Datasheet

LTC3561A
13
3561af
components and the output capacitor size. Typically, 3 to
4 cycles are required to respond to a load step, but only
in the fi rst cycle does the output drop linearly. The output
droop, V
DROOP
, is usually about 2 to 3 times the linear
drop of the fi rst cycle. Thus, a good place to start is with
the output capacitor value of approximately:
C
OUT
≈ 2.5
ΔI
OUT
f
O
•V
DROOP
More capacitance may be required depending on the duty
cycle and load step requirements.
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since the impedance
to the supply is very low. A 10μF ceramic capacitor is
usually enough for these conditions.
Setting the Output Voltage
The LTC3561A develops a 0.8V reference voltage between
the feedback pin, V
FB
, and the signal ground as shown in
Figure 4. The output voltage is set by a resistive divider
according to the following formula:
V
OUT
≈ 0.8V 1+
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
Keeping the current small (<5μA) in these resistors maxi-
mizes effi ciency, but making them too small may allow
stray capacitance to cause noise problems and reduce the
phase margin of the error amp loop.
To improve the frequency response, a feed-forward capaci-
tor C
F
may also 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.
Shutdown and Soft-Start
The SHDN/R
T
pin is a dual purpose pin that sets the oscil-
lator frequency and provides a means to shut down the
LTC3561A. This pin can be interfaced with control logic in
several ways, as shown in Figure 2 and Figure 3. In both
confi gurations, Run = “0” shuts down the LTC3561A and
Run = “1” activates the LTC3561A.
By activating the LTC3561A, an internal soft-start slowly
ramps the output voltage up until regulation. Soft-start
prevents surge currents from V
IN
by gradually ramping
the output voltage up during start-up. The output will ramp
from zero to full scale over a time period of approximately
0.8ms. This prevents the LTC3561A from having to quickly
charge the output capacitor and thus supplying an exces-
sive amount of instantaneous current.
APPLICATIONS INFORMATION
Figure 2. SHDN/R
T
Pin Activated with a Logic Input
Figure 3. SHDN/R
T
Pin Activated with a Switch
3561A F02
RUN
R
T
SHDN/R
T
3561A F03
RUN
R
T
SHDN/R
T
1M
SV
IN