Datasheet

R
A
=
- 1
V
TO
V
IH_EN
x R
B
EN
V
OUT1
R
A
R
B
LM20242
External
Power Supply
V
OUT2
t
SS
=
0.8V x C
SS
I
SS
LM20242
SNVS534E –OCTOBER 2007–REVISED MARCH 2013 www.ti.com
SETTING THE START UP TIME (C
SS
)
The addition of a capacitor connected from the SS pin to ground sets the time at which the output voltage will
reach the final regulated value. Larger values for C
SS
will result in longer start up times. Table 2 provides a list of
soft start capacitors and the corresponding typical start up times.
Table 2. 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 Equation 12 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 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 LM20242 can be used to address many
sequencing requirements. The turn-on of the LM20242 can be controlled with the precision enable pin by using
two external resistors as shown in Figure 13 .
Figure 13. Sequencing LM20242 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 Equation 13 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 need to be considered to insure proper turn-on of the device.
The LM20242 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.
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