LTC1265-3.3_15 Datasheet, PDF (8/16 Pages) Linear Technology – 1.2A, High Efficiency Step-Down DC/DC Converter

LTC1265/LTC1265-3.3/LTC1265-5
APPLICATIO S I FOR ATIO
P-channel switch duty cycle. At high input voltages, the
diode conducts most of the time. As V_IN approaches V_OUT,
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 I_SC(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 I_OUT(MAX) ≤ 500mA.
C_IN
In continuous mode, the input current of the converter is
a square wave of duty cycle V_OUT/ V_IN. To prevent large
voltage transients, a low ESR input capacitor must be
used. In addition, the capacitor must handle a high RMS
current. The C_IN RMS current is given by:
I_RMS
≈
I_OUT [V_OUT (V_IN – V_OUT)]^1/2
V_IN
(A_RMS)
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/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 V_IN for high frequency decoupling.
C_OUT
The selection of C_OUT is based upon the effective series
resistance (ESR) for proper operation of the LTC1265. The
required ESR of C_OUT is:
ESR_COUT < 50mV/I_RIPPLE
where I_RIPPLE is the ripple current of the inductor. For the
case where the I_RIPPLE is 25mV/R_SENSE, the required ESR
of C_OUT is:
ESR_COUT < 2(R_SENSE)
To avoid overheating, the output capacitor must be sized
to handle the ripple current generated by the inductor. The
worst-case RMS ripple current in the output capacitor is
given by:
I_RMS ≈
150mV
2(R_SENSE)
(A_RMS)
Generally, once the ESR requirement for C_OUT has been
met, the RMS current rating far exceeds the I_RIPPLE(P-P)
requirement.
ESR is a direct function of the volume of the capacitor.
Manufacturers such as Nichicon, AVX and Sprague should
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.
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
use in switching power supplies. An excellent choice is the
AVX TPS series of surface mount tantalums, available in
case heights ranging from 2mm to 4mm. Consult the
manufacturer for other specific recommendations.
When the capacitance of C_OUT is made too small, the
output ripple at low frequencies will be large enough to trip
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 R_SENSE and can be improved at
higher frequencies with lower values of L.
Low-Battery Detection
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:
) ^{VLB_TRIP = 1.25}
1 + ^R4
R3
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