October 1998

5-175

MIC5021

Micrel

5

The diode should have a peak forward current rating greater

than the load current. This is because the current through the

diode is the same as the load current at the instant the

MOSFET is turned off.

Input

Gnd

Gate

Sense

Sense

TTL Input

N-Channel

Power MOSFET

(IRF540)

+20V to +36V

MIC5021

1

2

3

4

8

7

6

5

10µF

Solenoid

(24V, 47

Ω)

0.01

µF

Schottky

Diode

(1N5822)

(+24V)

(< 0.08

Ω)

Figure 5. Solenoid Driver

with Current Sensing

Sense Pin Considerations

The sense pins of the MIC5021 are sensitive to negative

voltages. Forcing the sense pins much below –0.5V effec-

tively reverses the supply voltage on portions of the driver

resulting in unpredictable operation or damage.

MOSFET

Turnoff

0V

Negative

Spike

Input

Gate

MIC5021

1

2

3

4

8

7

6

5

Inductive

Load

Current flows from ground (0V)

through the diodes to the load

during negative transcients.

Forward drop across diodes

allows leads to go negative.

Figure 6. Inductive Load Turnoff

Figure 6 shows current flowing out of the sense leads of an

MIC5021 during a negative transient (inductive kick). Internal

Schottky diodes attempt to limit the negative transient by

maintaining a low forward drop.

Although the internal Schottky diodes can protect the driver

in low-current resistive applications, they are inadequate for

inductive loads or the lead inductance in high-current resis-

tive loads. Because of their small size, the diodes’ forward

voltage drop quickly exceeds 0.5V as current increases.

Circuits Without Current Sensing

Input

Gnd

Gate

Sense

−

Sense

+

TTL Input

Load

N-Channel

Power MOSFET

V+

MIC5021

1

2

3

4

8

7

6

5

10µF

0.01

µF

Figure 4a. Connecting Sense to Source

Input

Gnd

Gate

Sense

−

Sense

+

TTL Input

Load

N-Channel

Power MOSFET

V+

MIC5021

1

2

3

4

8

7

6

5

10µF

0.01

µF

Figure 4b. Connecting Sense to Supply

Inductive Load Precautions

Circuits controlling inductive loads, such as solenoids (Figure

5) and motors, require precautions when controlled by the

MIC5021. Wire wound resistors, which are sometimes used

to simulate other loads, can also show significant inductive

properties.

An inductive load releases stored energy when its current

flow is interrupted (when the MOSFET is switched off). The

voltage across the inductor reverses and the inductor at-

tempts to force current flow. Since the circuit appears open

(the MOSFET appears as a very high resistance) a very large

negative voltage occurs across the inductor.

Limiting Inductive Spikes

The voltage across the inductor can be limited by connecting

a Schottky diode across the load. The diode is forward biased

only when the load is switched off. The Schottky diode

clamps negative transients to a few volts. This protects the

MOSFET from drain-to-source breakdown and prevents the

transient from damaging the charge pump by way of the boost

capacitor. Also see

Sense Pin Considerations below.