MAX770MJA Просмотр технического описания (PDF) - Maxim Integrated

Номер в каталоге

Компоненты Описание

производитель

MAX770MJA

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

Table 1. Bootstrapped vs. Non-Bootstrapped Operation

PARAMETER

Gate Drive

FET On Resistance

Gate-Drive Capacitive Losses

No-Load Supply Current

Possible Input Voltage Range

BOOTSTRAPPED*

GND to VOUT

Lower

Higher

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)

Normally Recommended Input

Voltage Range

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

3V to 5V (MAX773)

Fixed Output Available

MAX770–MAX773(N)

Adjustable Output Available

MAX770–MAX773(N)

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

NON-BOOTSTRAPPED

GND to V+

Higher

Lower

Lower

3V to 16.5V

(MAX770/MAX771/MAX772),

3V and up (MAX773)

5V to 16.5V

(MAX770/MAX771/MAX772),

5V and up (MAX773)

MAX773(N)/MAX773(S)

MAX770/MAX771/MAX772/

MAX773(N)/MAX773(S)

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.

VIN

RSHUNT

C2

C1

0.1µF

3

V+ SGND 10

EXTH 13

L1

20µH

NPN

2N2222A D1

VOUT

EXTL 12

100Ω

N

MAX773

PNP

R2

C4

2N2907A

11

CS

6

FB

RSENSE

R1

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

Regulator

To prevent the shunt regulator from drawing current in

shutdown mode, place a switch in series with the shunt

resistor.

14 ______________________________________________________________________________________

Share Link: