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
·
¨
©
§
¸
¨
LM20146
SNVS563C –FEBRUARY 2008–REVISED APRIL 2013
www.ti.com
Since the Soft-Start charging current I
SS
is always present on the SS/TRK pin, the size of R2 should be less than
10 kΩ 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 35, to give the desired start up. Figure 35 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 35. 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 LM20146 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.
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