MAX770C/D 查看數據表（PDF） - Maxim Integrated

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MAX770C/D

5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers

Maxim Integrated 

5V/12V/15V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controllers

VIN = 24V TO 28V

C1

47µF

RSHUNT

3k

L1

250µH

C2

0.1µF

10 SGND

4 LBO

3

v+

EXTH 13

EXTL 12

8 REF

CS 11

C3

0.1µF

MAX773 V15 14

5 LBI

V12 1

V5 2

7

SHDN

FB 6

GND

R1

11.3k (1%)

9

D1

MUR115

VOUT = 100V

@ 10mA

N

Si9420DY

C4

100µF

RSENSE

1.0Ω

R2

732k (1%)

( ) R2 = (R1) VOUT -1

VREF

VREF = 1.5V

VIN

RSHUNT

3 V+

MAX773

C2

0.1µF

6V (typ)

10 SGND

RSHUNT = VIN (MIN) - VSHUNT (MAX)

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 on-

time one-shot turns it off (typically 16µs 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 volt-

age 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 volt-

ages, 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-com-

parator 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 low voltage start-up

circuitry is disabled if non-bootstrapped mode is select-

ed (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

12 ______________________________________________________________________________________

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