LTC3736 LINER [Linear Technology], LTC3736 Datasheet - Page 18

• Manufacturer Part Number: LTC3736
• Description: Dual 2-Phase, No RSENSE Synchronous Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
where f is the operating frequency, C
capacitance and I
tor. The output ripple is highest at maximum input voltage
since I
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:
To improve the frequency response, a feed-forward ca-
pacitor, C
route the V
inductor or the SW line.
LTC3736
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 0.7V puts the LTC3736 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 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
18
V
t
DELAY
OUT
RIPPLE
SS
FF
0 6
FB
, may be used. Great care should be taken to
.
to ramp up slowly providing the soft-start
0 7
increases with input voltage.
line away from noise sources, such as the
.
V
Figure 5. Setting Output Voltage
1/2 LTC3736
V
•
RIPPLE
•
1
U
0 7
C
.
V
R
SS
R
FB
is the ripple current in the induc-
B
A
A
U
1
V
s F C
OUT
/
R
R
3736 F05
B
A
W
•
SS
OUT
C
FF
is the output
Q
U
= 9 A). If
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.
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 regu-
lates the V
0.6V. The start-up of V
of V
(Figure 7c):
t
3.3V OR 5V
V
V
SS
OUT2
OUT1
OUT
OUT
1
1
2
, according to a ratio set by a resistor divider
to track that of V
C
FB2
SS
R
R
R
TRACKB
TRACKA
D1
voltage to the TRACK pin voltage instead of
Figure 6. RUN/SS Pin Interfacing
•
TRACKA
Figure 7a. Using the TRACK Pin
R A
600
2
0 7
OUT2
RUN/SS
.
mV
V
A
OUT1
C
is controlled by the voltage on the
•
OUT2
SS
R1B
R1A
R
TRACKA
OUT1
OUT1
may ratiometrically track that
V
TRACK
R B R A
FB1
LTC3736
2
as shown qualitatively in
to rise smoothly from 0V
V
FB2
OUT1
R
2
TRACKB
3736 F07a
V
OUT2
is controlled by
R2B
R2A
RUN/SS
3736 F06
C
SS
3736f