SC560CULTRT Datasheet, PDF (10/15 Pages) Semtech Corporation – Dual Output Low Noise LDO Linear Regulator

SC560
Applications Information
General Description
The SC560 is a family of dual output linear regulator devices
intended for applications where low dropout voltage, low
supply current, and low output noise are critical. Each
device provides a very simple, low cost solution for two
separate regulated outputs. Very little PCB area is required
due to the miniature package size and the need for only
four external capacitors.
The linear regulators LDOA and LDOB are powered from
a single input supply rail, and each provides 300mA of
output current. The SC560 can provide output voltages
in the range 1.2V to 5.0V. The output voltages for the
SC560A, SC560B and SC560C are set by connecting
external resistor dividers to the feedback pins of each
LDO. All other versions of the SC560 have fixed output
voltage values shown in the Pinout and Voltage Options
table on page 2.
Power On Control
The SC560A and SC560C devices have a single enable pin
(EN) that controls both LDO outputs. Pulling this pin low
causes the device to enter a low power shutdown mode
where it typically draws 100nA from the input supply.
When EN transitions high, the output of LDOA is enabled.
After a delay of 128μs, the output of LDOB is enabled. In
the SC560C, when the output voltage of LDOA reaches
87% of its regulation point, the delay timer starts and the
PGOOD signal transitions high after a delay of 200ms. The
power up/down sequence is shown in the timing diagram
in Figure 1.
EN
OUTA
PGOOD
OUTB
87%
200ms
87%
128 μs
Figure 1 — Timing Diagram
The SC560B and the fixed output variants provide a
separate enable pin for LDOB which allows LDOA and
LDOB to be enabled independently. The EN pin controls
the LDOA output and the ENB provides the same
functionality relative to the LDOB output. The table
shown below lists the effect of the polarity of the EN and
ENB signals on the outputs of LDOA and LDOB. Note from
the table that LDOB can only be enabled when LDOA is
already active. Since LDOB can be enabled separately,
there is no timing relationship between the two outputs
at startup.
EN
ENB
LDOA
LDOB
Low
Low
Off
Off
Low
High
Off
Off
High
Low
On
Off
High
High
On
On
The SC560C and the fixed output variants have a PGOOD
signal which monitors the output of LDOA and transitions
high 200ms after LDOA has reached 87% of its regulation
point. This can be used to hold a processor in reset when
the output voltage is out of regulation. Note that when
LDOA drops out of regulation and PGOOD is forced low,
LDOB is also disabled until PGOOD is reset.
Output Voltage Selection
The output voltage of each LDO for the SC560A, SC560B,
and SC560C version is set independently using external
resistor dividers. Figure 2 illustrates the proper connection
for LDOA.
OUTA
FBA
R₁
R₂
Figure 2 — Output Voltage Feedback Circuit
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