LTC3736-2 Linear Technology, LTC3736-2 Datasheet - Page 18

• Manufacturer Part Number: LTC3736-2
• Description: Synchronous Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3736-2
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
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:
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
function. This diode (and capacitor) can be deleted if the
external soft-start is not needed.
During soft-start, the start-up of V
slowly ramping the positive reference to the error amplifier
from 0V to 0.6V, allowing V
to its final value. The default internal soft-start time is 1ms.
18
t
3.3V OR 5V
DELAY
SS
=
to ramp up slowly providing the soft-start
0 65
D1
Figure 6. RUN/SS Pin Interfacing
.
Figure 5. Setting Output Voltage
1/2 LTC3736-2
RUN/SS
V
V
U
DD
•
0 7
≤ V
C
C
.
37362 F06
V
SS
IN
FB
SS
µ
U
A
OUT1
=
V
0 93
OUT
RUN/SS
.
to rise smoothly from 0V
R
R
37362 F05
B
A
W
(INTERNAL SOFT-START)
s F C
OUT1
/
µ
C
FF
•
is controlled by
SS
Q
U
RUN/SS
= 9µA). If
C
SS
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:
Tracking
The start-up of V
TRACK pin. Normally this pin is used to allow the start-up
of V
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
instead of 0.6V. The start-up of V
track that of V
divider (Figure 7c):
For coincident tracking (V
The ramp time for V
is:
R2A = R
R2B = R
t
t
V
V
SS
SS
OUT2
OUT
OUT
1
2
=
=
2
1
to track that of V
C
t
=
TRACKB
TRACKA
SS
SS
R
R
TRACKB
TRACKA
R
1
Figure 7a. Using the TRACK Pin
OUT1
TRACKA
•
R A
•
600
FB2
R
0 7
2
OUT2
.
TRACKA
, according to a ratio set by a resistor
R A
V
mV
µ
OUT2
1
OUT1
voltage to the TRACK pin voltage
A
is controlled by the voltage on the
•
R1B
R1A
R
to rise from 0V to its final value
TRACKA
OUT1
•
OUT1
V
TRACK
R
LTC3736-2
FB1
R B R A
TRACKA
2
= V
as shown qualitatively in
OUT2
V
R A R B
www.DataSheet4U.com
+
+
FB2
OUT2
1
R
37362 F07a
2
TRACKB
+
+
may ratiometrically
V
OUT2
R
during start-up),
R2B
R2A
1
TRACKB
37362fa