LM20133MHXSLASHNOPB datasheet(19/29 Pages) TI

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SS/TRK

VOUT1

LM20133

External

Power Supply

VOUT2

RA =

- 1

VTO

VIH_EN

x RB

VOUT1

LM20133

External

Power Supply

VOUT2

LM20133, LM20133Q

www.ti.com

SNVS526F – OCTOBER 2007 – REVISED MARCH 2013

Figure 34. Sequencing LM20133 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 LM20133 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 35, the

output of the LM20133 can be configured to track an external voltage source to obtain a simultaneous or

ratiometric start up.

Figure 35. 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 36, to give the desired start up. Figure 36 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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