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Our LCD has 10 series white LEDs for backlight with total Vf of 15V and current of 40ma. The backlight circuit is designed with SC4509. As per the datasheet its recommended output voltage is 20V and output current of 30ma. This translates to 20V * 30ma = 600mW. Our design also 15V * 40ma = 600mW. So is it correct assumption that the IC can be used in this design since the total power remain with in the limit? or the output current has to be less than 30ma irrespective of the output voltage? The max switch current limit of SC4509 is 500ma.

-bj

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The SC4509 feedback pin requires 0.2 volts across it to set the LED current. The current that flows thru the LEDs also flow thru a resistor (R1) that sets the 0.2 volts depending on the current flow.

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You say your design needs 40mA and this is well within the limits for the device so, alter the resistor value to set 40mA instead of 30mA.

So is it correct assumption that the IC can be used in this design since the total power remain with in the limit?

No, this is not correct. If R1 has a value that produces 30mA then it will produce 30mA and not 40mA. It's a constant current device not a constant power device.

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  • \$\begingroup\$ Hi Andy, Thanks. The datasheet says operating condition of max LED current of 30ma with max output voltage of 20V. It also says switch current limit of 500ma. I can set R1 for 40ma but is it safe to use for 40ma with 15V? \$\endgroup\$ – Bhav J Nov 27 '14 at 14:45
  • \$\begingroup\$ LEDs get destroyed with over current and if they are specified at 40mA (not absolute max) then you should be OK. The output voltage of the SC4509 adjusts itself to suit the LEDs current. \$\endgroup\$ – Andy aka Nov 27 '14 at 15:09
  • \$\begingroup\$ sorry for not putting the question clearly..LEDs max current is 50ma which I want to operate at 40ma only. This is fine. But SC4509 datasheet specifies max current of 30ma with switch current limit of 500ma. I want to set R1 for 40ma. Is it OK for SC4509 to operate at? \$\endgroup\$ – Bhav J Nov 27 '14 at 15:22

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