TLE6389_07 Datasheet, PDF (32/38 Pages) Infineon Technologies AG – Step-Down DC/DC Controller

TLE 6389
To achieve the same effect of slope compensation in the adjustable voltage version also
the inductance in µH is given by
^2,0
×
10^–4
⋅
V---H--Ω^---
⋅
V_OUT
⋅
R_S

E N S E_
<
L1
<
 ^4,0
×
10^–4
⋅
V---H--Ω^---
⋅
V
OUT
⋅
R_S
EN

SE_
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.
∆I
=
(_{---V----I--N-----}–_{-----V----O----U----T---})_----⋅_{---V----O----U----T-}
f_SW ⋅ V_IN ⋅ L1
In this equation f_sw 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 V_LIM and the sense resistor R_SENSE. With a maximum peak
current given by the application (I_{PEAK, PWM}=I_LOAD+0.5∆I) the sense resistor is calculated
to
R_SENSE = _2-----â‹---I--_P--V-_E---_A-^L---^IK--^M-_,---_P---_W-----_M--
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 R_SENSE that the load current doesn’t
reach the foldback part of the characteristic curve.
Datasheet Rev. 2.1
32
2007-08-13