LTC1435A Просмотр технического описания (PDF) - Linear Technology
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LTC1435A Datasheet PDF : 20 Pages
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LTC1435A
APPLICATIONS INFORMATION
CIN and COUT Selection
In continuous mode, the source current of the top
N-channel MOSFET is a square wave of duty cycle VOUT/
VIN. To prevent large voltage transients, a low ESR input
capacitor sized for the maximum RMS current must be
used. The maximum RMS capacitor current is given by:
[ ( )]1/2
VOUT VIN − VOUT
CIN required IRMS ≈ IMAX
VIN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monly 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
of life. This makes it advisable to further derate the capaci-
tor or to choose a capacitor rated at a higher temperature
than required. Several capacitors may also be paralleled to
meet size or height requirements in the design. Always
consult the manufacturer if there is any question.
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) is approximated by:
∆VOUT
≈
∆IL
ESR
+
1
4 fC OUT
where f = operating frequency, COUT = output capacitance
and ∆IL= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆IL increases
with input voltage. With ∆IL = 0.4IOUT(MAX) the output ripple
will be less than 100mV at max VIN assuming:
COUT required ESR < 2RSENSE
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(size)
product of any aluminum electrolytic at a somewhat
higher price. Once the ESR requirement for COUT has been
met, 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 electrolytic and
dry tantalum capacitors are both available in surface mount
configurations. In the 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 series of
surface mount tantalum, available in case heights ranging
from 2mm to 4mm. Other capacitor types include Sanyo
OS-CON, Nichicon PL series and Sprague 593D and 595D
series. Consult the manufacturer for other specific recom-
mendations.
INTVCC Regulator
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC1435A. The INTVCC pin can supply up to
15mA and must be bypassed to ground with a minimum
of 2.2µF tantalum or low ESR electrolytic. Good bypassing
is necessary to supply the high transient currents required
by the MOSFET gate drivers.
High input voltage applications, in which large MOSFETs
are being driven at high frequencies, may cause the maxi-
mum junction temperature rating for the LTC1435A to be
exceeded. The IC supply current is dominated by the gate
charge supply current when not using an output derived
EXTVCC source. The gate charge is dependent on operat-
ing frequency as discussed in the Efficiency Considerations
section. The junction temperature can be estimated by using
the equations given in Note 1 of the Electrical Character-
istics. For example, the LTC1435A is limited to less than
17mA from a 30V supply:
TJ = 70°C + (17mA)(30V)(100°C/W) = 126°C
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked when
operating in continuous mode at maximum VIN.
EXTVCC Connection
The LTC1435A contains an internal P-channel MOSFET
switch connected between the EXTVCC and INTVCC pins. The
switch closes and supplies the INTVCC power whenever the
EXTVCC pin is above 4.8V, and remains closed until EXTVCC
drops below 4.5V. This allows the MOSFET driver and
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