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SC4505MLTRT

High Efficiency Boost Converter for Backlight/Flash LED Driver

Semtech Corporation 

POWER MANAGEMENT

Applications Information

VIN

L1

D1

VOUT

COUT

SC4505

R3

R1

VO

R4

I2

COMP

R2

I1

VOVP

Figure 5. OVP Tweaking

As shown in Figure 5, R1 and R2 are the internal resistor

divider. R3 and R4 are external resistor divider for OVP

triggering point adjustment. It is noted that the resistance

of the internal resistor divider formed by R and R is

1

2

around 500kΩ. When OVP happens, a strong pulldown

will be applied to VO pin to quickly discharge the output

capacitor. Any large value of R3 will slow down the

discharge process at OVP condition. Generally speaking,

small value of R3 is preferred in applications.

However, small value of the resistor divider formed by

R and R would cause additional power loss. People

3

4

often use R4=100kΩ and then select R3 according to

the OVP specification. In this case, the OVP triggering

threshold estimation equation can be rewritten as

follows:

VOVP

=

R4//500KΩ + R3

R 4 //500KΩ

× 28V

Set

R4 = 100KΩ

We can get

R3

=





VOVP

28V

−1 × 83.3KΩ



The tolerance of the R3 and R4 should also be considered

in determining the OVP triggering point. Usually there is

about 5%~8% difference between the calculated value

and the measure OVP triggering threshold.

 2007 Semtech Corp.

11

SC4505

www.semtech.com

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