LTC3405 Linear Technology, LTC3405 Datasheet - Page 9

• Manufacturer Part Number: LTC3405
• Description: Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
where f = operating frequency, C
and I
voltage, the output ripple is highest at maximum input
voltage since I
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
be mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V
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.
TO 4.2V
2.7V
V
Figure 3. Paralleling a Ceramic with a Tantalum Capacitor
IN
L
IN
= ripple current in the inductor. For a fixed output
. At best, this ringing can couple to the output and
C
2.2 F
CER
IN
L
4
1
6
increases with input voltage.
IN
V
RUN
MODE
IN
U
, large enough to damage the part.
LTC3405
GND
2
SW
V
FB
U
3
5
1M
3405 F03
4.7 H
OUT
22pF
887k
W
= output capacitance
C
1 F
CER
OUT1
U
+
C
22 F
TANT
V
1.5V
OUT2
OUT
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.
A third solution is to use a high value resistor to inject a
feedforward signal at V
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:
TO 4.2V
R
V
TO 4.2V
OUT
2
2.7V
V
2.7V
V
IN
IN
R A R B
2
0 8
Figure 4. Using All Ceramic Capacitors
Figure 5. Feedforward Injection in an
All Ceramic Capacitor Application
.
C
2.2 F
CER
||
C
2.2 F
CER
IN
V
IN
2
1
4
1
6
4
1
6
R
R
V
RUN
MODE
V
RUN
MODE
IN
IN
2
R A R B
1
LTC3405
LTC3405
R A R B
FB
2
GND
GND
2
mimicking the ripple voltage of
2
2
V
V
SW
•
SW
FB
FB
2
2
3
5
3
5
887k
1M
R2B
1M
R1
200k
3405 F04
22pF
4.7 H
4.7 H
215k
22pF
R2A
LTC3405
C
4.7 F
CER
sn3405 3405fs
OUT
C
4.7 F
CER
V
1.5V
3405 F05
OUT1
OUT
V
1.5V
OUT
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