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
·
¨
©
§
¸
¨
LM20134, LM20134Q
www.ti.com
SNVS527G –OCTOBER 2007–REVISED MARCH 2013
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, 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.
THERMAL CONSIDERATIONS
The thermal characteristics of the LM20134 are specified using the parameter θ
JA
, which relates the junction
temperature to the ambient temperature. Although the value of θ
JA
is dependant on many variables, it still can be
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:
T
J
= P
D
θ
JA
+ T
A
(13)
and
P
D
= P
IN
x (1 - Efficiency) - 1.1 x I
OUT
2 x DCR
where
• T
J
is the junction temperature in °C
• P
IN
is the input power in Watts (P
IN
= V
IN
x I
IN
)
• θ
JA
is the junction to ambient thermal resistance for the LM20134
• T
A
is the ambient temperature in °C
• I
OUT
is the output load current
• DCR is the inductor series resistance (14)
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