Micrel, Inc.

MIC4100/1

March 2006

14

M9999-031506

The power dissipated inside the MIC4100/1 is equal to the

circuit

drive

gate

the

of

frequency

switching

the

is

fs

MOSFET

on the

voltage

source

to

gate

the

is

Vgs

Vgs

at

charge

gate

total

the

is

Qg

off

and

on

MOSFET

the

switching

by

dissipated

power

the

is

P

cycle

switching

per

dissipated

energy

the

is

E

Qg

Qg

E

driver

driver

dirver

where

fs

V

P

and

V

gs

driver

gs

×

×

=

×

=

ratio of Ron & Roff to the external resistive losses in Rg

and Rg_fet. Letting Ron =Roff, the power dissipated in the

MIC4100 due to driving the external MOSFET is:

fet

Rg

Rg

Ron

Ron

P

Pdiss

driver

drive

_

+

+

=

Supply Current Power Dissipation

s

ipated by the MIC4100 due to supply

Total power dissipation and Thermal Considerations

s

The die temperature may be calculated once the total

Power is dissipated in the MIC4100 even if is there i

nothing being driven. The supply current is drawn by the

bias for the internal circuitry, the level shifting circuitry and

shoot-through current in the output drivers. The supply

current is proportional to operating frequency and the Vdd

and Vhb voltages. The typical characteristic graphs show

how supply current varies with switching frequency and

supply voltage.

The power diss

current is

Ihb

Vhb

Idd

Vdd

Pdiss

ply

×

+

×

=

sup

Total power dissipation in the MIC4100 or MIC4101 i

equal to the power dissipation caused by driving the

external MOSFETs, the supply current and the internal

bootstrap diode.

total

drive

ply

total

Pdiode

Pdiss

Pdiss

Pdiss

+

+

=

sup

power dissipation is known.

JA

total

A

J

Pdiss

T

T

θ

×

+

=

C/W)

(

air

ambient

o

junction t

from

resistance

thermal

the

is

θ

MIC4100/1

the

of

n

dissipatio

power

the

is

Pdiss

C)

(

emperature

junction t

the

is

T

mperature

ambient te

maximum

the

is

T

:

JC

total

J

A

°

°

where

Propagation Delay and Delay Matching and other

Timing Considerations

Propagation delay and signal timing is an important

t only to minimize propagation

time between the control

e or a

t is less than the minimum pulse width may

ed for both the low side

(Vdd) and high side (HB) supply pins. These capacitors

external

consideration in a high performance power supply. The

MIC4100 is designed no

delay but to minimize the mismatch in delay between the

high-side and low-side drivers.

Fast propagation delay between the input and output drive

waveform is desirable. It improves overcurrent protection

by decreasing the response

signal

and

the

MOSFET

gate

drive.

Minimizing

propagation delay also minimizes phase shift errors in

power supplies with wide bandwidth control loops.

Many power supply topologies use two switching

MOSFETs operating 180º out of phase from each other.

These MOSFETs must not be on at the same tim

short circuit will occur, causing high peak currents and

higher power dissipation in the MOSFETs. The MIC4100

and MIC4101 output gate drivers are not designed with

anti-shoot-through protection circuitry. The output drives

signals simply follow the inputs. The power supply design

must include timing delays (dead-time) between the input

signals to prevent shoot-through. The MIC4100 &

MIC4101 drivers specify delay matching between the two

drivers to help improve power supply performance by

reducing the amount of dead-time required between the

input signals.

Care must be taken to insure the input signal pulse width

is greater than the minimum specified pulse width. An

input signal tha

result in no output pulse or an output pulse whose width is

significantly less than the input.

The maximum duty cycle (ratio of high side on-time to

switching period) is controlled by the minimum pulse width

of the low side and by the t

capacitor to charge during the off-time. Adequate time

the high-side driver is turned on.

Decoupling and Bootstrap Capacitor Selection

Decoupling capacitors are requir

supply the charge necessary to drive the

MOSFETs as well as minimize the voltage ripple on these

pins. The capacitor from HB to HS serves double duty by

providing decoupling for the high-side circuitry as well as

providing current to the high-side circuit while the high-side

external MOSFET is on. 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. A minimum value of 0.1uf is

required for each of the capacitors, regardless of the

MOSFETs being driven. Larger MOSFETs may require

larger 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. 25V rated X5R or X7R ceramic