Thermal Shutdown

The MAX891L/MAX892L feature thermal shutdown. The

switch turns off when the junction temperature exceeds

+135°C. Once the device cools by 10°C, the switch

turns back on. If the fault short-circuit condition is not

removed, the switch will cycle on and off, resulting in a

pulsed output.

Fault Indicator

The MAX891L/MAX892L provide a fault output (FAULT).

This open-drain output goes low when in current limit or

when the die temperature exceeds +135°C. During

start-up, FAULT is low until the switch is fully on and no

over-current condition exists. A 100k

Ω pull-up resistor

from FAULT to IN provides a logic-control signal.

__________Applications Information

Input Capacitor

To limit input voltage drop during momentary output

short-circuit conditions, connect a capacitor from IN to

GND. A 1µF ceramic capacitor is adequate for most

applications; however, higher capacitor values further

reduce voltage drop at the input.

Output Capacitor

Connect a 0.1µF capacitor from OUT to GND. One

function of this capacitor is to prevent inductive para-

sitics from pulling OUT negative during turn-off.

Layout and Thermal-Dissipation

Consideration

To take full advantage of the switch-response time to

output short-circuit conditions, it is very important to

keep all traces as short as possible to reduce the effect

of undesirable parasitic inductance. Place input and

output capacitors as close as possible to the device

(no more than 5mm).

Under normal operating conditions, the package dissi-

pates and channels heat away. Calculate maximum

power as follows:

When the output is short circuited, voltage drop across

the switch equals the input supply. Hence, the power

dissipated across the switch increases, as does the die

temperature. If the fault condition is not removed, the

thermal-overload-protection circuitry turns the switch off

until the die temperature falls by 10°C. A ground plane

in contact with the device helps dissipate additional

heat.

___________________Chip Information

TRANSISTOR COUNT: 396

SUBSTRATE CONNECTED TO GND

Current-Limited, High-Side

P-Channel Switches with Thermal Shutdown

_______________________________________________________________________________________

7

MAX891L

MAX892L

SET

GND

Figure 2. Setting the Current Limit