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R
KFF
+
ǒ
V
UVLO(on)
* 0.5
Ǔ
ǒ
0.018 ) ǒ
5
R
T
Ǔ
Ǔ
(34)
t
START
w 2p L C
O
Ǹ
(35)
C
SS
w
12 10
*6
0.7
t
START
(36)
I
SCP
w
C
O
V
OUT
t
START
) I
LOAD
DI
2
(37)
R
ILIM
w
I
SCP
R
DS(on)MAX
) V
ILIM(min)
I
SINK(max)
(38)
I
SCP(min)
+
I
SINK(min)
R
ILIM(min)
* V
ILIM(max)
R
DS(on)MIN
(39)
I
SCP(max)
+
I
SINK(max)
R
ILIM(max)
* V
ILIM(min)
R
DS(on)MAX
(40)
C
ILIM(max)
+
V
O
0.2
V
IN
R
ILIM
f
SW
(41)
TPS40075
SLUS676A – MAY 2006 – REVISED SEPTEMBER 2007
This equation gives a R
KFF
value of 136 k Ω . The closest lower standard value should be selected. For this design
and using E24 resistor range 133 k Ω was chosen. This yields a typical start voltage of 8.52 V.
3.2.3 Soft Start Capacitor
It is good practice to limit the rise time of the output voltage. This helps prevent output overshoot and possible
damage to the load. The selection of the soft start time is arbitrary, but it must meet one condition; it should be
greater than the time constant of the output filter, L and C
O
. This time is given by Equation 35
The soft-start time must be greater than 0.281 ms. A time of 1 ms was chosen, this time also helps keep the
initial input current during start up low. The value of C
SS
can be calculated using Equation 36 .
C
SS
should be greater than 17 nF, a 22 nF MLCC was chosen. The calculated start time using this capacitor is
1.28 ms.
3.2.4 Short Circuit Protection, R
ILIM
and C
ILIM
Short circuit protection is programmed using the R
ILIM
resistor. Selection of this resistor depends on the R
DS(on)
of
the switching MOSFET selected and the required short circuit current trip point, I
SCP
. The minimum I
SCP
is limited
by the inductor peak current, the output voltage, the output capacitor and the soft start time. Their relationship is
given by Equation 37 . A short circuit current trip point greater than that calculated by this equation should be
used.
The minimum short circuit current trip point for this design is 16.35 A. This value is used in Equation 38 to
calculate the minimum R
ILIM
value.
R
ILIM
is calculated to be 1.14 k Ω . The closest standard value greater than 1.14 k Ω is chose, this is 1.15 k Ω . To
verify that the short circuit current requirements are met the minimum and maximum short circuit current can be
calculated using Equation 39 and Equation 40 .
The minimum I
SCP
is 17.09 A and the maximum is 29.45 A.
It is recommended to add a small capacitor, C
ILIM
, across R
ILIM
. The value of this capacitor should be less than
that calculated in Equation 41 .
This equation yields a maximum C
ILIM
of 44 pF. A value half this is chosen, 22 pF.
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