STPS1L60 datasheet(11/34 Pages) STMICROELECTRONICS

11 / 34 page

AN3151

Overview of inverse buck converter

Doc ID 17051 Rev 1

11/34

The output voltage =VLED = N* VF(Typical)=40 V

Where N=Number of LEDs per string=10 and Vf is the typical forward voltage (4 V).

Forward voltage has some variation in respect to the typical value and the LED

manufacturer reports also the minimum forward voltage Vfmin and maximum forward voltage

Vfmax.

These values must be considered for dimensioning of the power converter.

The LED characteristics are strongly dependent on manufacturer and are reported in their

corresponding datasheets.

In particular, these are necessary to calculate the duty cycle variation range.

Design spec calculation:

Equation 2

Equation 3

δ=Ton/Ts and expressed in terms of voltage for a buck converter:

Equation 4

Equation 5

Equation 6

The current ripple admitted for this application is less than 10 % depending on the following

formula:

Equation 7

ILEDAVG = Average LED current

ΔI

LED

Ripple current on the LEDs

Table 1.

Electrical specs (continued)

Description

Symbol

Value

Unit

VOUTmin= VLEDmin=N*Vfmin=10*3.85 V=38.5 V

VOUTmax=VLEDmax=N*Vfmax=10*4.15 V=41.5 V

δ=Vout/Vin

δmin= V

OUTmin/Vin=80.2 %

δmax= V

OUTmax/Vin=86.4 %

⎪

⎩

⎪

⎨

⎧

⎟

⎠

⎞

⎜

⎝

⎛ −

≤

Vin

Vout

min

LED

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