESR input filter capacitor. With higher impedance batteries, such as alkaline and Li+, a higher value input capacitor may improve efficiency. The output filter capacitor reduces output ripple voltage and provides the load with transient peak currents when necessary. For the output, a 100F, low-equivalent-series-resistance (ESR) capacitor is recommended for most applications. Sanyo POSCAP, Panasonic SP/CB, and Kemet T510 are good low-ESR capacitors. Low-ESR tantalum capacitors offer a good tradeoff between price and performance. Do not exceed the ripple current ratings of tantalum capacitors. Avoid aluminum electrolytic capacitors; their high ESR typically results in higher output ripple voltage. bypass OUT to GND with a 0.68F ceramic capacitor, and connect OUT to POUT with a 4.7 resistor. Each of these components should be placed as close to its respective IC pins as possible, within 0.2in (5mm). Table 4 lists suggested suppliers.
MAX1760
Layout Considerations
High switching frequencies and large peak currents make PC board layout a critical part of design. Poor design will cause excessive EMI and ground bounce, both of which can cause instability or regulation errors by corrupting the voltage and current feedback signals. Power components--such as the inductor, converter IC, filter capacitors, and output diode--should be placed as close together as possible, and their traces should be kept short, direct, and wide. Keep the voltage feedback network very close to the IC, within 0.2in (5mm) of the FB pin. Keep noisy traces, such as those from the LX pin, away from the voltage feedback network and guarded from them using grounded copper. Refer to the MAX1760 EV kit for a full PC board example.
Chip Information
TRANSISTOR COUNT: 1361
Other External Components
Two ceramic bypass capacitors are required for proper operation. Bypass REF to GND with 0.22F. Also,
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11
0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter MAX1760
Package Information
10LUMAX.EPS
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) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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