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LTC3246 데이터시트(PDF) 12 Page - Analog Devices |
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LTC3246 데이터시트(HTML) 12 Page - Analog Devices |
12 / 20 page LTC3246 12 3246fa For more information www.linear.com/LTC3246 half of the total output current. The efficiency ( ) and chip power dissipation (PD) in 2:1 are approximately: POUT PIN = VOUT •IOUT VIN • 1 2 IOUT = 2VOUT VIN PD = VIN 2 – VOUT IOUT 1:1 Step-Down Charge Pump Operation When the input supply is less than about two times the output voltage, but more than the programmed output voltage, the LTC3246 will operate in 1:1 step-down mode. This method of regulation is very similar to a linear regula- tor. Charge is delivered directly from VIN to VOUT through most of the oscillator period. The charge transfer is briefly interruptedattheendoftheperiod.Whenin1:1step-down mode, the input current will be approximately equal to the total output current. Thus, efficiency ( ) and chip power dissipation (PD) in 1:1 are approximately: POUT PIN = VOUT •IOUT VIN •IOUT = VOUT VIN PD = VIN – VOUT IOUT 1:2 Step-Up Charge Pump Operation When the input supply is less than the output voltage, the LTC3246 will operate in 1:2 step-up mode. Charge transfer happens in two phases. On the first phase, the flying capacitor (CFLY) is connected between VIN and GND. On this phase, CFLY is charged up. On the second phase, the flying capacitor (CFLY) is connected between VIN and VOUT and the charge stored on CFLY during the first phase is transferred to VOUT. When in 1:2 step-up mode, the input current will be approximately twice the total output current. Thus, efficiency ( ) and chip power dissipation (PD) in 1:2 are approximately: POUT PIN = VOUT •IOUT VIN • 2IOUT = VOUT 2VIN PD = 2VIN – VOUT IOUT VOUT Ripple and Capacitor Selection The type and value of capacitors used with the LTC3246 determine output ripple and charge pump strength. The value of COUT directly controls the amount of output ripple for a given load current. Output ripple decreases with output capacitance until about 20µF, at which point output peak to peak ripple remains more or less constant. See Figure 3 for graph of output ripple vs output capacitance. Figure3. Typical VOUT Ripple Voltage vs COUT Capacitance 0 5 10 15 20 25 30 35 40 0 25 50 75 100 125 150 175 200 3246 TA01b BOOST, 500mA BOOST, 50mA BUCK, 500mA BUCK, 50mA LDO, 500mA LDO, 50mA COUT CAPACITANCE (µF) To reduce output noise and ripple, it is suggested that a low ESR (equivalent series resistance < 0.1) ceramic capacitor (10µF or greater) be used for COUT. For optimal performance, it is best to increase COUT for low VOUT as the ripple becomes a larger percentage of the regulation voltage degrading performance. Tantalum and aluminum capacitors can be used in parallel with a ceramic capacitor to increase the total capacitance but are not recommended to be used alone because of their high ESR. VOUT Overvoltage Protection An internal comparator monitors the voltage at VOUT and will prevent charge transfer in the event that VOUT exceeds the overvoltage threshold (5.9V typ.). Overvoltage protec- tion is added as a safety feature to prevent damage to the part in the event of a fault such as VOUTS/ADJ pin shorted to ground or not connected to VOUT. Charge transfer will start once the output falls to about 5.75V. APPLICATIONS INFORMATION |
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