MAX1771 Просмотр технического описания (PDF) - Maxim Integrated

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MAX1771

12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller

Maxim Integrated 

12V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controller

_______________Detailed Description

The MAX1771 is a BiCMOS, step-up, switch-mode pow-

er-supply controller that provides a preset 12V output,

in addition to adjustable-output operation. Its unique

control scheme combines the advantages of pulse-fre-

quency 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 tai-

lor 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

range of load currents; and 3) the maximum supply

current is only 110µA.

The device has a shutdown mode that reduces the

supply current to 5µA max.

Bootstrapped/Non-Bootstrapped Modes

Figure 2 shows the standard application circuits for

bootstrapped and non-bootstrapped modes. In boot-

strapped 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

REF

FB

DUAL-MODE

COMPARATOR

1.5V

REFERENCE

MIN OFF-TIME

ONE-SHOT

ERROR

COMPARATOR

Q TRIG

2.3µs

S

MAX ON-TIME

ONE-SHOT

TRIG Q

16µs

F/F

Q

R

LOW-VOLTAGE

OSCILLATOR

CURRENT-SENSE

AMPLIFIER

50mV

MAX1771

N

2.5V

0.1V

SHDN

BIAS

CIRCUITRY

V+

EXT

CS

Figure 1. Functional Diagram

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