LTC3736_15 Datasheet, PDF (18/28 Pages) Linear Technology – Dual 2-Phase, No RSENSE, Synchronous Controller with Output Tracking

LTC3736
APPLICATIO S I FOR ATIO
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_OUT
=
0.6V
•
⎛
⎝⎜ ^{1 +}
R_B
R_A
⎞
⎠⎟
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.
V_OUT
1/2 LTC3736
V_FB
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 = 9µA). 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_DELAY
=
0.65V
•
C_SS
0.7µA
=
0.93s/µF
• C_SS
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
18
3.3V OR 5V
RUN/SS
D1
C_SS
Figure 6. RUN/SS Pin Interfacing
RUN/SS
C_SS
3736 F06
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_SS1
=
C_SS
•
600mV
0.7µA
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_{OUT1 =} R2A _• R_TRACKA + R_TRACKB
V_OUT2 R_TRACKA
R2B + R2A
V_OUT1
V_OUT2
R_TRACKB
R_TRACKA
R1B LTC3736
R2B
V_FB1 V_FB2
R1A
R2A
TRACK
3736 F07a
Figure 7a. Using the TRACK Pin
3736fa