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TCM828ECT Datasheet(PDF) 3 Page - Microchip Technology |
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TCM828ECT Datasheet(HTML) 3 Page - Microchip Technology |
3 / 9 page 3 TCM828/829-4 5/22/00 TCM828 TCM829 Switched Capacitor Voltage Converters © 2001 Microchip Technology Inc. DS21488A DETAILED DESCRIPTION The TCM828/829 charge pump converters invert the voltage 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. This action connects C1 across C2, restoring charge to C2. APPLICATIONS INFORMATION Output Voltage Considerations The TCM828/829 perform voltage conversion but do 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 25 Ω nominal at +25 °C and VIN = +5V. VOUT is approximately – 5V at light loads, and droops according to the equation below: VDROOP = IOUT x ROUT VOUT = – (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 volt- age, temperature and oscillator frequency). (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). Figure 1. Ideal Switched Capacitor Charge Pump (4) Losses that occur during charge transfer (from the commutation capacitor to the output capacitor) when a voltage difference between the two capaci- tors exists. Most of the conversion losses are due to factors (2), (3) and (4) above. These losses are given by Equation 1. PLOSS (2, 3, 4) = IOUT 2 x ROUT ≅ IOUT2 x [ 1 +8RSWITCH+ 4ESRC1+ ESRC2 ] (fOSC) C1 The 1/(fOSC)(C1) term in Equation 1 is the effective output resistance of an ideal switched capacitor circuit (Figures 2a, 2b). The losses in the circuit due to factor (4) above are also shown in Equation 2. The output voltage ripple is given by Equation 3. PLOSS (4) = [(0.5)(C1)(VIN2– VOUT2) + (0.5)(C2)(VRIPPLE2 – 2VOUTVRIPPLE) ] x f OSC VRIPPLE = IOUT +2(IOUT)(ESRC2) (fOSC)(C2) Figure 2a. Ideal Switched Capacitor Model Figure 2b. Equivalent Output Resistance VOUT = – (VIN) C1 C2 TCM828/829 IN S1 S3 S4 S2 V+ V OUT R L C1 C2 f V+ V OUT R EQUIV R EQUIV = R L C2 f x C1 1 Equation 2. Equation 1. Equation 3. |
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