MIC5021_05 Datasheet, PDF (7/10 Pages) Micrel Semiconductor – High-Speed High-Side MOSFET Driver

MIC5021
Circuits Without Current Sensing
V+
10µF
TTL Input
MIC5021
1
8
V _DD
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, Inc.
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.
+20V to +36V
(+24V)
10µF
TTL Input
MIC5021
1
8
V _DD
V _BOOST
2
Input
7
Gate
3
CT
4
Gnd
6
Sense-
5
Sense+
0.01
µF
N-Channel
Power MOSFET
(IRF540)
R_{S E N S E}
(< 0.08Ω)
10µF
TTL Input
MIC5021
1
8
V _DD
V _BOOST
2
Input
7
Gate
3
C_T
4
Gnd
6
Sense-
5
Sense+
N-Channel
Power MOSFET
0.01
µF
Load
Figure 4b. Connecting Sense to Supply
Current sensing may be omitted by connecting the SENSE +
and SENSE – pins to the source of the MOSFET or to the sup-
ply. Connecting the SENSE pins to the supply is preferred for
inductive loads. Do not connect the SENSE pins to ground.
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 connect-
ing 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.
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 volt-
ages. Forcing the sense pins much below –0.5V effectively
reverses the supply voltage on portions of the driver resulting
in unpredictable operation or damage.
MIC5021
1
8
V_DD
2
7
Input
Gate
3
6
C_T
4
5
MOSFET
Turnoff
~VDD
0V
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
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.
July 2005
7
MIC5021