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

·

¨

©

§

¸

¨

LM20124, LM20124-Q1

www.ti.com

SNVS507F – OCTOBER 2007 – REVISED MARCH 2013

Figure 31. 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.

THERMAL CONSIDERATIONS

The thermal characteristics of the LM20124 are specified using the parameter

temperature to the ambient temperature. Although the value of

used to approximate the operating junction temperature of the device.

To obtain an estimate of the device, junction temperature, one may use the following relationship:

(13)

and

(14)

Where:

Copyright © 2007–2013, Texas Instruments Incorporated

Submit Documentation Feedback

19

Product Folder Links: LM20124 LM20124-Q1