MIC5012

Micrel

5-120

April 1998

Input

Source

Gate

1/2 MIC5012

V+

IRF541

1µF

Figure 6. Half-Bridge

Motor Driver

+

100nF

1N4001 (2)

1N5817

15V

220pF

1N4148

22k

Ω

M

12V,

10A Stalled

Input

Source

Gate

1/2 MIC5012

V+

Gnd

IRF541

10µF

+

1nF

10k

Ω

2N3904

22k

Ω

15V

PWM

INPUT

High-side drivers implemented with MIC501X drivers are

self-protected against inductive switching transients. Dur-

ing turn-off an inductive load will force the MOSFET source

5V or more below ground, while the MIC5012 holds the gate

at ground potential. The MOSFET is forced into conduction,

and it dissipates the energy stored in the load inductance.

The MIC5012 source pin is designed to withstand this

negative excursion without damage. External clamp diodes

are unnecessary.

Low-Side Driver (Figure 2). A key advantage of the low-

side topology is that the load supply is limited only by the

MOSFET BV

protect the MOSFET drain terminal from inductive switch-

ing transients. The MIC5012 supply should be limited to

15V in low-side topologies; otherwise, a large current will be

forced through the gate clamp zener. The switching speed

to 10V enhancement is 300

µs driving 1nF on a 5V supply.

On a 15V supply the turn-on time is less than 2

µs to 10V

Low-side drivers constructed with the MIC501X family are

also fast; the MOSFET gate is driven to near supply

immediately when commanded ON. Typical circuits achieve

10V enhancement in 10

µs or less on a 12 to 15V supply.

Modifying Switching Times. Do not add external capacitors

to the MOSFET gate. Add a resistor (1k

Ω to 51kΩ) in series

with the gate to slow down the switching time.

Bootstrapped High-Side Driver (Figure 3). The speed of

a high-side driver can be increased to better than 10

µs by

bootstrapping the supply off of the MOSFET source. This

topology can be used where the load is pulse-width modu-

lated (100Hz to 20kHz), or where it is energized continu-

ously. The Schottky barrier diode prevents the MIC5012

supply pin from dropping more than 200mV below the drain

supply, and it also improves turn-on time on supplies of less

than 10V. Since the supply current in the “off” state is only

a small leakage, the 100nF bypass capacitor tends to

remain charged for several seconds after the MIC5012 is

turned off. In a PWM application the chip supply is sustained

at a higher potential than the system supply, which im-

proves switching time.

Opto-Isolated Interface (Figure 4). Although the MIC5012

has no special input slew rate requirement, the lethargic

transitions provided by an opto-isolator may cause oscilla-

tions on the rise and fall of the output. The circuit shown

accelerates the input transitions from a 4N35 opto-isolator

by adding hysteresis. Opto-isolators are used where the

control circuitry cannot share a common ground with the

MIC5012 and high-current power supply, or where the

control circuitry is located remotely. This implementation is

intrinsically safe; if the control line is severed the MIC5012

will turn OFF.

Industrial Switch (Figure 5). The most common manual

control for industrial loads is a push button on/off switch.

The “on” button is physically arranged in a recess so that in

a panic situation the “off” button, which extends out from the

control box, is more easily pressed. This circuit is compat-

ible with control boxes such as the CR2943 series (GE).

The circuit is configured so that if both switches close

simultaneously, the “off” button has precedence.

This application also illustrates how two (or more) MOSFETs