The increase in the short circuit response time can be

computed using the various component values of the boost

converter. Consider a boost converter with a nominal output

current I

LED string voltage V

2.0mF. The disconnect FET is a TN2510N8 from Supertex

which has a saturation current I

increase in the short circuit response time due to the RC

filter can then be computed as:

This increase is found to be negligible (note that the equation

is valid for ΔT << R

the condition holds.

Sizing the Output Sense Resistor

To avoid exceeding the peak-current rating of the output

sense resistor during short circuit conditions, the power

rating of the resistor has to be chosen properly.

In this case, the maximum power dissipated in the sense

resistor is:

P

2

From the datasheet for a 1.24Ω, 1/4W resistor, the maximum

.

W

1

1

s

i

t

n

e

r

r

u

c

f

o

e

s

l

u

p

s

m

1

e

l

g

n

i

s

a

r

o

f

e

t

a

p

i

s

s

i

d

n

a

c

ti

r

e

w

o

p

Since the total short circuit time is about 350ns (including the

300ns time for turn off), the resistor should be able to handle

the current.

∆t ≈ R

I

=1kΩ • 470pF • In 1-

1 -

0.35A

≈ 66ns

I

O

3A - 0.35A

R

OVP2

C

O

R

S

Q2

FAULT

FDBK

C

LED

V

O

V

D

i

SENSE

R

OVP2

C

O

R

S

Q2

FAULT

FDBK

C

LED

V

O

V

D

i

SENSE

C

f

Fig. 4. Output of the Boost Converter Show-

ing LED Parsed Capacitance

Fig. 5. Adding a Low-pass Filter to Prevent

Pulse Triggering

FC9911

2012. 10. 31

13/13

Revision No : 0