Datasheet

SS/TRK
V
OUT1
R1
R2
EN
LM20323
External
Power Supply
V
OUT2
R
A
=
- 1
V
TO
V
IH_EN
x R
B
LM20323
www.ti.com SNVS557C –MAY 2008–REVISED APRIL 2013
The value for resistor R
B
can be selected by the user to control the current through the divider. Typically this
resistor will be selected to be between 1 kΩ and 49.9 kΩ. Once the value for R
B
is chosen the resistor R
A
can be
solved using the equation below to set the desired turn-on voltage.
(9)
When designing for a specific turn-on threshold (V
TO
) the tolerance on the input supply, enable threshold
(V
IH_EN
), and external resistors need to be considered to ensure proper turn-on of the device.
The LM20323 features an open drain power good (PGOOD) pin to sequence external supplies or loads and to
provide fault detection. This pin requires an external resistor (R
PG
) to pull PGOOD high when the output is within
the PGOOD tolerance window. Typical values for this resistor range from 10 kΩ to 100 kΩ.
TRACKING AN EXTERNAL SUPPLY
By using a properly chosen resistor divider network connected to the SS/TRK pin, as shown in Figure 28, the
output of the LM20323 can be configured to track an external voltage source to obtain a simultaneous or
ratiometric start up.
Figure 28. Tracking an External Supply
Since the soft-start charging current I
SS
is always present on the SS/TRK pin, the size of R2 should be less than
10 kΩ to minimize the errors in the tracking output. Once a value for R2 is selected the value for R1 can be
calculated using appropriate equation in Figure 29, to give the desired start up. Figure 28 shows two common
start up sequences; the top waveform shows a simultaneous start up while the waveform at the bottom illustrates
a ratiometric start up.
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