Dual-Slot PCMCIA Analog Power Controller

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7

If a mechanical interlock switch cannot be used, an

extra MOSFET must be added, as shown in Figure 1.

Placing two N-channel MOSFETs in series with their

be shut down to 0V without using a mechanical switch.

Switching Speed

The drive to the external MOSFETs ensures that the

3.3V supply is never connected to the 5V supply. This

is done by turning these transistors off quickly (using

active pull-down circuitry), and on more slowly (using

external pull-up resistors). The turn-on delay depends

on the value of the pull-up resistors, and on the gate

capacitance of the switching transistors. To save

power, use high-value resistors of up to 10M

Ω.

However, note that high-value resistors will increase the

time it takes to turn on the switched supplies

__________Applications Information

The MAX780 can be used with PCMCIA controllers

other than the Intel 82365SL DF. Figure 2 shows the

logic connections to the Cirrus Logic CL-PD6720

PCMCIA Host Adapter.

The MAX780 does not need a PCMCIA controller to

control from the system bus. Figure 3 shows the con-

nection to the system bus. Figure 4 shows the timing

requirements.

Reading from a PCMCIA Port

In the

provides the gate drive needed to turn on the external

N-channel MOSFETs. In some cases, the high-power

written to the PCMCIA port, not when data is being

several milliamps, so it consumes more power than is

necessary simply to provide gate drive for some FETs.

In these circumstances, a separate gate-drive supply is

needed to turn on the external FETs. Ideally it should

have a low quiescent current and be capable of being

turned off when read access to the PCMCIA port is not

required. Doubling or tripling charge pumps can easily

be built using a convenient clock signal from elsewhere

in the system. Buffering the clock signal with a suitable

gate provides on/off control, as shown in Figure 5.

When driven at 100kHz or more by a CMOS gate pow-

ered from 5V, the doubler circuit outputs 8.6V when

loaded with 25k

Ω (equivalent to four 100kΩ pull-up

resistors). Under similar conditions, but when running

from 3.3V, the tripler circuit produces 7.9V.

BDRV3

ADRV3

ADRV5

VCCIN

MAX780_

+12V

100k

+12V

100k

+5V

1

µF

AVCC

1

µF

BVCC

+12V

100k

+3.3V

BDRV5

* EXTRA MOSFET

NOTE: BODY DIODES OF MOSFETS SHOWN FOR CLARITY.

*

*

+12V

100k

Figure 1. Using an Extra MOSFET to Replace the Mechanical

Interlock