VOUT1

VOUT2

VEN

TIME

TIME

SIMULTANEOUS START UP

RATIOMETRIC START UP

=

1

R

VOUT1

VOUT2

VEN

OUT1

2

OUT

V

x

8

.

0

<

V

(

) x

-1

=

1

R

2

R

V

1

OUT

2

R

x

-1

V

2

OUT

V

8

.

0

¸

¹

·

¨

©

§

¸

¨

SS/TRK

VOUT1

R1

R2

EN

LM20144

External

Power Supply

VOUT2

LM20144

SNVS529G – OCTOBER 2007 – REVISED MARCH 2013

www.ti.com

Figure 33. 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 34, to give the desired start up. Figure 34 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.

Figure 34. Common Start Up Sequences

A simultaneous start up is preferred when powering most FPGAs, DSPs, or other microprocessors. In these

systems the higher voltage, VOUT1, usually powers the I/O, and the lower voltage, VOUT2, powers the core. A

simultaneous start up provides a more robust power up for these applications since it avoids turning on any

parasitic conduction paths that may exist between the core and the I/O pins of the processor.

The second most common power on behavior is known as a ratiometric start up. This start up is preferred in

applications where both supplies need to be at the final value at the same time.

Similar to the Soft-Start function, the fastest start up possible is 1ms regardless of the rise time of the tracking

voltage. When using the track feature the final voltage seen by the SS/TRACK pin should exceed 1V to provide

sufficient overdrive and transient immunity.

20

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