Datasheet
9
LTC3405
3405fa
where f = operating frequency, C
OUT
= output capacitance
and ∆I
L
= ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ∆I
L
increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalum. These are
specially constructed and tested for low ESR so they give
the lowest ESR for a given volume. Other capacitor types
include Sanyo POSCAP, Kemet T510 and T495 series, and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
the output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input, V
IN
. At best, this ringing can couple to the output and
be mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V
IN
, large enough to damage the part.
When ceramic capacitors are used at the output, their low
ESR cannot provide sufficient phase lag cancellation to
stabilize the loop. One solution is to use a tantalum
capacitor, with its higher ESR, to provide the bulk capaci-
tance and parallel it with a small ceramic capacitor to
reduce the ripple voltage as shown in Figure 3.
APPLICATIO S I FOR ATIO
WUUU
Another solution is to connect the feedback resistor to the
SW pin as shown in Figure 4. Taking the feedback informa-
tion at the SW pin removes the phase lag due to the output
capacitor resulting in a very stable loop. This configuration
lowers the load regulation by the DC resistance of the
inductor multiplied by the load current. This slight shift in
load regulation actually helps reduce the overshoot and
undershoot of the output voltage during a load transient.
V
IN
C
IN
2.2µF
CER
V
IN
2.7V
TO 4.2V
LTC3405
RUN
MODE
3
4.7µH
22pF
887k
1M
3405 F03
5
4
6
1
2
SW
V
FB
GND
C
OUT1
1µF
CER
C
OUT2
22µF
TANT
V
OUT
1.5V
+
Figure 3. Paralleling a Ceramic with a Tantalum Capacitor
V
IN
C
IN
2.2µF
CER
V
IN
2.7V
TO 4.2V
LTC3405
RUN
MODE
3
4.7µH
22pF
1M
887k
3405 F04
5
4
6
1
2
SW
V
FB
GND
C
OUT
4.7µF
CER
V
OUT
1.5V
Figure 4. Using All Ceramic Capacitors
A third solution is to use a high value resistor to inject a
feedforward signal at V
FB
mimicking the ripple voltage of
a high ESR output capacitor. The circuit in Figure 5 shows
how this technique can be easily realized. The feedforward
resistor, R2B, is connected to SW as in the previous
example. However, in this case, the feedback information
is taken from the resistive divider, R2A and R1, at the
output. This eliminates most of the load regulation degra-
dation due to the DC resistance of the inductor while
providing a stable operation similar to that obtained from
a high ESR tantalum type capacitor. Using this technique,
the extra feedforward resistor, R2B, must be accounted
for when calculating the resistive divider as follows:
RRARB
RA RB
RA RB
VV
R
R
OUT
22 2
22
22
08 1
2
1
==
+
=+
⎛
⎝
⎜
⎞
⎠
⎟
||
•
.
Figure 5. Feedforward Injection in an
All Ceramic Capacitor Application
V
IN
C
IN
2.2µF
CER
V
IN
2.7V
TO 4.2V
LTC3405
RUN
MODE
3
4.7µH
22pF
R1
200k
R2B
1M
3405 F05
5
4
6
1
2
SW
V
FB
GND
C
OUT1
4.7µF
CER
V
OUT
1.5V
R2A
215k