Dual-Output Power-Supply

Controller for Notebook Computers

8

_______________________________________________________________________________________

The MAX786 has two close relatives: the MAX782 and

the MAX783. The MAX782 and MAX783 each include

an extra flyback winding regulator and linear regulators

for dual, +12V/programmable PCMCIA VPP outputs.

The MAX782/MAX783 data sheet contains extra appli-

cations information on the MAX786 not found in this

data sheet.

+3.3V Switch-Mode Supply

The +3.3V supply is generated by a current-mode,

PWM step-down regulator using two N-channel

MOSFETs, a rectifier, and an LC output filter (Figure 1).

The gate-drive signal to the high-side MOSFET, which

must exceed the battery voltage, is provided by

a boost circuit that uses a 100nF capacitor connected

to BST3.

_________________Detailed Description

The MAX786 converts a 5.5V to 30V input to four outputs

(Figure 1). It produces two high-power, PWM, switch-

mode supplies, one at +5V and the other at +3.3V. The

two supplies operate at either 300kHz or 200kHz,

allowing for small external components. Output current

capability depends on external components, and can

exceed 6A on each supply. An internal 5V, 5mA supply

(VL) and a 3.3V, 5mA reference voltage are also gener-

ated via linear regulators, as shown in Figure 2. Fault

protection circuitry shuts off the PWMs when the inter-

nal supplies lose regulation.

Two precision voltage comparators are also included.

Their output stages permit them to be used as level

translators for driving external N-channel MOSFETs in

load-switching applications, or for more conventional

logic signals.

MAX786

V+

VL

23

22

INPUT

5.5V TO 30V

(NOTE 1)

C1

33

µF

D2A

1N4148

C5

0.1

µF

25

27

18

16

17

GND

REF

PGND

C3

1

µF

+3.3V AT 5mA

BST3

DH3

LX3

BST5

DH5

LX5

9

10

20

N1

C9

0.01

µF

+3.3V ON/OFF

+5V ON/OFF

SHUTDOWN

OSC SYNC

INPUT VOLTAGE RANGE 6.5V TO 30V

AS SHOWN. SEE

LOW-VOLTAGE

(6-CELL) OPERATION SECTION FOR

DETAILS.

NOTE 1:

USE SHORT, KELVIN-CONNECTED PC

BOARD TRACES PLACED VERY CLOSE

TO ONE ANOTHER.

NOTE 2:

COMPARATOR SUPPLY INPUT

IN

OUT

COMPARATOR 1

IN

OUT

COMPARATOR 2

(NOTE 2)

C8

0.01

µF

N4

D3

1N5819

L2

10

µH

R2

25m

+5V

AT

3A

C6

330

µF

+5V AT 5mA

4.7

µF

C10

33

µF

N2

D2B

1N4148

C4

0.1

µF

2

3

13

12

11

14

6

4

8

5

7

SS3

ON3

ON5

SHDN

SYNC

SS5

VH

D1

Q1

D2

Q2

19

15

21

DL5

CS5

FB5

(NOTE 2)

N3

26

D1

1N5819

L1

10

µH

R1

25m

Ω

+3.3V

AT

3A

C12

150

µF

24

1

28

DL3

CS3

FB3

C7

150

µF

Figure 1. MAX786 Application Circuit