Functional Description

The built in linear regulator of the FC9911 can operate up

to 250V at the VIN pin. The linear regulator provides an

internally regulated voltage of 7.75V (typ) at VDD if the

input voltage is in the range of 9.0 – 250V. This voltage is

used to power the IC and also provide the power to external

circuits connected at the VDD and VREF pins. This linear

regulator can be turned off by overdriving the VDD pin using

an external boostrap circuit at voltages higher than 8.25V

(up to 12V).

In practice, the input voltage range of the IC is limited by

the current drawn by the IC. Thus, it becomes important

to determine the current drawn by the IC to find out the

.

n

i

p

N

I

V

e

h

t

t

a

s

e

g

a

tl

o

v

g

n

it

a

r

e

p

o

m

u

m

i

n

i

m

d

n

a

m

u

m

i

x

a

m

The main component of the current drawn by the IC is the

.

n

i

p

E

T

A

G

e

h

t

t

a

r

e

v

ir

d

T

E

F

g

n

i

h

c

ti

w

s

e

h

t

y

b

n

w

a

r

d

t

n

e

r

r

u

c

To estimate this current, we need to know a few parameters

of the FET being used in the design and the switching

frequency.

The typical waveform of the current being sourced out of

GATE is shown in Fig. 1. Fig. 2 shows the equivalent circuit

of the GATE driver and the external FET. The values of V

DD

and R

Note: The equations given below are approximations and are

to be used only for estimation purposes. The actual values

will differ somewhat from the computed values.

Consider the case when the external FET is FDS3692 and

the switching frequency is f

voltage V

following parameters can be determined:

C

C

C

V

Power Topology

I

PK

I

1

I

avg

t

1

t

2

t

3

0

Fig. 1. Current Sourced Out of GATE at FET Turn-on Driver

Fig. 2. Equivalent Circuit of the GATE Driver

FC9911

C

GD

C

GS

V

DD

R

GATE

FC9911

2012. 10. 31

7/13

Revision No : 0