TLE6389_07 datasheet(32/38 Pages) INFINEON

32 / 38 page

TLE 6389

Datasheet Rev. 2.1

2007-08-13

To achieve the same effect of slope compensation in the adjustable voltage version also

the inductance in µH is given by

The inductance value determines together with the input voltage, the output voltage and

the switching frequency the current ripple which occurs during normal operation of the

step down converter. This current ripple is important for the all over ripple at the output

of the switching converter.

In this equation fsw is the actual switching frequency of the device, given either by the

internal oscillator or by an external source connected to the SYNC pin. When picking

finally the inductance of a certain supplier (Epcos, Coilcraft etc.) the saturation current

has to be considered. The saturation current value of the desired inductance has to be

higher than the maximum peak current which can appear in the actual application.

7.10.2

Determining the current limit

The peak current which the buck converter is able to provide is determined by the peak

current limit threshold voltage VLIM and the sense resistor RSENSE. With a maximum peak

current given by the application (IPEAK, PWM=ILOAD+0.5∆I) the sense resistor is calculated

The equation above takes account for the foldback characteristic of the current limit as

shown in the Fig. ’Output Voltage vs. Load Current’ on page 24/25 by introducing a factor

of 2. It must be assured by correct dimensioning of RSENSE that the load current doesn’t

reach the foldback part of the characteristic curve.

2,0 10

–

Ω

--------- V

OUT RSENSE

⋅⋅

⋅



 L1 4,0 10 4

–

Ω

--------- V

⋅

OUT

RSENSE

⋅⋅





∆I

VIN VOUT

–

() VOUT

⋅

fSW VIN L1

⋅⋅

------------------------------------------------------

RSENSE

VLIM

⋅ PEAK PWM

-------------------------------------

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