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Micrel
Grounding, Component Placement, and Circuit Layout
Nanosecond switching speeds and ampere peak currents
in and around the MIC4103 and MIC4104 drivers require
proper placement and trace routing of all components.
Improper placement may cause degraded noise immunity,
false switching, excessive ringing or circuit latch-up.
Figure 9 shows the critical current paths when the driver
outputs go high and turn on the external MOSFETs. It also
helps demonstrate the need for a low impedance ground
plane. Charge needed to turn-on the MOSFET gates
comes from the decoupling capacitors C VDD and C B .
Current in the low-side gate driver flows from C VDD through
the internal driver, into the MOSFET gate and out the
Source. The return connection back to the decoupling
capacitor is made through the ground plane. Any
inductance or resistance in the ground return path causes
a voltage spike or ringing to appear on the source of the
MOSFET. This voltage works against the gate drive
voltage and can either slow down or turn off the MOSFET
during the period where it should be turned on.
Current in the high-side driver is sourced from capacitor C B
and flows into the HB pin and out the HO pin, into the gate
of the high side MOSFET. The return path for the current
is from the source of the MOSFET and back to capacitor
C B . The high-side circuit return path usually does not have
a low impedance ground plane so the trace connections in
this critical path should be short and wide to minimize
parasitic inductance. As with the low-side circuit,
impedance between the MOSFET source and the
decoupling capacitor causes negative voltage feedback
which fights the turn-on of the MOSFET.
It is important to note that capacitor CB must be placed
close to the HB and HS pins. This capacitor not only
provides all the energy for turn-on but it must also keep HB
pin noise and ripple low for proper operation of the high-
side drive circuitry.
Low -side drive turn-on
current path
MIC4103/4104
Figure 10 shows the critical current paths when the driver
outputs go low and turn off the external MOSFETs. Short,
low impedance connections are important during turn-off
for the same reasons given in the turn-on explanation.
Remember that during turn-off current flowing through the
internal diode replenishes charge in the bootstrap
capacitor, CB.
Figure 10. Turn-Off Current Paths
The following circuit guidelines should be adhered to for
optimum circuit performance:
1. The V DD and HB bypass capacitors must be placed
close to the supply and ground pins. It is critical that
the trace length between the high side decoupling
capacitor (C B ) and the HB & HS pins be minimized to
reduce trace inductance.
2. A ground plane should be used to minimize parasitic
inductance and impedance of the return paths. The
MIC4103 is capable of greater than 2A peak currents
and any impedance between the MIC4103, the
decoupling capacitors, and the external MOSFET will
degrade the performance of the driver.
3. Trace out the high di/dt and dv/dt paths, as shown in
Vdd
LO
Figures 9 and 10 and minimize trace length and loop
area for these connections. Minimizing these
gnd
plane
C Vdd
HB
Vss
parameters decreases the parasitic inductance and
the radiated EMI generated by fast rise and fall times.
gnd
HO
LI
plane
C B
HS
Level
shift
HI
High-side drive turn-on
current path
Figure 9. Turn-On Current Paths
November 2010
16
M9999-110910-B
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