AN4140 datasheet(6/17 Pages) STMICROELECTRONICS

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Component calculation

AN4140

6/17

Doc ID 023412 Rev 1

Component calculation

4.1

CIN selection

It is recommended to use 10 µF as the input capacitor to achieve good stability of the device

and low noise on the VIN track.

4.2

COUT selection

It is recommended to use 2.2 µF as the optimal value of the output capacitor to get the best

compromise between output voltage ripple and load transient response.

4.3

RSENSE value

RSENSE = VREF / ILED

RSENSE sensing resistor

VREF reference voltage = 160 mV typical

ILED total LED output current

PRSENSE = RSENSE * ILED²

PRSENSE power dissipation of the sensing resistor

4.4

Inductor selection

A thin shielded inductor with a low DC series resistance of winding is recommended for this

application. To achieve a good efficiency in step-up mode, it is recommended to use an

inductor with a DC series resistance RDCL = RD / 10 [Ω, Ω, 1], where RD is the dynamic

resistance of the LED.

Equation 1

Table 2.

RSENSE calculated values

ILED

100

150

200

250

300

350

400

RSENSE

1.6

Ω

1.1

Ω

0.8

Ω

0.65

Ω

0.55

Ω

0.45

Ω

0.4

Ω

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