Datenblatt-Suchmaschine für elektronische Bauteile |
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TC1240ECH Datenblatt(PDF) 3 Page - Microchip Technology |
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TC1240ECH Datenblatt(HTML) 3 Page - Microchip Technology |
3 / 11 page 3 TC1240 Positive Doubling Charge Pump with Shutdown in SOT Package TC1240-1 7/7/00 DS21333A © 2001 Microchip Technology Inc. DETAILED DESCRIPTION The TC1240 charge pump converter doubles the volt- age applied to the VIN pin. Conversion consists of a two- phase operation (Figure 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, and 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. APPLICATIONS INFORMATION Output Voltage Considerations TheTC1240 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 approximately 17 Ω nominal at +25 °C and VIN = +2.8V. VOUT is approximately +5.6V at light loads, and droops according to the equation below: VDROOP = IOUT x ROUT VOUT = 2 x VIN – VDROOP Charge Pump Efficiency The overall power efficiency of the charge pump is affected by four factors: (1) Losses from power consumed by the internal oscil- lator, switch drive, etc. (which vary with input voltage, temperature and oscillator frequency). Figure 1. Ideal Swiched Capacitor Charge Pump Doubler (2) I 2R 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 exists. Most of the conversion losses are due to factors (2) and (3) above. These losses are given by Equation 1(b). (a) PLOSS (2, 3) = IOUT 2 x ROUT (b) ≅ I OUT 2 x [ 1 +8RSWITCH + 4ESRC1 + ESRC2 ] (fPUMP) C1 Equation 1. The pump frequency in Equation 1(b) is defined as one- half the oscillator frequency (i.e. fPUMP = fOSC/2). The 1/(fPUMP)(C1) term in Equation 1(b) is the effective output resistance of an ideal switched capacitor circuit (Figures 2a, 2b). The value of RSWITCH can be approximated at 1.4 Ω for the TC1240. The remaining losses in the circuit are due to factor (4) above, and are shown in Equation 2. The output voltage ripple is given by Equation 3. PLOSS(4) = [(0.5)(C1) (4VIN 2– VOUT2 ) + (0.5)(C2)(2VOUT VRIPPLE – VRIPPLE 2 )] x fOSC Equation 2. VRIPPLE = IOUT +2(IOUT)(ESRC2) (fOSC)(C2) Equation 3. Figure 2a. Ideal Swiched Capacitor Model V+ V OUT R L C1 C2 f Figure 2b. Equivalent Output Resistance C1 C2 TC1240 S1 S3 S4 S2 OSC VIN OUT = 2 x VIN VIN V+ V OUT R EQUIV R EQUIV = R L C2 f x C1 1 |
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