Datasheet

LTC3736-2
18
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APPLICATIONS INFORMATION
Run/Soft-Start Function
The RUN/SS pin is a dual purpose pin that provides the
optional external soft-start function and a means to shut
down the LTC3736-2.
Pulling the RUN/SS pin below 0.65V puts the LTC3736-2
into a low quiescent current shutdown mode (I
Q
= 9μA). If
RUN/SS has been pulled all the way to ground, there will
be a delay before the LTC3736-2 comes out of shutdown
and is given by:
tV
C
A
sFC
DELAY
SS
SS
=
μ
= μ065
07
093.•
.
./•
This pin can be driven directly from logic as shown in
Figure 6. Diode D1 in Figure 6 reduces the start delay
but allows C
SS
to ramp up slowly providing the soft-start
function. This diode (and capacitor) can be deleted if the
external soft-start is not needed.
During soft-start, the start-up of V
OUT1
is controlled by
slowly ramping the positive reference to the error ampli-
fi er from 0V to 0.6V, allowing V
OUT1
to rise smoothly from
0V to its fi nal value. The default internal soft-start time
is 1ms.
Figure 5. Setting Output Voltage
1/2 LTC3736-2
V
FB
V
OUT
R
B
C
FF
R
A
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This can be increased by placing a capacitor between the
RUN/SS pin and SGND. In this case, the soft-start time
will be approximately:
tC
mV
A
SS SS1
600
07
=
μ
•
.
Tracking
The start-up of V
OUT2
is controlled by the voltage on the
TRACK pin. Normally this pin is used to allow the start-up
of V
OUT2
to track that of V
OUT1
as shown qualitatively in
Figures 7a and 7b. When the voltage on the TRACK pin
is less than the internal 0.6V reference, the LTC3736-2
regulates the V
FB2
voltage to the TRACK pin voltage in-
stead of 0.6V. The start-up of V
OUT2
may ratiometrically
track that of V
OUT1
, according to a ratio set by a resistor
divider (Figure 7c):
V
V
RA
R
RR
RB RA
OUT
OUT TRACKA
TRACKA TRACKB1
2
2
22
=
+
+
•
For coincident tracking (V
OUT1
= V
OUT2
during start-up),
R2A = R
TRACKA
R2B = R
TRACKB
The ramp time for V
OUT2
to rise from 0V to its fi nal value
is:
tt
R
RA
RA RB
RR
SS SS
TRACKA
TRACKA TRACKB
21
1
11
=
+
+
••
Figure 6. RUN/SS Pin Interfacing
Figure 7a. Using the TRACK Pin
3.3V OR 5V RUN/SS RUN/SS
C
SS
C
SS
(INTERNAL SOFT-START)
D1
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V
DD
≤ V
IN
RUN/SS
LTC3736-2
V
FB2
V
OUT2
V
OUT1
V
FB1
TRACK
R2B
R2A
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R1B
R1A
R
TRACKA
R
TRACKB