LM20123 datasheet(18/29 Pages) TI

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SS/TRK

VOUT1

LM20123

External

Power Supply

VOUT2

RA =

- 1

VTO

VIH_EN

x RB

VOUT1

LM20123

External

Power Supply

VOUT2

LM20123

SNVS524E – OCTOBER 2007 – REVISED MARCH 2013

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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 CSS to completely discharge resulting in start up times less than predicted. To aid in discharging

the Soft-Start capacitor during long disable periods an external 1 M

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 LM20123 can be used to address many

sequencing requirements. The turn-on of the LM20123 can be controlled with the precision enable pin by using

two external resistors as shown in Figure 29.

Figure 29. Sequencing LM20123 with Precision Enable

The value for resistor RB can be selected by the user to control the current through the divider. Typically this

resistor will be selected to be between 10 k

Ω and 1 MΩ. Once the value for RB is chosen the resistor RA can be

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

(12)

When designing for a specific turn-on threshold (VTO) the tolerance on the input supply, enable threshold

(VIH_EN), and external resistors needs to be considered to insure proper turn-on of the device.

The LM20123 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 (RPG) to pull PGOOD high while when the output is

within 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 LM20123 can be configured to track an external voltage source to obtain a simultaneous or

ratiometric start up.

Figure 30. Tracking an External Supply

Since the Soft-Start charging current ISS is always present on the SS/TRK pin, the size of R2 should be less than

10 k

Ω 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 31, to give the desired start up. Figure 31 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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