MIC4103 Datasheet, PDF (14/17 Pages) Micrel Semiconductor – 100V Half Bridge MOSFET Drivers 3/2A Sinking/Sourcing Current

Micrel
MIC4103/4104
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
Vdd
gnd
C_Vdd
plane
HB
HO
C_B
HS
High-side drive turn-on
current path
LO
Vss
LI
Level
shift
HI
gnd
plane
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 Figures 9 and 10 and minimize trace length and
loop area for these connections. Minimizing these
parameters decreases the parasitic inductance
and the radiated EMI generated by fast rise and
fall times.
Figure 9. Turn-On Current Paths
Figure 10 shows the critical current paths when the driver
outputs go low and turn off the external MOSFETs. Short,
October 2007
14
M9999-100107-B