Datasheet

SS/TRK
V
OUT1
R1
R2
EN
LM20333
External
Power Supply
V
OUT2
R
A
=
- 1
V
TO
V
IH_EN
x R
B
EN
V
OUT1
R
A
R
B
LM20333
External
Power Supply
V
OUT2
22222
t
SS
=
0.8V x C
SS
I
SS
LM20333
www.ti.com SNVS558D –MAY 2008–REVISED APRIL 2013
If different start up times are needed the equation shown below can be used to calculate the start up time.
(8)
As shown above, the start up time is influenced by the value of the soft-start capacitor C
SS
and the 4.5 µA soft-
start pin current I
SS
.
While the soft-start capacitor can be sized to meet many start up requirements, there are limitations to its size.
The soft-start time can never be faster than 1 ms due to the internal default 1 ms start up time. When the device
is enabled there is an approximate time interval of 50 µs when the soft-start capacitor will be discharged just
prior to the soft-start ramp. If the enable pin is rapidly pulsed or the soft-start capacitor is large there may not be
enough time for C
SS
to completely discharge resulting in start up times less than predicted. To aid in discharging
of soft-start capacitor during long disable periods an external 1MΩ resistor from SS/TRK to ground can be used
without greatly affecting the start up time.
USING PRECISION ENABLE AND POWER GOOD
The precision enable (EN) and power good (PGOOD) pins of the LM20333 can be used to address many
sequencing requirements. The turn-on of the LM20333 can be controlled with the precision enable pin by using
two external resistors as shown in Figure 29 .
Figure 29. Sequencing LM20333 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 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 LM20333 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 30, the
output of the LM20333 can be configured to track an external voltage source to obtain a simultaneous or
ratiometric start up.
Figure 30. Tracking an External Supply
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