LTC3836_15 Datasheet, PDF (17/30 Pages) Linear Technology – Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller

LTC3836
APPLICATIONS INFORMATION
A small (0.1μF to 1μF) bypass capacitor between the chip
V_IN pin and ground, placed close to the LTC3836, is also
suggested. A 10Ω resistor placed between C_IN (C1) and
the V_IN pin provides further isolation between the two
channels.
The selection of C_OUT is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfied, the capacitance is adequate for filtering. The
output ripple (∆V_OUT) is approximated by:

V_OUT

I_RIPPLE


ESR
+
1
8fC_OUT


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 LTC3836 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


1/2 LTC3836
V_FB
V_OUT
R_B
C_FF
R_A
3836 F05
Figure 5. Setting Output Voltage
To improve the frequency response, a feedforward 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.
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 LTC3836.
Pulling the RUN/SS pin below 0.65V puts the LTC3836 into
a low quiescent current shutdown mode (I_Q = 6.5μA). If
RUN/SS has been pulled all the way to ground, there will
be a delay before the LTC3836 comes out of shutdown
and is given by:
t_DELAY
=
0.65V
•
C_SS
0.65μA
=
1s/μF
•
C_SS
This pin can be driven directly from logic as shown in
Figure 6. Diode D_SS in Figure 6 reduces the start delay
but 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.
3.3V OR 5V
RUN/SS
D_SS
RUN/SS
C_SS
C_SS
V_DD ≤ V_IN
RUN/SS
(INTERNAL SOFT-START)
3836 F06
Figure 6. RUN/SS Pin Interfacing
3836fb
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