AN2259 datasheet(7/38 Pages) STMICROELECTRONICS

7 / 38 page

AN2259

2 INPUT CAPACITOR SELECTION

7/38

INPUT CAPACITOR SELECTION

For DC input, the input capacitor, C1, is selected based on its ripple current rating for the

capacitor. The ripple current is calculated based on its duty cycle as outlined below.

Where D = duty cycle

Vo = output voltage

Vin = input voltage

The RMS current through the capacitor therefore is:

µ= efficiency

For an AC input voltage, the input capacitor is selected primarily to have enough capacity to

supply the LED between the peaks of the AC input. The capacitor must be selected so that the

minimum voltage at the input to the L5973D is maintained during each half cycle of the AC

input.

If the application is driving only one LED, the Vmin is determined by the minimum operating

voltage specification for the L5973D (4.4V). When driving more than one LED in series, the

minimum input voltage is determined by the output voltage and the minimum differential input to

output voltage for the regulator (the drop out voltage). In this case Vmin = (x * Vf) + (Iout * Rsense)

+ VDO

Where: x = number of LED in series

Vf = forward voltage of one LED

Io = LED drive current

VDO = Drop out voltage

The capacitor can then be selected using the equation:

The ripple current rating will have two parts where in the low frequency range, the capacitor will

be charged by 120Hz while at the high frequency range the capacitor is discharged by 250kHz.

For the low frequency part, it is approximately the same as the input RMS current and the

power factor is approximately 0.7 for a full wave rectifier.

Vin

Iripple

⋅

−

Vin

Vpeak

⋅

)

min

Vpeak

(

⋅

−

⋅

−

AN2259 датащи(PDF) 7 Page - STMicroelectronics

AN2259 датащи(HTML) 7 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

датащи скачать

ссылки URL