SS/TRK

R1

R2

EN

LM20333

External

Power Supply

- 1

EN

LM20333

External

Power Supply

LM20333

SNVS558C – MAY 2008 – REVISED JANUARY 2009

www.ti.com

If different start up times are needed the equation shown below can be used to calculate the start up time.

(8)

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

of soft-start capacitor during long disable periods an external 1M

Ω 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

resistor will be selected to be between 10 k

be solved using the equation below to set the desired turn-on voltage.

(9)

The LM20333 features an open drain power good (PGOOD) pin to sequence external supplies or loads and to

the PGOOD tolerance window. Typical values for this resistor range from 10 k

Ω 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

18

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