MIC4605 Datasheet, PDF (22/25 Pages) Micrel Semiconductor – 85V Half-Bridge MOSFET Drivers

Micrel, Inc.
Figure 17 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 when 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 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
that 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.
Figure 17. Turn-On Current Paths
MIC4605
Figure 18 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, C_B.
Figure 18. Turn-Off Current Paths
Use the following layout guidelines for optimum circuit
performance:
• Use a ground plane to minimize parasitic inductance
and impedance of the return paths. The MIC4605 is
capable of greater than 1A peak currents and any
impedance between the MIC4605, the decoupling
capacitors, and the external MOSFET will degrade the
performance of the driver.
• A typical layout of a synchronous buck converter power
stage is shown in Figure 19.
The high-side MOSFET drain connects to the input
supply voltage (drain) and the source connects to the
switching node. The low-side MOSFET drain connects to
the switching node and its source is connected to ground.
The buck converter output inductor (not shown) connects
to the switching node. The high-side drive trace, HO, is
routed on top of its return trace, HS, to minimize loop
area and parasitic inductance. The low-side drive trace
LO is routed over the ground plane to minimize the
impedance of that current path. The decoupling
capacitors, C_B and C_VDD, are placed to minimize etch
length between the capacitors and their respective pins.
This close placement is necessary to efficiently charge
capacitor C_B when the HS node is low. All traces are
0.025in wide or greater to reduce impedance. C_IN is used
to decouple the high current path through the MOSFETs.
November 11, 2013
22
Revision 1.0