MAX771CSLASHD datasheet(14/20 Pages) MAXIM

14 / 20 page

Use an N-channel FET as the power switch when using

the shunt regulator (see

MAX773 Shunt-Regulator

Operation in the Detailed Description). The shunt-regu-

lator current powers the MAX773 and also provides the

FET gate-drive current, which depends largely on the

FET’s total gate charge at VGS = 5V. To determine the

shunt-resistor value, first determine the maximum shunt

current required.

ISHUNT = ISUPP + IGATE

See

N-Channel MOSFETs in the Power-Transistor

Selection section to determine IGATE.

Determine the shunt-resistor value using the following

equation:

VIN(min) - VSHUNT(max)

RSHUNT(max) = ————————————

ISHUNT

where VSHUNT(max) is 6.3V.

The shunt regulator is not disabled in shutdown

mode, and continues to draw the calculated shunt

current.

If the calculated shunt regulator current exceeds 20mA,

or if the shunt current exceeds 5mA and less shunt reg-

ulator current is desired, use the circuit of Figure 6 to

provide increased drive and reduced shunt current

when driving N-FETs with large gate capacitances.

Select ISHUNT = 3mA. This provides adequate biasing

current for this circuit, although higher shunt currents

can be used.

To prevent the shunt regulator from drawing current in

shutdown mode, place a switch in series with the shunt

resistor.

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

Low IQ, Step-Up DC-DC Controllers

______________________________________________________________________________________

MAX773(N)/MAX773(S)

MAX770–MAX773(N)

Fixed Output Available

Higher

Lower

GND to VOUT

2V to 5V (MAX770/MAX771/MAX772),

3V to 5V (MAX773)

MAX770–MAX773(N)

Higher

2V to 16.5V (MAX770/MAX771/MAX772),

(internal feedback resistors)

3V to 16.5V (MAX770/MAX771/MAX772),

(external feedback resistors)

3V to 16.5V (MAX773)

BOOTSTRAPPED*

MAX770/MAX771/MAX772/

MAX773(N)/MAX773(S)

Adjustable Output Available

Lower

Gate-Drive Capacitive Losses

Higher

FET On Resistance

GND to V+

Gate Drive

5V to 16.5V

(MAX770/MAX771/MAX772),

5V and up (MAX773)

Normally Recommended Input

Voltage Range

Lower

No-Load Supply Current

3V to 16.5V

(MAX770/MAX771/MAX772),

3V and up (MAX773)

Possible Input Voltage Range

NON-BOOTSTRAPPED

PARAMETER

Table 1. Bootstrapped vs. Non-Bootstrapped Operation

MAX773

SGND

RSHUNT

EXTL

100

Ω

V+

0.1

µF

VIN

µH

NPN

2N2222A

VOUT

RSENSE

PNP

2N2907A

EXTH

Figure 6. Increased N-FET Gate Drive when Using the Shunt

Regulator

*MAX773(S) indicates shunt mode; MAX773(N) indicates NOT in shunt mode.

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