MIC5021

Micrel

5-174

October 1998

Logic-Level MOSFET Precautions

Logic-level MOSFETs have lower maximum gate-to-source

voltage ratings (typically

±10V) than standard MOSFETs

(typically

±20V). When an external MOSFET is turned on, the

doubling effect of the boost capacitor can cause the gate-to-

source voltage to momentarily exceed 10V. Internal zener

diodes clamp this voltage to 16V maximum which is too high

for logic-level MOSFETs. To protect logic-level MOSFETs,

connect a zener diode (5V

≤V

Overcurrent Limiting

A 50mV comparator is provided for current sensing. The low

used for current sensing.

The adjustable retry feature can be used to handle loads with

high initial currents, such as lamps or heating elements, and

can be adjusted from the C

C

overcurrent condition.

C

The default duty cycle (C

to the electrical characteristics when selecting a capacitor for

reduced duty cycle.

C

Increasing the duty cycle increases the power dissipation in

the load and MOSFET under a “fault” condition. Circuits may

become unstable at a duty cycle of about 75% or higher,

depending on conditions.

Caution: The MIC5021 may be

damaged if the voltage applied to C

maximum voltage rating.

Boost Capacitor Selection

The boost capacitor value will vary depending on the supply

voltage range.

Input

Gnd

Gate

Sense

Sense

TTL Input

+12V to +20V

MIC5021

1

2

3

4

8

7

6

5

10µF

0.01

µF

Figure 1. 12V to 20V Configuration

A 0.01

µF boost capacitor is recommended for best perfor-

mance in the 12V to 20V range. Refer to figure 1. Larger

capacitors may damage the MIC5021.

Input

Gnd

Gate

Sense

Sense

TTL Input

+12V to +36V

MIC5021

1

2

3

4

8

7

6

5

10µF

2.7

nF

Figure 2. 12V to 36V Configuration

If the full 12V to 36V voltage range is required, the boost

capacitor value must be reduced to 2.7nF. Refer to Figure 2.

The recommended configuration for the 20V to 36V range is

to place the capacitor is placed between V

shown in Figure 3.

Input

Gnd

Gate

Sense

Sense

TTL Input

+12V to +36V

MIC5021

1

2

3

4

8

7

6

5

10µF

0.1

µF

Figure 3. Preferred 20V to 36V Configuration

Do not use both boost capacitor between V

MOSFET source and V

Current Sense Resistors

Lead length can be significant when using low value (< 1

Ω)

resistors for current sensing. Errors caused by lead length

can be avoided by using four-teminal current sensing resis-

tors.

Four-terminal resistors are available from several

manufacturers.