July 2005 7 MIC5011
MIC5011 Micrel, Inc.
and it dissipates the energy stored in the load inductance.
The MIC5011 source pin (3) 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 BVDSS rating. Clamping may be required to
protect the MOSFET drain terminal from inductive switching
transients. The MIC5011 supply should be limited to 15V in
low-side topologies, otherwise a large current will be forced
through the gate clamp zener.
Low-side drivers constructed with the MIC501X family are
also fast; the MOSFET gate is driven to near supply imme-
diately when commanded ON. Typical circuits achieve 10V
enhancement in 10µs or less on a 12 to 15V supply.
Modifying Switching Times (Figure 3). High-side switch-
ing times can be improved by a factor of 2 or more by
adding external charge pump capacitors of 1nF each. In
cost-sensitive applications, omit C1 (C2 has a dominant
effect on speed).
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 4). 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
modulated (100Hz to 20kHz), or where it is energized con-
tinuously. The Schottky barrier diode prevents the MIC5011
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 MIC5011 is turned
off. In a PWM application the chip supply is sustained at
a higher potential than the system supply, which improves
switching time.
Applications Information (Continued)
Input
Source
Gate
1
2
3
4
8
MIC5011
7
6
5
V+
Gnd
C1
Com
C2
+
10µF
Control Input
IRF531
14.4V
ON
O F F
LOAD
1n
F
1nF
Construction Hints
High current pulse circuits demand equipment and assembly
techniques that are more stringent than normal, low current
lab practices. The following are the sources of pitfalls most
often encountered during prototyping. Supplies: many bench
power supplies have poor transient response. Circuits that
are being pulse tested, or those that operate by pulse-width
modulation will produce strange results when used with a
supply that has poor ripple rejection, or a peaked transient
response. Always monitor the power supply voltage that
appears at the drain of a high-side driver (or the supply
side of the load in a low-side driver) with an oscilloscope.
It is not uncommon to find bench power supplies in the
1 kW class that overshoot or undershoot by as much as
50% when pulse loaded. Not only will the load current and
voltage measurements be affected, but it is possible to
over-stress various components—especially electrolytic
capacitors—with possibly catastrophic results. A 10µF sup-
ply bypass capacitor at the chip is recommended.
Residual Resistances: Resistances in circuit connections
may also cause confusing results. For example, a circuit
may employ a 50mΩ power MOSFET for low drop, but
careless construction techniques could easily add 50 to
100mΩ resistance. Do not use a socket for the MOSFET. If
the MOSFET is a TO-220 type package, make high-current
drain connections to the tab. Wiring losses have a profound
effect on high-current circuits. A floating millivoltmeter can
identify connections that are contributing excess drop
under load.
Circuit Topologies
The MIC5011 is suited for use with standard MOSFETs in
high- or low-side driver applications. In addition, the MIC5011
works well in applications where, for faster switching times,
the supply is bootstrapped from the MOSFET source
output. Low voltage, high-side drivers (such as shown in
Figure 1) are the slowest; their speed is reflected in the
gate turn-on time specifications. The fastest drivers are
the low-side and bootstrapped high-side types (Figures 2
and 4). Load current switching times are often much faster
than the time to full gate enhancement, depending on the
circuit type, the MOSFET, and the load. Turn-off times are
essentially the same for all circuits (less than 10µs to V
GS
= 1V). The choice of one topology over another is based on
a combination of considerations including speed, voltage,
and desired system characteristics.
High-Side Driver (Figure 1). The high-side topology works
well down to V
+
= 7V with standard MOSFETs. From 4.75 to
7V supply, a logic-level MOSFET can be substituted since
the MIC5011 will not reach 10V gate enhancement (10V is
the maximum rating for logic-compatible MOSFETs).
High-side drivers implemented with MIC501X drivers are
self-protected against inductive switching transients. During
turn-off an inductive load will force the MOSFET source 5V
or more below ground, while the MIC5011 holds the gate at
ground potential. The MOSFET is forced into conduction,
Figure 3. High Side Driver with
External Charge Pump Capacitors
