9
LTC1771
ripple current ratings are often based on 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. Do not underspecify this component. An addi-
tional 0.1µF ceramic capacitor is also helpful on V
IN
for
high frequency decoupling.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering.
The output ripple (∆V
OUT
) in continuous mode is approxi-
mated by:
∆≈ +
V I ESR
fC
OUT RIPPLE
OUT
1
8
where f is the operating frequency, C
OUT
is the output
capacitance and I
RIPPLE
is the ripple current in the
inductor. For output ripple less than 100mV, assure C
OUT
required ESR is <2R
SENSE
.
The first condition relates to the ripple current into the ESR
of the output capacitance while the second term guaran-
tees that the output capacitance does not significantly
discharge during the operating frequency period due to
ripple current. The choice of using smaller output capaci-
tance increases the ripple voltage due to the discharging
term but can be compensated for by using capacitors of
very low ESR to maintain the ripple voltage at or below
50mV. The I
TH
pin OPTI-LOOP
TM
compensation compo-
nents can be optimized to provide stable, high perfor-
mance transient response regardless of the output
capacitors selected.
When running into dropout, extra input and output capaci-
tance may be necessary for optimal performance due to
the drop in frequency as the duty cycle approaches 100%.
Compare Figure 1 to the low dropout regulators shown in
the Typical Applications section for recommended C
IN
,
C
OUT
, C
FF
and C
C
values for low dropout regulators vs
regulators not requiring low dropout.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR for its
size of any aluminum electrolytic at a somewhat higher
price. Typically once the ESR requirement is satisfied, the
RMS current rating generally far exceeds the I
RIPPLE(P-P)
requirement.
In surface mount applications multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum
electrolytics and dry tantalum capacitors are both available
in surface mount configurations. In 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, available in case heights ranging from
2mm to 4mm. Other capacitor types include Sanyo
OS-CON, Sanyo POSCAP, Nichicon PL series and
Panasonic SP.
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
often useful to analyze individual losses to determine what
is limiting efficiency and which change would produce the
most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 +L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in the LTC1771 circuits: the LTC1771 DC bias
current, MOSFET gate charge current, I
2
R losses and
catch diode losses.
1. The DC bias current is 9µA at no load and increases
proportionally with load up to a constant 150µA during
continuous mode. This bias current is so small that this
loss is negligible at loads above a milliamp but at no
load accounts for nearly all of the loss.
2. The MOSFET gate charge current results from switch-
ing the gate capacitance of the power MOSFET switch.
Each time the gate is switched from high to low to high
again, a packet of charge dQ moves from V
IN
to ground.
The resulting dQ/dt is the current out of V
IN
which is
typically much larger than the DC bias current. In
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