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MAX894LC_D

Dual, Current-Limited, High-Side P-Channel Switches with Thermal Shutdown

Maxim Integrated 

Dual, Current-Limited, High-Side P-Channel

Switches with Thermal Shutdown

_______________Detailed Description

The MAX894L/MAX895L P-channel MOSFET power

switches limit output current to a user-programmed

level. When the output current is increased beyond the

set current level, the current also increases through the

replica switch (IOUT/1050) (MAX895) and through RSET

(Figure 1). The current-limit error amplifier compares

the voltage across RSET to the internal 1.24V reference,

and regulates the current back to the lesser of the pro-

grammed current limit (ILIMIT) or the maximum current

limit (IMAX).

These switches are not bidirectional; therefore, the

input voltage must be higher than the output voltage.

Setting the Current Limit

The MAX894L/MAX895L feature internal current-limiting

circuitry with maximum programmable values (IMAX) of

500mA, and 250mA, respectively. For best perfor-

mance, set the current limit (ILIMIT) between 0.2IMAX ≤

ILIMIT ≤ IMAX. This current limit remains in effect

throughout the input supply-voltage range.

Program the current limit with a resistor (RSET_) con-

nected from SET_ to ground (Figure 2) as follows:

ISET = ILIMIT / IRATIO

RSET = 1.24V / ISET = (1.24V x IRATIO) / ILIMIT

where ILIMIT is the desired current limit for either switch

and IRATIO is the IOUT to ISET current ratio (1085mA for

the MAX894L and 1050mA for the MAX895L.

Short-Circuit Protection

The MAX894L/MAX895L are short-circuit-protected

switches. In the event of an output short circuit or cur-

rent-overload condition, the current through either switch

is limited by the internal current-limiting error amplifier to

1.5 x ILIMIT. When the fault condition is removed, the

replica error amplifier sets the current limit back to ILIMIT.

For a high ∆VDS/∆t during an output short-circuit condi-

tion, the switch turns off and disconnects the input supply

from the output. The current-limiting amplifier then slowly

turns the switch on with the output current limited to

1.5 x ISET. When the fault condition is removed, the current

limit is set back to ILIMIT. Refer to Output Short-Circuit

Fast-Loop Response and Output Short-Circuit Slow-Loop

Response in the Typical Operating Characteristics.

Thermal Shutdown

The MAX894L/MAX895L 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 cycles on and off, resulting in a

pulsed output.

SET

MAX894L

RSET

MAX895L

GND

Figure 2. Setting the Current Limit

__________Applications Information

Input Capacitor

To limit input voltage drop during momentary output

short-circuit conditions, place 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. This

capacitor prevents inductive parasitics 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, reducing 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 can

dissipate and channel heat away. Calculate the maxi-

mum power as follows:

P = I2LIMITA x R ONA + I2LIMITB x R ONB

where RONA and RONB are the on-resistances of

switches A and B, respectively.

When one or both outputs are short circuited, the volt-

age drop across the switch equals the input supply.

Hence, the power dissipated across the switch increas-

es, 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

to further dissipate heat.

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