Datasheet

AVIN
nPOR
170 ms
LM26484
www.ti.com SNVS573F –SEPTEMBER 2008–REVISED MARCH 2013
FLEXIBLE POWER SEQUENCING OF MULTIPLE POWER SUPPLIES
The two bucks and the LDO in the LM26484 can be individually controlled with ENSW1, ENSW2, and ENLDO,
respectively. All the enable inputs need to be either grounded or tied to V
IH
.
Power-On Reset
The LM26484 provides an active low reset output nPOR. Typical waveform is as shown in Figure 17 below:
Figure 17. Power-On Reset Waveform
LDO Functional Description
The LDO is a linear regulator which targets analog loads characterized by low noise requirements. The LDO is
enabled through the ENLDO pin. The output voltage is determined by the configuration of the external feedback
resistors, as seen in Typical Application Circuit, R5 and R6.
NO-LOAD STABILITY
The LDO will remain stable and in regulation with no external load. This is an important consideration in some
circuits, for example CMOS RAM keep-alive applications.
Table 2. LDO Configuration and Component Selection Guide
Feedback
Target Ideal Resistor Values Common R Values
Actual V
OUT
with
Capacitor
Com R (V)
V
OUT
(V) R5 (KΩ) R6 (KΩ) R5 (KΩ) R6 (KΩ) C11 (pF)
0.8 120 200 120 200 0.8 15
0.9 160 200 162 200 0.905 15
1 200 200 200 200 1 15
1.1 240 200 240 200 1.1 15
1.2 280 200 280 200 1.2 12
1.3 320 200 324 200 1.31 12
1.4 360 200 357 200 1.393 10
1.5 400 200 402 200 1.505 10
RESISTOR SELECTION FOR LDO
The output voltage of the LDO on the LM26484 is established by the feed back resistor divider R5 and R6 shown
on the typical application circuit (page 1). The equation for determining V
OUT
is: V
OUT
= V
FB
*(R5+R6)/R6, where
V
FB
is the voltage on the LDO_FB pin.
The LDO control loop will force the voltage on V
FB
to be 0.50V.
Table 2 shows ideal resistor values to establish LDO voltages from 0.8V to 1.5V along with common resistor
values to establish these voltages. Common resistors do not always produce the target value. The resulting
output voltage using common resistors is also found in Table 2. To keep the power consumed by the feedback
network low it is recommended that R6 be established as about 200 kΩ. Lesser values of R6 are OK and can be
used at the user’s discretion.
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