Datasheet

t
OFF
= -
R1 x R3
x C21 x ln
1 -
1.23 x (R1 + R3)
V
IN
x R1
R1 + R3
R3 = 1.23 x
V
IN(MIN)
+ 5 PA x R1 - 1.23
R1
- 1
R9
R8
1.23V
V
OUT
=
t
SS
=
10 PA
C16 x 1.23V
LM5118, LM5118-Q1
www.ti.com SNVS566G –APRIL 2008–REVISED FEBRUARY 2013
C16 = C
SS
The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage and the output
voltage, to reach the final regulated value. The time is determined from:
(22)
and assumes a current limit>Iload + ICout
For this application, a C16 value of 0.1 µF was chosen which corresponds to a soft-start time of about 12 ms.
R8, R9
R8 and R9 set the output voltage level, the ratio of these resistors is calculated from:
(23)
For a 12V output, the R8/R9 ratio calculates to 9.76. The resistors should be chosen from standard value
resistors and a good starting point is to select resistors within power ratings appropriate for the output voltage.
Values of 309Ω for R9 and 2.67 kΩ for R8 were selected.
R1, R3, C21
A voltage divider can be connected to the UVLO pin to set a minimum operating voltage VIN
(UVLO)
for the
regulator. If this feature is required, the easiest approach to select the divider resistor values is to choose a value
for R1 between 10 kΩ and 100 kΩ, while observing the minimum value of R1 necessary to allow the UVLO
switch to pull the UVLO pin low. This value is:
R1 ≥ 1000 x V
IN(MAX)
R1 ≥ 75k in our example
R3 is then calculated from
(24)
Since VIN
(MIN)
for our example is 5V, set VIN
(UVLO)
to 4.0V for some margin in component tolerances and input
ripple.
R1 = 75k is chosen since it is a standard value
R3 = 29.332k is calculated from the equation above. 29.4k was used since it is a standard value.
Capacitor C21 provides filtering for the divider and the off time of the “hiccup” duty cycle during current limit. The
voltage at the UVLO pin should never exceed 15V when using an external set-point divider. It may be necessary
to clamp the UVLO pin at high input voltages.
Knowing the desired off time during “hiccup” current limit, the value of C21 is given by:
(25)
Notice that t
OFF
varies with V
IN
In this example, C21 was chosen to be 0.1 µF. This will set the t
OFF
time to 956 µs with VIN = 12V.
R2
A 1M pull-up resistor connected from the EN pin to the VIN pin is sufficient to keep enable in a high state if on-off
control is not used.
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