MIC5021 Micrel Semiconductor, MIC5021 Datasheet - Page 7

• Manufacturer Part Number: MIC5021
• Description: High-Speed High-Side MOSFET Driver
• Manufacturer: Micrel Semiconductor

Circuits Without Current Sensing
Current sensing may be omitted by connecting the
and
supply. Connecting the
for inductive loads. Do not connect the
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.
October 1998
MIC5021
TTL Input
TTL Input
SENSE
Figure 4b. Connecting Sense to Supply
Figure 4a. Connecting Sense to Source
10µF
10µF
– pins to the source of the MOSFET or to the
1
2
3
4
1
2
3
4
V
Input
C
Gnd
V
Input
C
Gnd
DD
T
DD
T
SENSE
MIC5021
MIC5021
Sense
Sense
Sense
Sense
V
V
BOOST
BOOST
Gate
Gate
pins to the supply is preferred
8
7
6
5
8
7
6
5
SENSE
0.01
µF
0.01
µF
V+
V+
N-Channel
Power MOSFET
Load
N-Channel
Power MOSFET
Load
pins to ground.
SENSE
+
5-175
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.
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.
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.
TTL Input
10µF
Forward drop across diodes
allows leads to go negative.
Current flows from ground (0V)
through the diodes to the load
during negative transcients.
Figure 6. Inductive Load Turnoff
1
2
3
4
Figure 5. Solenoid Driver
V
Input
C
DD
T
with Current Sensing
MIC5021
1
2
3
4
V
Input
C
Gnd
DD
T
Gate
MIC5021
Sense
Sense
V
8
7
6
5
BOOST
Gate
8
7
6
5
(24V, 47 )
Solenoid
MOSFET
Turnoff
+20V to +36V
Inductive
Load
0.01
µF
~V
0V
Negative
Spike
(+24V)
N-Channel
Power MOSFET
(IRF540)
R
(< 0.08 )
DD
SENSE
Schottky
Diode
(1N5822)
Micrel
5