Datasheet
ENABLE
2.0M
21 PA
INT-VCC (5V)
1.279V
RUN
RENB
12.7k
RENT
42.2k
VIN
100:
RENH
5.1V
LMZ23603
SNVS711I –MARCH 2011–REVISED OCTOBER 2013
www.ti.com
Figure 51. Enable Input Detail
OUTPUT VOLTAGE SELECTION
Output voltage is determined by a divider of two resistors connected between V
O
and ground. The midpoint of
the divider is connected to the FB input.
The regulated output voltage determined by the external divider resistors R
FBT
and R
FBB
is:
V
O
= 0.796V * (1 + R
FBT
/ R
FBB
) (4)
Rearranging terms; the ratio of the feedback resistors for a desired output voltage is:
R
FBT
/ R
FBB
= (V
O
/ 0.796V) - 1 (5)
These resistors should generally be chosen from values in the range of 1.0 kΩ to 10.0 kΩ.
For V
O
= 0.8V the FB pin can be connected to the output directly and R
FBB
can be set to 8.06kΩ to provide
minimum output load.
A table of values for R
FBT
, and R
FBB
, is included in the Simplified Applications Schematic.
SOFT-START CAPACITOR SELECTION
Programmable soft-start permits the regulator to slowly ramp to its steady state operating point after being
enabled, thereby reducing current inrush from the input supply and slowing the output voltage rise-time.
Upon turn-on, after all UVLO conditions have been passed, an internal 1.6mSec circuit slowly ramps the SS/TRK
input to implement internal soft start. If 1.6mSec is an adequate turn–on time then the Css capacitor can be left
unpopulated. Longer soft-start periods are achieved by adding an external capacitor to this input.
Soft start duration is given by the formula:
t
SS
= V
REF
* C
SS
/ Iss = 0.796V * C
SS
/ 50uA (6)
This equation can be rearranged as follows:
C
SS
= t
SS
* 50μA / 0.796V (7)
Using a 0.22μF capacitor results in 3.5 msec typical soft-start duration; and 0.47μF results in 7.5 msec typical.
0.47 μF is a recommended initial value.
As the soft-start input exceeds 0.796V the output of the power stage will be in regulation and the 50 μA current is
deactivated. Note that the following conditions will reset the soft-start capacitor by discharging the SS input to
ground with an internal current sink.
• The Enable input being pulled low
• Thermal shutdown condition
• Internal Vcc UVLO (Approx 4.3V input to V
IN
)
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