Datasheet

R
A
=
- 1
V
TO
V
IH_EN
x R
B
EN
V
OUT1
R
A
R
B
LM20145
External
Power Supply
V
OUT2
t
SS
=
0.8V x C
SS
I
SS
LM20145
SNVS530E –OCTOBER 2007–REVISED MARCH 2013 www.ti.com
Table 3. Start Up Times for Different Soft-Start Capacitors
Start Up Time (ms) C
SS
(nF)
1 none
5 33
10 68
15 100
20 120
If different start up times are needed the equation shown below can be used to calculate the start up time.
(12)
As shown above, the start up time is influenced by the value of the Soft-Start capacitor C
SS
(F) and the 5 µA Soft-
Start pin current I
SS
(A). that may be found in the electrical characteristics table.
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 1ms 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 1 MΩ 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 LM20145 can be used to address many
sequencing requirements. The turn-on of the LM20145 can be controlled with the precision enable pin by using
two external resistors as shown in Figure 30.
Figure 30. Sequencing LM20145 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.
(13)
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 LM20145 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 31, the
output of the LM20145 can be configured to track an external voltage source to obtain a simultaneous or
ratiometric start up.
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