Rainbow Electronics MAX782 User Manual

Page 22

MAX782

Triple-Output Power-Supply
Controller for Notebook Computers

______________________________________________________________________________________

Figure 8 shows a more complex example of power
sequencing. On power-up, the Intel 486SL computer
requires the +5V supply to come up before the +3.3V
supply. A power-ready signal is required

≥

50ms later.

This circuit’s ON3 output connects to the MAX782’s
ON3 pin, and can be wire-OR connected with an open-
drain output to enable another circuit to turn the +3.3V
supply off.

PCMCIA Slot +3.3V/+5V VCC Switching

The MAX782 contains level shifters that simplify driving
external power MOSFETs to switch PCMCIA card VCC
to 3.3V and 5V. While a PCMCIA card is being inserted
into the socket, the VCC pins on the card edge should
be powered down to 0V so “hot insertion” does not
damage the PCMCIA card. The simplest way to do this
is to use a mechanical switch that has to be physically
opened before the PCMCIA card can be inserted. The
switch, which disconnects VCC, can be closed only
when the card has been fitted snugly into its socket.
Figure 9’s circuit illustrates this approach and correctly
shows the connections to both MOSFETs: N2 appears
to be inserted with drain and source the wrong way
around, but this is necessary to prevent its body diode
from pulling the +3.3V supply up to 5V when VCC is
connected to the +5V supply.

Figure 10’s circuit provides an alternative method of
connecting the VCC supply to the PCMCIA slot. While
it avoids using a mechanical switch, it does not provide
the security of a physical interlock. Placing the two
MOSFETs N1 and N2 with their body diodes facing in
opposite directions allows VCC to be shut down to 0V
without using a mechanical switch, and allows VCC to
be driven to 5V without the +3.3V supply being pulled
up to 5V.

HYST1

C4
100nF

VCC

R6
1M

C3
100nF

POWER READY

ON3

C1
22nF

OUT2

R2
390k

R1
1M

R5
820k

100

+5V SUPPLY

+3.3V SUPPLY

SET1

SET2

GND

1.3V

ICL7665

Figure 8. Power-Up Sequencing for the Intel 486SL

PCMCIA 2.0

DIGITAL

CONTROLLER

VCC_EN0

VCC_EN1

FB5

FB3

VDD

+5V SUPPLY

MECHANICAL

SWITCH

SLOT
VCC

+3.3V SUPPLY

MAX782

NOTE:

MOSFET BODY DIODES SHOWN FOR CLARITY.

Figure 9. Simple Switching for PCMCIA Slot VCC