Datasheet

LTC3891
18
3891fa
APPLICATIONS INFORMATION
maximum RMS current of one channel must be used. The
maximum RMS capacitor current is given by:
C
IN
Required I
RMS
≈
I
MAX
V
IN
V
OUT
( )
V
IN
– V
OUT
( )




1/2
(1)
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet
size or height requirements in the design. Due to the high
operating frequency of the LTC3891, ceramic capacitors
can also be used for C
IN
. Always consult the manufacturer
if there is any question.
A small (0.1μF to 1μF) bypass capacitor between the chip
V
IN
pin and ground, placed close to the LTC3891, is also
suggested. A small (≤10Ω) resistor placed between C
IN
(C1) and the V
IN
pin provides further isolation.
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:
ΔV
OUT
≈ ΔI
L
ESR +
1
8 • f • C
OUT






where f is the operating frequency, C
OUT
is the output
capacitance and ∆I
L
is the ripple current in the inductor.
The output ripple is highest at maximum input voltage
since ∆I
L
increases with input voltage.
Setting Output Voltage
The LTC3891 output voltage is set by an external feed-
back resistor divider carefully placed across the output,
as shown in Figure 3. The regulated output voltage is
determined by:
V
OUT
= 0.8V 1+
R
B
R
A






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 Pin
The LTC3891 is enabled using the RUN pin. It has a rising
threshold of 1.21V with 50mV of hysteresis. Pulling the
RUN pin below 1.16V shuts down the main control loop.
Pulling it below 0.7V disables the controller and most
internal circuits, including the INTV
CC
LDOs. In this state,
the LTC3891 draws only 14μA of quiescent current.
Releasing the RUN pin allows a small 7μA internal current
to pull up the pin to enable the controller. The RUN pin may
be externally pulled up or driven directly by logic. When
driving the RUN pin with a low impedance source, do not
exceed the absolute maximum rating of 8V. The RUN pin
has an internal 11V voltage clamp that allows the RUN pin
to be connected through a resistor to a higher voltage (for
example, V
IN
), so long as the maximum current into the
RUN pin does not exceed 100μA.
The RUN pin can be implemented as a UVLO by connecting
it to the output of an external resistor divider network off
V
IN
, as shown in Figure 4.
LTC3891
V
FB
V
OUT
R
B
C
FF
R
A
3891 F03
Figure 3. Setting Output Voltage
LTC3891
RUN
V
IN
R
B
R
A
3891 F04
Figure 4. Using the RUN Pin as a UVLO