Rainbow Electronics MAX786 - Manual (Page 8)

MAX786
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 µ F
D2A
1N4148
C5
0.1 µ F
25
27
18
16
17
GND REF PGND
C3
1
µ F
+3.3V AT 5mA
BST3
DH3
LX3
BST5
DH5
LX5
9 10 20
N1
C9
0.01 µ F
+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
µ F
N4
D3
1N5819
L2
10 µ H
R2
25m
+5V
AT
3A
C6
330
µ F
+5V AT 5mA
4.7 µ F
C10
33
µ F
N2
D2B
1N4148
C4
0.1
µ F
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 µ H
R1
25m Ω
+3.3V
AT
3A
C12
150 µ F
24
1
28
DL3
CS3
FB3
C7
150 µ F
Figure 1. MAX786 Application Circuit