MAX761C 데이터 시트보기 (PDF) - Maxim Integrated

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MAX761C

12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters

Maxim Integrated 

12V/15V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Converters

1A

500mA

1A

500mA

0A

Figure 4a. CCM, Heavy Load Current Waveform (500mA/div)

Figure 4b. Light/Medium Load Current Waveform (500mA/div)

used in a bootstrapped circuit (Figure 5), undervoltage

lockout prevents start-up at low input voltages; but

once started, operation can continue down to a lower

voltage that depends on the load.

There is no undervoltage lockout when the internal feed-

back resistors are used (Figure 2), and special circuitry

guarantees start-up at 2.0V. The start-up circuitry fixes

the duty cycle at 50% until V+ is driven to 2.5V, above

which the normal control system takes over.

Shutdown Mode

The MAX761/MAX762 enter shutdown mode when

SHDN is high. In this mode, the internal biasing circuitry

is turned off (including the reference) and VOUT equals

V+ minus a diode drop (due to the DC path from the

input to the output). In shutdown mode, the supply cur-

rent drops to less than 5µA. SHDN is a TTL/CMOS logic

level input. Connect SHDN to GND for normal operation.

LBO is high impedance during shutdown.

Modes of Operation

When delivering high output currents, the MAX761/

MAX762 operate in CCM. In this mode, current always

flows in the inductor, and the control circuit adjusts the

switch’s duty cycle on a cycle-by-cycle basis to maintain

regulation without exceeding the switch-current capabili-

ty. This provides excellent load-transient response and

high efficiency.

In discontinuous-conduction mode (DCM), current

through the inductor starts at zero, rises to a peak value,

then ramps down to zero on each cycle. Although effi-

ciency is still excellent, the switch waveforms contain

ringing (the inductor's self-resonant frequency). This

ringing is normal and poses no operational problems.

Low-Battery Detector

The MAX761/MAX762 provide a low-battery comparator

that compares the voltage on LBI to the 1.5V reference

voltage. When the LBI voltage is below VREF, LBO (an

open-drain output) goes low. The low-battery compara-

tor’s 20mV of hysteresis adds noise immunity, prevent-

ing repeated triggering of LBO. Use a resistor-divider

network between V+, LBI, and GND to set the desired

trip voltage VTRIP (Figure 3). When SHDN is high, LBI is

ignored and LBO is high impedance. The value of

resistor R3 should be no larger than 500kΩ to ensure

the LBI leakage current does not cause inaccuracies in

VTRIP.

__________________Design Procedure

Setting the Output Voltage

The MAX761/MAX762’s output voltage can be adjusted

from 5V to 16.5V using external resistors R1 and R2

configured as shown in Figures 3 and 5. For adjustable-

output operation, select feedback resistor R1 in the

10kΩ to 250kΩ range. Higher R1 values within this

range give lowest supply current and best light-load

efficiency. R2 is given by:

R2 = (R1)( VOUT - 1)

VREF

where VREF = 1.5V.

Note: Tie FB to GND for fixed-output operation

(bootstrapped mode only).

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