Datasheet

V
OUT1
V
OUT2
V
EN
VOLTAGE
TIME
VOLTAGE
TIME
SIMULTANEOUS START UP
RATIOMETRIC START UP
=1R
V
OUT1
V
OUT2
V
EN
OUT12OUT
Vx8.0<V
( )
x
-1
=1R
2R
V
1OUT
2R
x
-1
V
2OUT
V8.0
¸
¹
·
¨
©
§
¸
¨
LM20123
www.ti.com SNVS524E –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
systems the higher voltage, V
OUT1
, usually powers the I/O, and the lower voltage, V
OUT2
, 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.
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