# LED Backlight driver output current / voltage

Started by November 26, 2014
```Our LCD has 10 series white LEDs for backlight with total Vf of 15V and cur=
rent of 40ma. The backlight circuit is designed with SC4509. As per the dat=
asheet its recommended output voltage is 20V and  output current of 30ma. T=
his translates to 20V * 30ma =3D 600mW. Our design also 15V * 40ma =3D 600m=
W.  So is it correct assumption that the IC can be used in this design sinc=
e 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 =20
```
```On 11/26/2014 6:59 AM, bhav.jnk@gmail.com wrote:
> 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.

According to the data sheet, you can get no more than 30mA. If your Vf
is 15V, then you cannot get more than 450mW output power.
```
```On Wednesday, November 26, 2014 7:18:59 PM UTC+5:30, John S wrote:
> On 11/26/2014 6:59 AM, bhav.jnk@gmail.com wrote:
> > 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.
>
> According to the data sheet, you can get no more than 30mA. If your Vf
> is 15V, then you cannot get more than 450mW output power.

Thanks but the switch max current limit 500ma. So can't we get 30ma + 10ma more ?

-bj
```
```On 11/26/2014 12:35 PM, bhav.jnk@gmail.com wrote:
> On Wednesday, November 26, 2014 7:18:59 PM UTC+5:30, John S wrote:
>> On 11/26/2014 6:59 AM, bhav.jnk@gmail.com wrote:
>>> 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.
>>
>> According to the data sheet, you can get no more than 30mA. If your Vf
>> is 15V, then you cannot get more than 450mW output power.
>
> Thanks but the switch max current limit 500ma. So can't we get 30ma + 10ma more ?
>
> -bj
>

The data sheet says 30mA max LED current. Go ahead and try it.
```
```Am Mittwoch, 26. November 2014 14:00:02 UTC+1 schrieb bhav...@gmail.com:
> Our LCD has 10 series white LEDs for backlight with total Vf of 15V and c=
urrent of 40ma. The backlight circuit is designed with SC4509. As per the d=
atasheet its recommended output voltage is 20V and  output current of 30ma.=
This translates to 20V * 30ma =3D 600mW. Our design also 15V * 40ma =3D 60=
0mW.  So is it correct assumption that the IC can be used in this design si=
nce 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 curre=
nt limit of SC4509 is 500ma.
>=20
>=20
>=20
> -bj

Sometimes is the thermal power dissipation from the internal switch resista=
nce is the limiting factor.=20
In this case the output voltage is not important, only the current counts. =
If you take the bigger package (MLPD-W8) with better thermal disspation and=
a good layout, you would have a better chance.

The datasheet in not clear about the limitations, if it is internally limit=
```On 11/26/2014 4:59 AM, bhav.jnk@gmail.com wrote: