MIC5021 Просмотр технического описания (PDF) - Micrel
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MIC5021 Datasheet PDF : 10 Pages
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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≤VZener<10V) from
gate to source.
Overcurrent Limiting
A 50mV comparator is provided for current sensing. The low
level trip point minimizes I2R 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 CT connection.
CT to ground maintains gate drive shutdown following an
overcurrent condition.
CT open, or a capacitor to ground, causes automatic retry.
The default duty cycle (CT open) is approximately 20%. Refer
to the electrical characteristics when selecting a capacitor for
reduced duty cycle.
CT through a pull-up resistor to VDD 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 CT 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, Inc.
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
10µF
TTL Input
MIC5021
1
8
V DD
V BOOST
2
Input
7
Gate
3
CT
6
Sense-
2.7
nF
4
5
Gnd Sense+
Load
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 VDD and VBOOST
as shown in Figure 3.
+12V to +36V
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
Load
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
Load
Figure 3. Preferred 20V to 36V Configuration
Do not use both boost capacitor between VBOOST and the
MOSFET source and VBOOST and VDD 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 re-
sistors. Four-terminal resistors are available from several
manufacturers.
Figure 1. 12V to 20V Configuration
MIC5021
6
July 2005
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