mic4102 Micrel Semiconductor, mic4102 Datasheet - Page 13

• Manufacturer Part Number: mic4102
• Description: 100v Half Bridge Mosfet Driver With Anti-shoot Through Protection
• Manufacturer: Micrel Semiconductor

The power dissipated by the MIC4102 due to supply
current is
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 .
The die temperature may be calculated once the total
power dissipation is known.
Anti Shoot-Through, Propagation Delay and other
Timing Considerations
The block diagram in Figure 1 illustrates how the
MIC4102 drives the power stage of a synchronous buck
converter.
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
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
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.
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
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disadvantage is the driver cannot monitor the gate
voltage inside the MOSFET.
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.
MOSFET data sheet.
external damping resistor is used.
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
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
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Figure 8. Gate Drive Circuit with Parasitics
This turn-off delay is usually specified in the
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Rg
This delay increases when an
Rg_fet
Rg_fet
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Cgd
Cgs
Cgs
Figure 8 shows an
Vin
M9999-112806
MIC4102
HS FET
LS FET
Switching
Node
LOOFF