LTC1265/LTC1265-3.3/LTC1265-5
W U U
U
APPLICATIO S I FOR ATIO
P-channel switch duty cycle. At high input voltages, the
worst-case RMS ripple current in the output capacitor is
given by:
diode conducts most of the time. As VIN approaches VOUT
,
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output
is short circuited. Under this condition, the diode must
safely handle ISC(PK) at close to 100% duty cycle. Most
LTC1265 circuits will be well served by either a 1N5818 or
a MBRS130LT3 Schottky diode. An MBRS0520 is a good
choice for IOUT(MAX) ≤ 500mA.
150mV
I
≈
(A
)
RMS
RMS
2(R
)
SENSE
Generally, once the ESR requirement for COUT has been
met, the RMS current rating far exceeds the IRIPPLE(P-P)
requirement.
ESR is a direct function of the volume of the capacitor.
ManufacturerssuchasNichicon,AVXandSpragueshould
be considered for high performance capacitors. The
OS-CON semiconductor dielectric capacitor available
from Sanyo has the lowest ESR for its size at a somewhat
higher price.
CIN
In continuous mode, the input current of the converter is
a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low ESR input capacitor must be
used. In addition, the capacitor must handle a high RMS
current. The CIN RMS current is given by:
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 tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, it is critical that the capacitors are both available
in surface mount configuration and are surge tested for
useinswitchingpowersupplies. Anexcellentchoiceisthe
AVX TPS series of surface mount tantalums, available in
case heights ranging from 2mm to 4mm. Consult the
manufacturer for other specific recommendations.
1
/
2
I
[V
(V – V )]
OUT OUT IN OUT
(A
)
I
≈
RMS
RMS
V
IN
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/2. This simple worst case is commonly used for
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 to choose a capacitor rated at a higher temperature
than required. Do not underspecify this component. An
additional 0.1µF ceramic capacitor is also required on
PWR VIN for high frequency decoupling.
When the capacitance of COUT is made too small, the
outputrippleatlowfrequencieswillbelargeenoughtotrip
the voltage comparator. This causes Burst Mode opera-
tion to be activated when the LTC1265 would normally be
in continuous operation. The effect will be most pro-
nounced with low value of RSENSE and can be improved at
higher frequencies with lower values of L.
COUT
The selection of COUT is based upon the effective series
resistance(ESR)forproperoperationoftheLTC1265.The
required ESR of COUT is:
Low-Battery Detection
ESRCOUT < 50mV/IRIPPLE
The low-battery comparator senses the input voltage
through an external resistive divider. This divided voltage
connects to the (–) input of a voltage comparator (Pin 4)
which is compared with a 1.25V reference voltage. Ne-
glecting Pin 4 bias current, the following expression is
used for setting the trip limit:
where IRIPPLE is the ripple current of the inductor. For the
case where the IRIPPLE is 25mV/RSENSE, the required ESR
of COUT is:
ESRCOUT < 2(RSENSE
)
To avoid overheating, the output capacitor must be sized
to handle the ripple current generated by the inductor. The
R4
R3
1 +
= 1.25
V
LB_TRIP
)
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