Datasheet

5V
6.8 PH x 250 kHz
(1 + 0.1) x (7A + 0.5 x 2.8) +
R
S
=
=
V
OUT
L x f
SW
(1 + margin) x (I
OUT
+ 0.5 x I
PP
) +
V
CS

/A
0.12
#
10 m:
L =
5V
0.4 x 7A x 250 kHz
5V
1 -
36V
x
= 6.2 PH
0
I
PP
T = 1/F
SW
I
O
LM25088
LM25088-Q1
www.ti.com SNVS609H –DECEMBER 2008–REVISED MARCH 2013
Figure 23. Inductor Current
The maximum ripple current occurs at the maximum input voltage. Typically, I
PP
is selected between 20% and
40% of the full load current. Higher ripple current will result in a smaller inductor. However, it places more burden
on the output capacitor to smooth out the ripple current to achieve low output ripple voltage. For this example
40% ripple was chosen for a smaller sized inductor.
(10)
The nearest standard value of 6.8 µH will be used. To prevent saturation, the inductor must be rated for the peak
current. During normal operation, the peak current occurs at maximum load current (plus maximum ripple). With
properly scaled component values, the peak current is limited to V
CS(TH)
/R
S
During overload conditions. At the
maximum input voltage with a shorted output, the chosen inductor must be evaluated at elevated temperature. It
should be noted that the saturation current rating of inductors drops significantly at elevated temperatures.
CURRENT SENSE RESISTOR
The current limit value (I
LIM
) is set by the current sense resistor (R
S
).
R
S
can be calculated by
(11)
Some ‘margin’ beyond the maximum load current is recommended for the current limit threshold. In this design
example, the current limit is set at 10% above the maximum load current, resulting in a R
S
value of 10 mΩ. The
CS and CSG pins should be Kelvin connected to the current sense resistor.
RAMP CAPACITOR
With the inductor and sense resistor value selected, the value of the ramp capacitor (C
RAMP
) necessary for the
emulation ramp circuit is given by:
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