Datasheet

VIN - 1.25
1.25V x R
UV2
R
UV1
=
20 PA
V
HYS
R
UV2
=
V
OUT
COMP
FB
LM5119
+
-
R
FB2
R
FB1
R
COMP
C
COMP
C
HF
0.8V
R
FB2
- 1
R
FB1
V
OUT
0.8V
=
1.25V
10 PA x t
RES
C
RES
=
0.8V
t
SS
x 10 PA
C
SS
=
LM5119/LM5119Q
www.ti.com SNVS676F –AUGUST 2010–REVISED FEBRUARY 2013
SOFT START CAPACITOR
The capacitor at the SS pin (C
SS
) determines the soft-start time (t
SS
), which is the time for the output voltage to
reach the final regulated value. The value of C
SS
for a given time is determined from:
(31)
For this application, a value of 0.047μF was chosen for a soft-start time of 3.8ms.
RESTART CAPACITOR
The restart pin sources 10µA into the external restart capacitor (C
RES
). The value of the restart capacitor is given
by:
(32)
Where t
RES
is the time the LM5119 remains off before a restart attempt in hiccup mode current limiting. For this
application, a value of 0.47µF was chosen for a restart time of 59ms.
OUTPUT VOLTAGE DIVIDER
R
FB1
and R
FB2
set the output voltage level, the ratio of these resistors is calculated from:
(33)
1.33kΩ was chosen for R
FB1
in this design which results in a R
FB2
value of 6.98kΩ for V
OUT2
of 5V. A reasonable
guide is to select the value of R
FB1
in the range between 500Ω and 10kΩ. The value of R
FB1
should be large
enough to keep the total divider power dissipation small.
Figure 8. Feedback Configuration
UVLO DIVIDER
The UVLO threshold is internally set to 1.25V at the UVLO pin. The LM5119 is enabled when the system input
voltage VIN causes the UVLO pin to exceed the threshold voltage of 1.25V. When the UVLO pin voltage is below
the threshold, the internal 20μA current source is disabled. When the UVLO pin voltage exceeds the 1.25V
threshold, the 20μA current source is enabled causing the UVLO pin voltage to increase, providing hysteresis.
The values of R
UV1
and R
UV2
can be determined from the following equation:
(34)
(35)
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