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LTC3246 ๋ฐ์ดํ„ฐ์‹œํŠธ(HTML) 10 Page - Analog Devices

๋ถ€ํ’ˆ๋ช… LTC3246
์ƒ์„ธ๋‚ด์šฉ  Wide VIN Range Buck-Boost Charge Pump with Watchdog Timer
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LTC3246 ๋ฐ์ดํ„ฐ์‹œํŠธ(HTML) 10 Page - Analog Devices

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LTC3246
10
3246fa
For more information www.linear.com/LTC3246
APPLICATIONS INFORMATION
General Operation
The LTC3246 uses switched capacitor based DC/DC
conversion to provide the efficiency advantages associ-
ated with inductor based circuits as well as the cost and
simplicity advantages of a linear regulator. The LTC3246
uses an internal switch network and fractional conversion
ratios to achieve high efficiency and regulation over widely
varying VIN and output load conditions.
Internalcontrolcircuitryselectstheappropriateconversion
ratio based on VIN and load conditions. The device has
threepossibleconversionmodes:2:1step-downmode,1:1
step-down mode and 1:2 step-up mode. Only one external
flying capacitor is needed to operate in all three modes.
2:1 mode is chosen when VIN is greater than two times the
desired VOUT. 1:1 mode is chosen when VIN falls between
two times VOUT and VOUT. 1:2 mode is chosen when VIN
falls below the desired VOUT. The internal mode control
logic maintains output regulation over all load conditions.
Regulation is achieved by sensing the output voltage
and enabling charge transfer when the output falls below
regulation. When the charge pump is enabled, it controls
the current into the flying capacitor to limit the output
ripple beyond that of conventional switched capacitor
charge pumps. The part has two SEL pins that select the
output regulation (fixed 5V, fixed 3.3V or adjustable) as
well as shutdown.
Thechargepumpoperatesatanominalfrequencyofabout
450kHz, though actual output ripple frequency will vary
with output load, operating mode and output capacitance.
The LTC3246 is designed for applications requiring high
systemreliability.Thepartincludesoutputsupplymonitor-
ing and watchdog timing circuitry as well as overvoltage,
short-circuit and overtemperature protection.
VOUT Regulation and Mode Selection
Regulation is achieved by sensing the output voltage and
enabling charge transfer when the output falls below the
programmed regulation voltage. The amount of charge
transferred per cycle is controlled over the full input range
to minimize output ripple. The regulation voltage (fixed
5V, fixed 3.3V or adjustable) is selected through the SEL1
and SEL2 pins per Table 1 in the Pin Function section.
The optimal conversion ratio is chosen based on VIN, VOUT
and output conditions. Two internal comparators are used
to select the default conversion ratio. The conversion ratio
switchpointisoptimizedtoprovidepeakefficiencyoverall
supply and load conditions while maintaining regulation.
Each comparator also has built-in hysteresis to reduce the
tendency of oscillating between modes when a transition
point is reached.
The LTC3246 will attempt to regulate its output over the
full operating range (2.7V to 38V), but like any regulator
the output will drop out of regulation if inadequate supply
voltage exists to the operating load. As the input voltage
drops, the LTC3246 will eventually end up in the 1:2 step
up mode. As the input voltage drops further, the output
will eventually drop out of regulation. At this point, the 1:2
step-up charge pump impedance can be calculated as:
ROUT =
2 โ€ข VIN โ€“ VOUT
IOUT
This equation can be rewritten to determine the output
current at which the output will drop out for a given input
voltage as:
IOUT =
2 โ€ข VIN โ€“ VOUT
ROUT
IOUT 500mA
For a typical 1:2 step-up charge pump impedance of 4
with 5V output voltage and 3V input voltage, the output
current at dropout will be about:
IOUT =
2 โ€ข 3 โ€“ 4.8
4
mA = 300mA
Thus, typically the part should be able to output 300mA
without dropping out. To be conservative, the max 1:2
step-up charge pump impedance of 8 should be used
whichgivesamoreconservativeoutputcurrentof150mA.
Any supply impedance in series with the LTC3246 must
be doubled and added to the 1:2 step-up charge pump
impedance. It is also important to have the specified COUT
andCFLYcapacitancetoachievethespecifiedoutputimped-
ance. Observing dropout will allow the user to calculate
the output impedance for their specific application.


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