MIC5021
Logic-Level MOSFET Precautions
Logic-level MOSFETs have lower maximum gate-to-source
voltage ratings (typically
±10V)
than standard MOSFETs
(typically
±20V).
When an external MOSFET is turned on, the
doubling effect of the boost capacitor can cause the gate-to-
source voltage to momentarily exceed 10V. Internal zener
diodes clamp this voltage to 16V maximum which is too high
for logic-level MOSFETs. To protect logic-level MOSFETs,
connect a zener diode (5V≤V
Zener
<10V) from gate to source.
Overcurrent Limiting
A 50mV comparator is provided for current sensing. The low
level trip point minimizes I
2
R losses when a power resistor is
used for current sensing.
The adjustable retry feature can be used to handle loads with
high initial currents, such as lamps or heating elements, and
can be adjusted from the C
T
connection.
C
T
to ground maintains gate drive shutdown following an
overcurrent condition.
C
T
open, or a capacitor to ground, causes automatic retry.
The default duty cycle (C
T
open) is approximately 20%. Refer
to the electrical characteristics when selecting a capacitor for
reduced duty cycle.
C
T
through a pull-up resistor to V
DD
increases the duty cycle.
Increasing the duty cycle increases the power dissipation in
the load and MOSFET under a “fault” condition.
Circuits may
become unstable at a duty cycle of about 75% or higher,
depending on conditions.
Caution: The MIC5021 may be
damaged if the voltage applied to C
T
exceeds the absolute
maximum voltage rating.
Boost Capacitor Selection
The boost capacitor value will vary depending on the supply
voltage range.
+12V to +20V
Micrel
A 0.01µF boost capacitor is recommended for best perfor-
mance in the 12V to 20V range. Refer to figure 1. Larger
capacitors may damage the MIC5021.
+12V to +36V
MIC5021
10µF
TTL Input
1
2
3
4
V
DD
Input
C
T
Gnd
V
BOOST
Gate
Sense
Sense
8
7
6
5
2.7
nF
Figure 2. 12V to 36V Configuration
If the full 12V to 36V voltage range is required, the boost
capacitor value must be reduced to 2.7nF. Refer to Figure 2.
The recommended configuration for the 20V to 36V range is
to place the capacitor is placed between V
DD
and V
BOOST
as
shown in Figure 3.
+12V to +36V
MIC5021
10µF
TTL Input
1
2
3
4
V
DD
Input
C
T
Gnd
V
BOOST
Gate
Sense
Sense
8
7
6
5
0.1
µF
MIC5021
10µF
TTL Input
1
2
3
4
V
DD
Input
C
T
Gnd
V
BOOST
Gate
Sense
Sense
8
7
6
5
0.01
µF
Figure 3. Preferred 20V to 36V Configuration
Do not use both boost capacitor between V
BOOST
and the
MOSFET source and V
BOOST
and V
DD
at the same time.
Current Sense Resistors
Lead length can be significant when using low value (< 1Ω)
resistors for current sensing. Errors caused by lead length
can be avoided by using four-teminal current sensing resis-
tors. Four-terminal resistors are available from several
manufacturers.
Figure 1. 12V to 20V Configuration
Load
5-174
Load
Load
October 1998
MICREL [ MICREL SEMICONDUCTOR ]
