8

LT1161

1161fa

Low Voltage/Wide Supply Range Operation

When the supply is <12V, the LT1161 charge pumps do

not produce sufficient gate voltage to fully enhance stan-

dard N-channel MOSFETs. For these applications, logic-

level MOSFETs can be used to extend operation down to

greater, then it can also be used up to the 60V (absolute

maximum) rating of the LT1161. MOSFETs are available

from both Motorola and Siliconix which meet these

criteria.

Protecting Against Supply Transients

The LT1161 is 100% tested and guaranteed to be safe

pins. However, when this voltage is exceeded, even for a

few microseconds, the result can be a catastrophic failure.

For this reason

it is imperative that the LT1161 not be

exposed to supply transients above 60V.

quired to be connected to the positive side of the drain

sense resistors (see Drain Sense Configuration). There-

fore, the best way to prevent supply transients is to ensure

that the supply is adequately decoupled at the point where

hundred microfarads may be required with high current

switches.

When operating voltages approach the 60V absolute maxi-

mum rating of the LT1161, local supply decoupling be-

recommended. A small ferrite bead between the supply

connection and local capacitor can also be effective in

suppressing transients. Note however, that resistance

will cause an error in the current sense threshold.

Fault Feedback

Two methods can be used to derive switch status. First,

the timer pin voltage can be monitored to indicate when

the switch is turned off due to current limit. During normal

operation (ON or OFF), the timer voltage is 3.5V and only

during current limit does the voltage drop below 3V.

The second method shown in Figure 8 uses a quad

exclusive-NOR gate to indicate when the output of the

switch has not obeyed the input command (i.e., output low

when it should be high or vice versa). In addition to current

limit, this gives a fault indication if the switch is shorted or

if the load is open.

Figure 8. Fault Feedback Using Exclusive-NOR Gate

24V

ADD FOR

OPEN-LOAD

DETECTION

1161 F08

FAULT

INPUT

1/4 MM74HC266A

100k

LT1161

DS

G

IN

LOAD

Low-Side Driving

Although the LT1161 is primarily targeted at high-side

(grounded load) switch applications, it can also be used

for low-side (supply-connected load), or mixed high- and

low-side switch applications. Figures 9a and 9b illustrate

LT1161 switch channels driving low-side power MOSFETs.

Because the LT1161 charge pump tries to pump the gate

of the N-channel MOSFET

above supply, a clamp zener is

MOSFET from being exceeded. The LT1161 gate drive is

current limited for this purpose so that no resistance is

needed between the gate pin and zener.

Figure 9a. Low-Side Driver with Load Current Sensing

100

µF

IRFZ44

1

µF

0.01

Ω

(PTC)

1161 F09a

15V

1N4744

LT1161

12V TO 48V

DS

G

4A

LOAD

T

+