Datasheet

IN
BIAS
EN
OUT
PG
SS
TPS74201LDO1
(1)
DSP
IN
I/O
CORE
BIAS
EN
OUT
PG
TRACK
TPS74301LDO2
(1)
R
3
32.4kW
R
4
10kW
R
1
R
2
5V
3.3V
1.2V
NOTES:(1)CapacitorsonIN,BIAS,andOUTalongwiththeresistors
necessarytosettheoutputvoltagehavebeenomittedforsimplification.
(2)LowestvalueforV andhighestvalueforR shouldbeused
inthiscalculation.R mustbethecloseststandardvaluebelowthe
calculatedvalueforproperratiometricsequencing.
CORE 2
1
I/O
T mei
C REO
V
OUT
SIMU TL A E USSEQUENCINGN O
RATIOMETRICSEQUENCING
(2)
C REO
I/O
xR
2
V CC
IO
- 0.808
R
1
=
0.808
xR
2
V CC
CORE
- 0.8
R
1
=
0.8
TPS743xx
SBVS065K –DECEMBER 2005–REVISED AUGUST 2010 www.ti.com
PROGRAMMABLE SEQUENCING WITH The maximum recommended value for R
2
is 100kΩ.
TRACK Once R
2
is selected, R
1
is calculated using one of the
equations given in Figure 32.
The TPS743xx features a track pin that allows the
output to track an external supply at start-up. While
SEQUENCING REQUIREMENTS
the TRACK input is below 0.8V, the error amplifier
regulates the FB pin to the TRACK input. Properly
The device can have V
IN
, V
BIAS
, V
EN
, and V
TRACK
choosing the resistor divider network (R
1
and R
2
) as
sequenced in any order without causing damage to
shown in Figure 32 enables the regulator output to
the device. However, for the track function to work as
track the external supply to obtain a simultaneous or
intended, certain sequencing rules must be applied.
ratiometric start-up. Once the TRACK input reaches
V
BIAS
must be present and the device enabled before
0.8V, the error amplifier regulates the FB pin to the
the track signal starts to ramp. V
IN
should ramp up
0.8V internal reference. Further increases to the
faster than the external supply being tracked so that
TRACK input have no effect.
the tracking signal will not drive the device into V
IN
dropout as V
OUT
ramps up. The preferred method to
sequence the tracking device is to have V
IN
, V
BIAS
,
and V
EN
above the minimum required voltages before
enabling the master supply to initiate the startup
sequence. This method is illustrated in Figure 32.
Resistors R
3
and R
4
disable the master supply until
the input voltage is above 3.52V (typical).
If the TRACK pin is not needed it should be
connected to V
IN
. Configured in this way, the device
starts up typically within 40ms, which may result in
large inrush current that could cause the input supply
to droop. If soft-start is needed, consider the
TPS742xx or TPS744xx devices.
NOTE: When V
BIAS
and V
EN
are present and V
IN
is
not supplied, this device outputs approximately 50mA
of current from OUT. Although this condition will not
cause any damage to the device, the output current
may charge up the OUT node if total resistance
between OUT and GND (including external feedback
resistors) is greater than 10kΩ.
Figure 32. Various Sequencing Methods Using
the TRACK Pin
14 Submit Documentation Feedback Copyright © 2005–2010, Texas Instruments Incorporated