MIC5012
Micrel
ously. The Schottky barrier diode prevents the MIC5012
supply pin from dropping more than 200mV below the drain
supply, and it also improves turn-on time on supplies of less
than 10V. Since the supply current in the “off” state is only
a small leakage, the 100nF bypass capacitor tends to
remain charged for several seconds after the MIC5012 is
turned off. In a PWM application the chip supply is sustained
at a higher potential than the system supply, which im-
proves switching time.
Opto-Isolated Interface
(Figure 4). Although the MIC5012
has no special input slew rate requirement, the lethargic
transitions provided by an opto-isolator may cause oscilla-
tions on the rise and fall of the output. The circuit shown
accelerates the input transitions from a 4N35 opto-isolator
by adding hysteresis. Opto-isolators are used where the
control circuitry cannot share a common ground with the
MIC5012 and high-current power supply, or where the
control circuitry is located remotely. This implementation is
intrinsically safe; if the control line is severed the MIC5012
will turn OFF.
Industrial Switch
(Figure 5). The most common manual
control for industrial loads is a push button on/off switch.
The “on” button is physically arranged in a recess so that in
a panic situation the “off” button, which extends out from the
control box, is more easily pressed. This circuit is compat-
ible with control boxes such as the CR2943 series (GE).
The circuit is configured so that if both switches close
simultaneously, the “off” button has precedence.
This application also illustrates how two (or more) MOSFETs
Applications Information
(Continued)
High-side drivers implemented with MIC501X drivers are
self-protected against inductive switching transients. Dur-
ing turn-off an inductive load will force the MOSFET source
5V or more below ground, while the MIC5012 holds the gate
at ground potential. The MOSFET is forced into conduction,
and it dissipates the energy stored in the load inductance.
The MIC5012 source pin is designed to withstand this
negative excursion without damage. External clamp diodes
are unnecessary.
Low-Side Driver
(Figure 2). A key advantage of the low-
side topology is that the load supply is limited only by the
MOSFET BV
DSS
rating. Clamping may be required to
protect the MOSFET drain terminal from inductive switch-
ing transients. The MIC5012 supply should be limited to
15V in low-side topologies; otherwise, a large current will be
forced through the gate clamp zener. The switching speed
to 10V enhancement is 300µs driving 1nF on a 5V supply.
On a 15V supply the turn-on time is less than 2µs to 10V
Low-side drivers constructed with the MIC501X family are
also fast; the MOSFET gate is driven to near supply
immediately when commanded ON. Typical circuits achieve
10V enhancement in 10µs or less on a 12 to 15V supply.
Modifying Switching Times. Do not add external capacitors
to the MOSFET gate. Add a resistor (1kΩ to 51kΩ) in series
with the gate to slow down the switching time.
Bootstrapped High-Side Driver
(Figure 3). The speed of
a high-side driver can be increased to better than 10µs by
bootstrapping the supply off of the MOSFET source. This
topology can be used where the load is pulse-width modu-
lated (100Hz to 20kHz), or where it is energized continu-
15V
1N5817
1N4001 (2)
1N4148
100nF
1/2 MIC5012
V+
Input
+
1µF
22kΩ
220pF
Source
Gate
IRF541
PWM
INPUT
15V
+
10µF
M
10A Stalled
12V,
1/2 MIC5012
10kΩ
22kΩ
1nF
V+
Input
Source
Gnd
2N3904
Gate
IRF541
Figure 6. Half-Bridge
Motor Driver
5-120
April 1998
MICREL [ MICREL SEMICONDUCTOR ]
