LTC1771
W U U
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APPLICATIO S I FOR ATIO
ripplecurrentratingsareoftenbasedon2000hoursoflife.
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 VIN for
high frequency decoupling.
electrolyticsanddrytantalumcapacitorsarebothavailable
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.
The selection of COUT 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 (∆VOUT) in continuous mode is approxi-
mated by:
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
oftenusefultoanalyzeindividuallossestodeterminewhat
is limiting efficiency and which change would produce the
most improvement. Efficiency can be expressed as:
1
∆VOUT ≈IRIPPLE ESR +
8fCOUT
where f is the operating frequency, COUT is the output
capacitance and IRIPPLE is the ripple current in the
inductor. For output ripple less than 100mV, assure COUT
Efficiency = 100% – (L1 + L2 +L3 + ...)
required ESR is <2RSENSE
.
whereL1, L2, etc. aretheindividuallossesasapercentage
of input power.
ThefirstconditionrelatestotheripplecurrentintotheESR
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 ITH pin OPTI-LOOPTM compensation compo-
nents can be optimized to provide stable, high perfor-
mance transient response regardless of the output
capacitors selected.
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, I2R 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 VIN to ground.
The resulting dQ/dt is the current out of VIN which is
typically much larger than the DC bias current. In con-
tinuousmode,IGATECHG =fQPwhereQPisthegatecharge
of the internal switch. Both the DC bias and gate charge
losses are proportional to VIN and thus their effects will
be more pronounced at higher supply voltages.
Manufacturers such as Nichicon, United Chemicon and
Sanyoshouldbeconsideredforhighperformancethrough-
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 IRIPPLE(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
3. I2Rlossesarepredictedfromtheinternalswitch,induc-
tor and current sense resistor. In continuous mode the
averageoutputcurrentflowsthroughLbutis“chopped”
OPTI-LOOP is a trademark of Linear Technology Corporation.
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