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TC1240ECHTR Folha de dados(PDF) 7 Page - Microchip Technology |
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TC1240ECHTR Folha de dados(HTML) 7 Page - Microchip Technology |
7 / 16 page 2003 Microchip Technology Inc. DS21516C-page 7 TC1240/TC1240A 4.0 DETAILED DESCRIPTION The TC1240/TC1240A charge pump converter dou- bles the voltage applied to the VIN pin. Conversion con- sists of a two-phase operation (Figure 4-1). During the first phase, switches S2 and S4 are open and S1 and S3 are closed. During this time, C1 charges to the voltage on VIN and load current is supplied from C2. During the second phase, S2 and S4 are closed, while S1 and S3 are open. During this second phase, C1 is level-shifted upward by VIN volts. This connects C1 to the reservoir capacitor C2, allowing energy to be delivered to the output as needed. The actual voltage is slightly lower than 2 x VIN since the four switches (S1-S4) have an on-resistance and the load drains charge from reservoir capacitor C2. FIGURE 4-1: Ideal Switched Capacitor Charge Pump Doubler. 5.0 TYPICAL APPLICATIONS 5.1 Output Voltage Considerations The TC1240/TC1240A performs voltage doubling but does not provide regulation. The output voltage will droop in a linear manner with respect to load current. The value of this equivalent output resistance is approx- imately 12 Ω nominal at +25°C and VIN = +5.0V for the TC1240A and 17 Ω nominal at +25°C and VIN = +2.8V for the TC1240. VOUT is approximately +10.0V at light loads for the TC1240A and +5.6V for the TC1240, and droops according to the equation below: EQUATION 5.2 Charge Pump Efficiency The overall power efficiency of the charge pump is affected by four factors: 1. Losses from power consumed by the internal oscillator, switch drive, etc. (which vary with input voltage, temperature and oscillator frequency). 2. I2R losses due to the on-resistance of the MOSFET switches on-board the charge pump. 3. Charge pump capacitor losses due to effective series resistance (ESR). 4. Losses that occur during charge transfer (from commutation capacitor to the output capacitor) when a voltage difference between the two capacitors exist. Most of the conversion losses are due to factors (2) and (3) above. These losses are given by Equation 5-1. EQUATION 5-1: VOUT = 2 x VIN C1 C2 TC1240/TC1240A VIN S1 S3 S4 S2 OSC VIN VOUT 2VIN VDROOP – × = VDROOP IOUT ROUT × = a) b) PLOSS(2,3) IOUT 2 ROUT × = ROUT 1 FSW C1 () ---------------------- 8RSWITCH 4ESRC1 ESRC2 ++ + = |
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