LTC3736-1 Datasheet, PDF (18/28 Pages) Linear Technology – Dual 2-Phase, No RSENSE Synchronous Controller with Spread Spectrum

LTC3736-1
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 LTC3736-1 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:
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. 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:
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 7 reduces the start delay but
V_OUT
1/2 LTC3736-1
V_FB
R_B
C_FF
R_A
37361 F06
Figure 6. Setting Output Voltage
18
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:
t_SS1
=
C_SS
•
600mV
0.7µA
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_{OUT1 =} R2A _• R_TRACKA + R_TRACKB
V_OUT2 R_TRACKA
R2B + R2A
For coincident tracking (V_OUT1 = V_OUT2 during start-up),
R2A/R2B = R_TRACKA/R_TRACKB
3.3V OR 5V
RUN/SS
D1
RUN/SS
C_SS
Figure 7. RUN/SS Pin Interfacing
C_SS
37361 F07
37361f