TIME

TIME

SIMULTANEOUS START UP

RATIOMETRIC START UP

=

1

R

VOUT2

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

R1

R2

EN

LM20145

External

Power Supply

LM20145

www.ti.com

SNVS530E – OCTOBER 2007 – REVISED MARCH 2013

Figure 31. Tracking an External Supply

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 32, to give the desired start up. Figure 32 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 32. Common Start Up Sequences

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

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.

Copyright © 2007–2013, Texas Instruments Incorporated

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