LTC1701(1999) Ver la hoja de datos (PDF) - Linear Technology
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LTC1701 Datasheet PDF : 12 Pages
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LTC1701
APPLICATIO S I FOR ATIO
Most LTC1701 circuits will be well served by either an
MBR0520L or an MBRM120L. An MBR0520L is a good
choice for IOUT(MAX) ≤ 500mA, as long as the output
doesn’t need to sustain a continuous short.
Input Capacitor (CIN) Selection
In continuous mode, the input current of the converter is
a square wave with a duty cycle of approximately VOUT/
VIN. 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:
( ) VOUT VIN − VOUT
IRMS ≈ IMAX
VIN
where the maximum average output current IMAX equals
the peak current (1 Amp) minus half the peak-to-peak
ripple current, IMAX = 1 – ∆IL/2.
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/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 VIN for high frequency decoupling.
Output Capacitor (COUT) Selection
The selection of COUT is driven by the required ESR.
Typically, once the ESR requirement is satisfied, the
capacitance is adequate for filtering. The output ripple
(∆VOUT) is determined by:
∆VOUT
≈
∆IL ESR
+
1
8fCOUT
where f = operating frequency, COUT = output capacitance
and ∆IL = ripple current in the inductor. With ∆IL = 0.4
IOUT(MAX) the output ripple will be less than 100mV with:
ESRCOUT < 100mΩ
Once the ESR requirements for COUT have been met, the
RMS current rating generally far exceeds the IRIPPLE(P-P)
requirement.
When the capacitance of COUT is made too small, the
output ripple at low frequencies will be large enough to trip
the ITH 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. The OS-CON
semiconductor dielectric capacitor available from Sanyo
has the lowest ESR(size) product of any aluminum elec-
trolytic at a somewhat higher price. In the case of tanta-
lum, 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.
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