MIC5021 datasheet(13/24 Pages) MICROCHIP

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DS20005677B-page 13

MIC5021

FIGURE 6-6:

Solenoid Driver with Current

Sensing.

6.9.1

LIMITING INDUCTIVE SPIKES

The voltage across the inductor can be limited by con-

necting 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 (see

Sense Pin Considerations).

The diode should have a peak forward current rating

greater than the load current. This is because the cur-

rent through the diode is the same as the load current

at the instant the MOSFET is turned off.

6.9.2

SENSE PIN CONSIDERATIONS

The sense pins of the MIC5021 are sensitive to nega-

tive voltages. Forcing the sense pins much below

–0.5V effectively reverses the supply voltage on por-

tions of the driver resulting in unpredictable operation

or damage.

Figure 6-7 shows current flowing out of the sense leads

of an MIC5021 during a negative transient (inductive

kick). Internal Schottky diodes attempt to limit the neg-

ative transient by maintaining a low forward drop.

FIGURE 6-7:

Inductive Load Turn-Off.

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 resistive loads. Because of their small

size, the diodes’ forward voltage drop quickly exceeds

0.5V as current increases.

6.9.3

EXTERNAL PROTECTION

Resistors placed in series with each SENSE connec-

tion limit the current drawn from the internal Schottky

diodes during a negative transient. This minimizes the

forward drop across the diodes.

During normal operation, sensing current from the

sense pins is unequal (5 µA and 15 µA). The internal

Schottky diodes are reverse-biased and have no effect.

To avoid skewing the trip voltage, the current limiting

resistors must drop equal voltages at the trip point cur-

rents (see Figure 6-8). To minimize resistor tolerance

error, use a voltage drop lower than the trip voltage of

50 mV. 5 mV is suggested.

FIGURE 6-8:

Resistor Voltage Drop.

VDD

INPUT

GND

V BOOST

GATE

SENSE-

SENSE+

TTL INPUT

RSENSE

N-CHANNEL

POWER MOSFET

(IRF540)

+20V TO +36V

MIC5021

10μF

SOLENOID

(24V, 47Ω)

0.01

μF

SCHOTTKY

DIODE

(1N5822)

(+24V)

(<0.08Ω)

MOSFET

TURN-OFF

NEGATIVE

SPIKE

~VDD

VDD

INPUT

GATE

MIC5021

INDUCTIVE

LOAD

FORWARD DROP ACROSS DIODES

ALLOWS LEADS TO GO NEGATIVE

CURRENT FLOWS FROM GROUND (0V)

THROUGH THE DIODES TO THE LOAD

DURING NEGATIVE TRANSCIENTS.

V DD

INPUT

C T

GND

V BOOST

GATE

SENSE-

SENSE+

N-CHANNEL

POWER MOSFET

MIC5021

LOAD

5μA

15μA

VR2

VR1 =VR2

TO AVOID SKEWING

THE 50mV TRIP POINT.

(5mV SUGGESTED)

R1 3 × R2

VR1

50mV NOMINAL

(@ TRIP)

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