MIC5011 Datasheet, PDF (7/12 Pages) Micrel Semiconductor – Minimum Parts High- or Low-Side MOSFET Driver

MIC5011
Applications Information (Continued)
Micrel
Construction Hints
High current pulse circuits demand equipment and assem-
bly techniques that are more stringent than normal, low
current lab practices. The following are the sources of
pitfalls most often encountered during prototyping. Sup-
plies: 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 under-
shoot 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—espe-
cially electrolytic capacitors—with possibly catastrophic
results. A 10µF supply 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 un-
der 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 out-
put. 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 sys-
tem 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. Dur-
ing 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,
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 switch-
ing 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
immediately 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.
ON
+
10µF
Control Input
OFF
MIC5011
1 _V+
C1 ⁸
2 _{Input Com} 7
3 _{Source C2} 6
4 _{Gnd Gate} 5
14.4V
1nF
1nF
IRF531
LOAD
Figure 3. High Side Driver with
External Charge Pump Capacitors
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 modu-
lated (100Hz to 20kHz), or where it is energized continu-
ously. 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 im-
proves switching time.
July 2000
7
MIC5011