Datasheet

18
LTC3736-1
37361f
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 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:
tC
mV
A
SS SS1
600
07
=
µ
•
.
It is recommended that C
SS
have a value of at least twice
that of the frequency filtering capacitor connected to the
FREQ pin when spread sprectrum operation is enabled
(see Operation Frequency section).
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 8a and 8b. When the voltage on the TRACK pin is
less than the internal 0.6V reference, the LTC3736-1
regulates 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 8c):
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/R2B = R
TRACKA
/R
TRACKB
APPLICATIO S I FOR ATIO
WUUU
3.3V OR 5V RUN/SS RUN/SS
C
SS
C
SS
D1
37361 F07
Figure 7. 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 LTC3736-1 output voltages are each set by an external
feedback resistor divider carefully placed across the out-
put, as shown in Figure 6. The regulated output voltage is
determined by:
VV
R
R
OUT
B
A
=+
⎛
⎝
⎜
⎞
⎠
⎟
06 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. When spread spectrum operation
is enabled, it is recommended that R
A
and R
B
be large-
valued, preferably on the order of hundreds of kilohms.
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 LTC3736-1.
Pulling the RUN/SS pin below 0.65V puts the LTC3736-1
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 LTC3736-1 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 7 reduces the start delay but
1/2 LTC3736-1
V
FB
V
OUT
R
B
C
FF
R
A
37361 F06
Figure 6. Setting Output Voltage