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

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

12

______________________________________________________________________________________

GND

MAX773

C3

0.1

µF

v+

EXTL

CS

V5

8

5

7

V15

V12

LBO

REF

LBI

SHDN

4

12

11

2

14

1

9

1.0

Ω

C4

100

µF

D1

MUR115

EXTH

13

3

L1

250

µH

@ 10mA

SGND

N

10

C1

47

µF

FB

6

R2

732k (1%)

R1

11.3k (1%)

Si9420DY

3k

C2

0.1

µF

R2 = (R1)

(

-1

)

MAX773

C2

0.1

µF

SGND

6V (typ)

V+

3

10

Figure 3e. 100V Output, Shunt Regulator, N-Channel Power

MOSFET

Figure 4. MAX773 Shunt Regulator