18
LTC3736
3736fa
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 amplifier
from 0V to 0.6V, allowing V
OUT1
to rise smoothly from 0V
to its final value. The default internal soft-start time is 1ms.
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:
t
C
mV
A
SS
SS
1
600
0 7
=
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 regu-
lates the V
FB2
voltage to the TRACK pin voltage instead 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
R A
R
R
R
R B R A
OUT
OUT
TRACKA
TRACKA
TRACKB
1
2
2
2
2
=
+
+
•
APPLICATIO S I FOR ATIO
W
U
U
U
3.3V OR 5V
RUN/SS
RUN/SS
C
SS
C
SS
D1
3736 F06
Figure 6. RUN/SS Pin Interfacing
where f is the operating frequency, C
OUT
is the output
capacitance and I
RIPPLE
is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since I
RIPPLE
increases with input voltage.
Setting Output Voltage
The LTC3736 output voltages are each set by an external
feedback resistor divider carefully placed across the out-
put, as shown in Figure 5. The regulated output voltage is
determined by:
V
V
R
R
OUT
B
A
=
+
⎛
⎝
⎜
⎞
⎠
⎟
0 6
1
.
•
To improve the frequency response, a feed-forward ca-
pacitor, C
FF
, may be used. Great care should be taken to
route the V
FB
line away from noise sources, such as the
inductor or the SW line.
1/2 LTC3736
V
FB
V
OUT
R
B
C
FF
R
A
3736 F05
Figure 5. Setting Output Voltage
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.
Pulling the RUN/SS pin below 0.65V puts the LTC3736
into a low quiescent current shutdown mode (I
Q
= 9
µ
A). If
RUN/SS has been pulled all the way to ground, there will
be a delay before the LTC3736 comes out of shutdown and
is given by:
t
V
C
A
s F C
DELAY
SS
SS
=
µ
=
µ
0 65
0 7
0 93
.
•
.
.
/
•
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
LTC3736
V
FB2
V
OUT2
V
OUT1
V
FB1
TRACK
R2B
R2A
3736 F07a
R1B
R1A
R
TRACKA
R
TRACKB
Figure 7a. Using the TRACK Pin
