MIC5021 Datasheet, PDF (7/9 Pages) Micrel Semiconductor – High-Speed High-Side MOSFET Driver

MIC5021
Circuits Without Current Sensing
V+
10µF
TTL Input
MIC5021
1
V_DD
8
V_BOOST
2
Input
7
Gate
3
C_T
4
Gnd
6
Sense−
5
Sense+
N-Channel
Power MOSFET
0.01
µF
Load
Figure 4a. Connecting Sense to Source
V+
Micrel
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.
10µF
TTL Input
MIC5021
1
V_DD
8
V_BOOST
2
Input
7
Gate
3
C_T
4
Gnd
6
Sense
5
Sense
+20V to +36V
(+24V)
0.01
µF
N-Channel
Power MOSFET
(IRF540)
R_SENSE
(< 0.08Ω)
10µF
TTL Input
MIC5021
1
V_DD
8
V_BOOST
2
Input
7
Gate
3
C_T
4
Gnd
6
Sense−
5
Sense+
N-Channel
Power MOSFET
0.01
µF
Load
Solenoid
(24V, 47Ω)
Schottky
Diode
(1N5822)
Figure 5. Solenoid Driver
with Current Sensing
Sense Pin Considerations
The sense pins of the MIC5021 are sensitive to negative
Figure 4b. Connecting Sense to Supply
voltages. Forcing the sense pins much below –0.5V effec-
Current sensing may be omitted by connecting the SENSE +
and SENSE – pins to the source of the MOSFET or to the
tively reverses the supply voltage on portions of the driver
resulting in unpredictable operation or damage.
supply. Connecting the SENSE pins to the supply is preferred
MIC5021
for inductive loads. Do not connect the SENSE pins to ground.
1
8
V_DD
5
Inductive Load Precautions
2
Input
7
Gate
MOSFET
3
6
Turnoff
Circuits controlling inductive loads, such as solenoids (Figure
C_T
~VDD
5) and motors, require precautions when controlled by the
4
5
0V
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
Forward drop across diodes
allows leads to go negative.
Current flows from ground (0V)
through the diodes to the load
during negative transcients.
Negative
Spike
Inductive
Load
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
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.
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
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.
capacitor. Also see Sense Pin Considerations below.
October 1998
5-175