MIC4100 Datasheet, PDF (15/18 Pages) Micrel Semiconductor – 100V Half Bridge MOSFET Drivers

Micrel, Inc.
MIC4100/1
capacitors are recommended for most applications. The
minimum capacitance value should be increased if low
voltage capacitors are use since even good quality
dielectric capacitors, such as X5R, will lose 40% to 70% of
their capacitance value at the rated voltage.
Placement of the decoupling capacitors is critical. The
bypass capacitor for Vdd should be placed as close as
possible between the Vdd and Vss pins. The bypass
capacitor (C_B) for the HB supply pin must be located as
close as possible between the HB and HS pins. The etch
connections must be short, wide and direct. The use of a
ground plane to minimize connection impedance is
recommended. Refer to the section on layout and
component placement for more information.
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 etch 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.
The voltage on the bootstrap capacitor drops each time it
delivers charge to turn on the MOSFET. The voltage drop
depends on the gate charge required by the MOSFET.
Most MOSFET specifications specify gate charge vs. Vgs
voltage. Based on this information and a recommended
∆V_HB of less than 0.1V, the minimum value of bootstrap
capacitance is calculated as:
C_B
≥
Q_gate
∆V_HB
where : Q_gate = Total Gate Charge at V_HB
∆V_HB = Voltage drop at the HB pin
The decoupling capacitor for the Vdd input may be
calculated in with the same formula; however, the two
capacitors are usually equal in value.
Grounding, Component Placement and Circuit Layout
Nanosecond switching speeds and ampere peak currents
in and around the MIC4100 and MIC4101 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.
Low-side drive turn-on
current path
Vdd
gnd
C_Vdd
plane
HB
LO
Vss
HO
LI
C_B
HS
Level
shift
HI
High-side drive turn-on
current path
Turn-On Current Paths
Figure 9
gnd
plane
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.
Current flowing through the internal diode replenishes
charge in the bootstrap capacitor, CB.
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
March 2006
15
M9999-031506