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Micrel, Inc.
The power dissipated by the MIC4102 due to supply
MIC4102
disadvantage is the driver cannot monitor the gate
current is
voltage inside the MOSFET.
Figure 8 shows an
Pdiss sup ply = Vdd × Idd + Vhb × Ihb
Total power dissipation and Thermal Considerations
Total power dissipation in the MIC41032 equals the
power dissipation caused by driving the external
MOSFETs, the supply current and the internal bootstrap
diode .
Pdiss total = Pdiss sup ply + Pdiss drive + Pdiode total
equivalent circuit, including parasitics, of the gate driver
section. The internal gate resistance (Rg_gate) and any
external damping resistor (Rg) isolate the MOSFET’s
gate from the driver output. There is a delay between
when the driver output goes low and the MOSFET turns
off. This turn-off delay is usually specified in the
MOSFET data sheet. This delay increases when an
external damping resistor is used.
The die temperature may be calculated once the total
power dissipation is known.
Vdd
Ron
HB
Vin
T J = T A + Pdiss total × θ JA
where :
T A is the maximum ambient temperatur e
T J is the junction temperatur e ( ° C)
Pdiss total is the power dissipatio n of the MIC4102
Roff
HO
Rg
HS
Cgd
Rg_fet
Cgs
HS FET
Switching
Node
θ JC is the thermal resistance from junction to ambient air ( ° C/W)
Ron
Cgd
Anti Shoot-Through, Propagation Delay and other
LO
Rg_fet
LS FET
Timing Considerations
The block diagram in Figure 1 illustrates how the
MIC4102 drives the power stage of a synchronous buck
converter. It is important that only one of the two
MOSFETs is on at any given time. If both MOSFETs are
simultaneously on they will short Vin to ground, causing
high current from the Vin supply to “shoot through” the
MOSFETs into ground. Excessive shoot-through causes
Roff
Vss
Cgs
higher power dissipation in the MOSFETs, voltage
spikes and ringing in the circuit. The high current and
voltage ringing generate conducted and radiated EMI.
Minimizing shoot-through can be done passively,
actively or though a combination of both. Passive shoot-
through protection uses delays between the high and
low gate drivers to prevent both MOSFETs from being
on at the same time. These delays can be adjusted for
different applications. Although simple, the
disadvantage of this approach is the long delays
required to account for process and temperature
variations in the MOSFET and MOSFET driver.
Active shoot-though monitors voltages on the gate drive
outputs and switch node to determine when to switch
the MOSFETs on and off. This active approach adjusts
the delays to account for some of the variations, but it
too has its disadvantages. High currents and fast
switching voltages in the gate drive and return paths can
cause parasitic ringing that may turn the MOSFETs back
on even though the gate driver output is low. Another
Figure 8. Gate Drive Circuit with Parasitics
The MIC4102 uses a combination of active sensing and
passive delay to insure that both MOSFETs are not on at
the same time and to minimize shoot-through current.
The timing diagram helps illustrate how the anti-shoot-
through circuitry works. A high level on the PWM pin
causes the LO pin to go low. The MIC4102 monitors the
LO pin voltage and prevents the HO pin from turning on
until the voltage on the LO pin reaches the V LOOFF
threshold. After a short delay, the MIC4102 drives the
HO pin high. Monitoring the LO voltage eliminates any
excessive delay due to the MOSFET drivers turn-off time
and the short delay accounts for the MOSFET turn-off
delay as well as letting the LO pin voltage settle out. An
external resistor between the LO output and the
MOSFET may affect the performance of the LO pin
monitoring circuit and is not recommended.
A low on the PWM pin causes the HO pin to go low after
a short delay (T HOOFF ). Before the LO pin can go high,
November 2006
13
M9999-112806
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