April 1998

5-121

MIC5012

Micrel

5

Figure 8. Motor Stall

Shutdown

M

T

STOP

RUN

START

12V

100nF

1N4148

Input

Source

Gate

1/2 MIC5012

V+

Gnd

12V

IRFZ44

10µF

+

330k

Ω

330k

Ω

R

can be paralleled. This reduces the switch drop, and distrib-

utes the switch dissipation into multiple packages.

Half-Bridge Motor Driver (Figure 6). Closed loop control of

motor speed requires a half-bridge driver. This topology

presents an extra challenge since the two output devices

should not cross conduct (shoot-through) when switching.

Cross conduction increases output device power dissipa-

tion. Speed is also important, since PWM control requires

the outputs to switch in the 2 to 20kHz range.

The circuit of Figure 6 utilizes fast configurations for both the

top- and bottom-side drivers. Delay networks at each input

provide a 2 to 3

µs dead time effectively eliminating cross

conduction. Two of these circuits can be connected to-

gether to form an H-bridge for locked antiphase or sign/

magnitude control.

Time-Delay Relay (Figure 7). The MIC5012 forms the

basis of a simple time-delay relay. As shown, the delay

commences when power is applied, but the 100k

Ω/1N4148

could be independently driven from an external source such

as a switch or another high-side driver to give a delay

relative to some other event in the system. Hysteresis has

been added to guarantee clean switching at turn-on.

Motor Driver with Stall Shutdown (Figure 8). Tachometer

feedback can be used to shut down a motor driver circuit

when a stall condition occurs. The control switch is a 3-way

type; the “START” position is momentary and forces the

driver ON. When released, the switch returns to the “RUN”

position, and the tachometer's output is used to hold the

MIC5012 input ON. If the motor slows down, the tach output

is reduced, and the MIC5012 switches OFF. Resistor “R”

sets the shutdown threshold.

Input

Source

Gate

1/2 MIC5012

V+

Gnd

12V

IRFZ44

10µF

Figure 7. 30 Ampere

Time-Delay Relay

+

OUTPUT

(Delay=2s)

10k

Ω

100

Ω

47µF

+

100k

Ω

1N4148

Electronic Governor (Figure 9). The output of an ac

tachometer can be used to form a PWM loop to maintain the

speed of a motor. The tachometer output is rectified,

partially filtered, and fed back to the input of the MIC5012.

When the motor is stalled there is no tachometer output,

and MIC5012 input is pulled high delivering full power to the

motor. If the motor spins fast enough, the tachometer output

is sufficient to pull the MIC5012 input low, shutting the

output off. Since the rectified waveform is only partially

filtered, the input oscillates around its threshold causing the

MIC5012 to switch on and off at the frequency of the

tachometer signal. A PWM action results since the average

dc voltage at the input decreases as the motor spins faster.

The 1k

Ω potentiometer is used to set the running speed of

the motor. Loop gain (and speed regulation) is increased by

increasing the value of the 100nF filter capacitor.

The performance of such a loop is imprecise, but stable and

inexpensive. A more elaborate loop would consist of a PWM

controller and a half-bridge.

Figure 9. Electronic Governor

M

T

100nF

1N4148

Input

Source

Gate

1/2 MIC5012

V+

Gnd

15V

IRF541

10µF

+

330k

Ω

330k

Ω

1k

Ω

15V