MIC4223_11 Datasheet, PDF (12/20 Pages) MIC GROUP RECTIFIERS – Dual 4A, 4.5V to 18V, 15ns Switch Time, Low-Side MOSFET Drivers with Enable

Micrel, Inc.
MIC4223/MIC4224/MIC4225
Bypass Capacitor Selection
Bypass capacitors are required for proper operation by
supplying the charge necessary to drive the external
MOSFETs as well as minimize the voltage ripple on the
supply pins.
Ceramic capacitors are recommended because of their
low impedance and small size. Z5U type ceramic capacitor
dielectrics are not recommended due to the large change
in capacitance over temperature and voltage.
Manufacturer specifications should be checked to insure
voltage and temperature do not reduce the capacitance
below the value needed. A minimum value of 1µF is
required regardless of the MOSFETs being driven. Larger
MOSFETs, with their higher input capacitance may require
larger decoupling capacitance values for proper operation.
The voltage rating of the capacitors depends on the supply
voltage, ambient temperature and the voltage derating
used for reliability.
Placement of the decoupling capacitors is critical. The
bypass capacitor for V_DD should be placed as close as
possible between the VDD and GND pins. The etch
connections must be short, wide and direct. The use of a
ground plane to minimize connection impedance is
recommended. Multiple vias insure a low inductance path
and help with power dissipation. Refer to the section on
layout and component placement for more information.
Grounding, Component Placement and Circuit Layout
Nanosecond switching speeds and ampere peak currents
in and around the MOSFET driver necessitate proper
placement and trace routing of all components. Improper
placement may cause degraded noise immunity, false
switching and excessive ringing.
Figure 6 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. Current in the
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.
Figure 6. Driver Turn-On Current Path
Figure 7 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 from the V_DD supply replenishes charge in the
decoupling capacitor, C_VDD.
Figure 7. Driver Turn-Off Current Path
The following circuit guidelines should be adhered to for
optimum circuit performance:
The V_DD bypass capacitor must be placed close to the
VDD and ground pins. It is critical that the etch length
between the decoupling capacitor and the VDD and GND
pins be minimized to reduce pin inductance. Multiple vias
in parallel help minimize inductance in the ground and V_DD
paths.
A ground plane is recommended to minimize parasitic
inductance and impedance of the return paths. The
MIC4223 family of drivers is capable of high peak currents
and very fast transition times. Any impedance between
the driver, the decoupling capacitors and the external
MOSFET will degrade the performance of the circuit.
Trace out the high di/dt and dv/dt paths, as shown in
Figures 6 and 7 and minimize etch 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.
June 2009
12
M9999-061109-A
[phone]