7
LTC1701/LTC1701B
Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter is
a square wave with a duty cycle of approximately V
OUT
/
V
IN
. To prevent large voltage transients, a low equivalent
series resistance (ESR) input capacitor sized for the maxi-
mum RMS current must be used. The maximum RMS
capacitor current is given by:
II
VVV
V
RMS MAX
OUT IN OUT
IN
≈
−
()
where the maximum average output current I
MAX
equals
the peak current (1 Amp) minus half the peak-to-peak
ripple current, I
MAX
= 1 – ∆I
L
/2.
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case is commonly used to
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours life-
time. This makes it advisable to further derate the capaci-
tor, or choose a capacitor rated at a higher temperature
than required. Several capacitors may also be paralleled to
meet the size or height requirements of the design. An
additional 0.1µF to 1µF ceramic capacitor is also recom-
mended on V
IN
for high frequency decoupling.
Output Capacitor (C
OUT
) Selection
The selection of C
OUT
is driven by the required ESR.
Typically, once the ESR requirement is satisfied, the
capacitance is adequate for filtering. The output ripple
(∆V
OUT
) is determined by:
∆≈∆ +
V I ESR
fC
OUT L
OUT
1
8
where f = operating frequency, C
OUT
= output capacitance
and ∆I
L
= ripple current in the inductor. With ∆I
L
= 0.4
I
OUT(MAX)
the output ripple will be less than 100mV with:
ESR
COUT
< 100mΩ
Once the ESR requirements for C
OUT
have been met, the
RMS current rating generally far exceeds the I
RIPPLE(P-P)
requirement.
When the capacitance of C
OUT
is made too small, the
output ripple at low frequencies will be large enough to trip
the I
TH
comparator. This causes Burst Mode operation to
be activated when the LTC1701 would normally be in
continuous mode operation. The effect can be improved at
higher frequencies with lower inductor values.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Alumi-
num electrolyte and dry tantulum 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, AVX TPSV and KEMET T510 series of
surface mount tantalums, avalable in case heights ranging
from 2mm to 4mm. Other capacitor types include Nichicon
PL series, Sanyo POSCAP and Panasonic SP.
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. These are tempt-
ing for switching regulator use because of their very low
ESR. Unfortunately, the ESR is so low that it can cause
loop stability problems. Solid tantalum capacitor ESR
generates a loop “zero” at 5kHz to 50kHz that is instrumen-
tal in giving acceptable loop phase margin. Ceramic ca-
pacitors remain capacitive to beyond 300kHz and usually
resonate with their ESL before ESR becomes effective.
Also, ceramic caps are prone to temperature effects which
requires the designer to check loop stability over the
operating temperature range.
For these reasons, most of the input and output capaci-
tance should be composed of tantalum capacitors for
stability combined with about 0.1µF to 1µF of ceramic
capacitors for high frequency decoupling. Great care must
be taken when using only ceramic input and output capaci-
tors. The OPTI-LOOP compensation allows transient re-
sponse to be optimized for all types of output capacitors,
including low ESR ceramics.
Setting the Output Voltage
The LTC1701 develops a 1.25V reference voltage between
the feedback pin, V
FB
, and the signal ground as shown in
APPLICATIO S I FOR ATIO
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