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
·
¨
©
§
¸
¨
SS/TRK
V
OUT1
R1
R2
EN
LM20242
External
Power Supply
V
OUT2
LM20242
www.ti.com SNVS534E –OCTOBER 2007–REVISED MARCH 2013
TRACKING AN EXTERNAL SUPPLY
By using a properly chosen resistor divider network connected to the SS/TRK pin, as shown in Figure 14, the
output of the LM20242 can be configured to track an external voltage source to obtain a simultaneous or
ratiometric start up.
Figure 14. Tracking an External Supply
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 15, to give the desired start up. Figure 15 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 15. 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.
Copyright © 2007–2013, Texas Instruments Incorporated Submit Documentation Feedback 19
Product Folder Links: LM20242