5

LTC1157

APPLICATIO S I FOR ATIO

MOSFET Selection

The LTC1157 is designed to operate with both standard

and logic level N-channel MOSFET switches. The choice of

switch is determined primarily by the operating supply

voltage.

Logic Level MOSFET Switches at 3.3V

Logic level switches should be used with the LTC1157

when powered from 2.7V to 4V. Although there is some

variation among manufacturers, logic level MOSFET

generally little price differential. Logic level MOSFETs are

frequently designated by an “L” and are usually available

in surface mount packaging. Some logic level MOSFETs

are rated up to

±15Vandcanbeusedinapplicationswhich

require operation over the entire 2.7V to 5.5V range.

Standard MOSFET Switches at 5V

Standard N-channel MOSFET switches should be used

with the LTC1157 when powered from 4V to 5.5V supply

as the built-in charge pump produces ample gate drive to

fully enhance these switches when powered from a 5V

nominal supply. Standard N-channel MOSFET switches

maximum of

±20V.

Powering Large Capacitive Loads

Electrical subsystems in portable battery-powered equip-

ment are typically bypassed with large filter capacitors to

reduce supply transients and supply induced glitching. If

not properly powered however, these capacitors may

themselves become the source of supply glitching.

For example, if a 100

µF capacitor is powered through a

switch with a slew rate of 0.1V/

µs, the current during start-

up is:

= (100

= 10A

Obviously, this is too much current for the regulator (or

output capacitor) to supply and the output will glitch by as

much as a few volts.

The start-up current can be substantially reduced by

limiting the slew rate at the gate of an N-channel switch as

shown in Figure 1. The gate drive output of the LTC1157

is passed through a simple RC network, R1 and C1, which

substantially slows the slew rate of the MOSFET gate to

approximately 1.5

operating as a source follower, the slew rate at the source

is essentially the same as that at the gate, reducing the

start-up current to approximately 15mA which is easily

managed by the system regulator. R2 is required to

eliminate the possibility of parasitic MOSFET oscillations

during switch transitions. Also, it is good practice to

isolate the gates of paralleled MOSFETs with 1k resistors

to decrease the possibility of interaction between switches.

Reverse Battery Protection

The LTC1157 can be protected against reverse battery

conditions by connecting a 300

Ω resistor in series with

the ground pin. The resistor limits the supply current to

less than 12mA with – 3.6V applied. Since the LTC1157

draws very little current while in normal operation, the

drop across the ground resistor is minimal. The 3.3V

µP

(or control logic) can be protected by adding 10k resistors

in series with the input pins.

+

GND

G1

IN1

1/2 LTC1157

MTD3055EL

3.3

µF

LTC1157 • TA02

LT1129-3.3

+

3.3V

LOAD

C1

0.1

µF

100

µF

3.3V

R2

1k

R1

100k

ON/0FF

Figure 1. Powering a Large Capacitive Load