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LTC3246 데이터시트(HTML) 13 Page - Analog Devices

부품명 LTC3246
상세내용  Wide VIN Range Buck-Boost Charge Pump with Watchdog Timer
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LTC3246 데이터시트(HTML) 13 Page - Analog Devices

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LTC3246
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For more information www.linear.com/LTC3246
VIN Capacitor Selection
ThefinitechargetransferarchitectureusedbytheLTC3246
makes input noise filtering much less demanding than the
sharp current spikes of conventional regulated charge
pumps. Depending on the mode of operation, the input
current of the LTC3246 can step from about 1A to 0A on
a cycle-by-cycle basis. Low ESR will reduce the voltage
steps caused by changing input current, while the ab-
solute capacitor value will determine the level of ripple.
The total amount and type of capacitance necessary for
input bypassing is very dependent on the applied source
impedance as well as existing bypassing already on the
VIN node. For optimal input noise and ripple reduction, it
is recommended that a low ESR ceramic capacitor be used
for CIN bypassing. An electrolytic or tantalum capacitor
may be used in parallel with the ceramic capacitor on CIN
to increase the total capacitance, but, due to the higher
ESR, it is not recommended that an electrolytic or tan-
talum capacitor be used alone for input bypassing. The
LTC3246 will operate with capacitors less than 1µF, but,
depending on the source impedance, input noise can feed
through to the output causing degraded performance.
For best performance 1µF or greater total capacitance is
suggested for CIN.
Flying Capacitor Selection
Ceramic capacitors should always be used for the flying
capacitor. The flying capacitor controls the strength of
the charge pump. In order to achieve the rated output
current, it is necessary for the flying capacitor to have
at least 1µF of capacitance over operating temperature
with a bias voltage equal to the programmed VOUT (see
Ceramic Capacitor Selection Guidelines). If only 100mA
or less of output current is required for the application,
the flying capacitor minimum can be reduced to 0.2µF.
The voltage rating of the ceramic capacitor should be
VOUT +1V or greater.
Ceramic Capacitor Selection Guidelines
Capacitors of different materials lose their capacitance
with higher temperature and voltage at different rates.
For example, a ceramic capacitor made of X5R or X7R
material will retain most of its capacitance from –40°C
to 85°C, whereas a Z5U or Y5V style capacitor will lose
considerable capacitance over that range (60% to 80%
loss typical). Z5U and Y5V capacitors may also have a
very strong voltage coefficient, causing them to lose an
additional60%ormoreoftheircapacitancewhentherated
voltage is applied. Therefore, when comparing different
capacitors, it is often more appropriate to compare the
amount of achievable capacitance for a given case size
rather than discussing the specified capacitance value. For
example, over rated voltage and temperature conditions,
a 4.7µF, 10V, Y5V ceramic capacitor in an 0805 case may
not provide any more capacitance than a 1µF, 10V, X5R
or X7R available in the same 0805 case. In fact, over bias
and temperature range, the 1µF, 10V, X5R or X7R will
provide more capacitance than the 4.7µF, 10V, Y5V. The
capacitor manufacturer’s data sheet should be consulted
to determine what value of capacitor is needed to ensure
minimum capacitance values are met over operating
temperature and bias voltage. Below is a list of ceramic
capacitor manufacturers and how to contact them:
MANUFACTURER
WEBSITE
AVX
www.avxcorp.com
Kemet
www.kemet.com
Murata
www.murata
Taiyo Yuden
www.t-yuden.com
TDK
www.tdk.com
Wurth Elektronik
www.we-online.com
BIAS Pin and Capacitor Selection
TheBIASpinoftheLTC3246isa5Voutputthatisgenerated
by an internal Low Drop-Out (LDO) regulator supplied by
VIN. The BIAS voltage is used as a supply for the internal
low voltage circuitry. A capacitor on the BIAS pin is neces-
sary to stabilize the LDO output and minimize ripple during
transient conditions. A low ESR ceramic capacitor with a
minimum capacitance of 2µF over temperature with 5V
bias should be used. Since the BIAS voltage comes from
an LDO, the BIAS voltage will drop with VIN as VIN goes
below 5V. This is normal and expected operation. The
BIAS pin voltage is for internal circuitry only and should
not be loaded externally.
APPLICATIONS INFORMATION


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