Datasheet

SS/TRK
V
OUT1
R1
R2
EN
LM20133
External
Power Supply
V
OUT2
R
A
=
- 1
V
TO
V
IH_EN
x R
B
EN
V
OUT1
R
A
R
B
LM20133
External
Power Supply
V
OUT2
LM20133, LM20133Q
www.ti.com SNVS526F –OCTOBER 2007–REVISED MARCH 2013
Figure 34. Sequencing LM20133 with Precision Enable
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 10 kΩ and 1 MΩ. 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.
(12)
When designing for a specific turn-on threshold (V
TO
) the tolerance on the input supply, enable threshold
(V
IH_EN
), and external resistors needs to be considered to insure proper turn-on of the device.
The LM20133 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 while 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 35, the
output of the LM20133 can be configured to track an external voltage source to obtain a simultaneous or
ratiometric start up.
Figure 35. 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 36, to give the desired start up. Figure 36 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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