MIC4102 datasheet(13/17 Pages) MICREL

13 / 17 page

Micrel, Inc.

MIC4102

November 2006

M9999-112806

The power dissipated by the MIC4102 due to supply

current is

Ihb

Vhb

Idd

Vdd

Pdiss

ply

sup

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 .

total

drive

ply

total

Pdiode

Pdiss

sup

The die temperature may be calculated once the total

power dissipation is known.

total

Pdiss

C/W)

(

air

ambient

junction

from

resistance

thermal

the

MIC4102

the

dissipatio

power

the

Pdiss

(

temperatur

junction

the

temperatur

ambient

maximum

the

total

where

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.

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

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

disadvantage is the driver cannot monitor the gate

voltage inside the MOSFET.

Figure 8 shows an

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.

HS FET

Vdd

Rg_fet

Ron

Roff

Cgd

Cgs

Rg_fet

Cgd

Cgs

Vin

LS FET

Ron

Roff

Vss

Switching

Node

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 VLOOFF

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 (THOOFF). Before the LO pin can go high,

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