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TC1220ECH Folha de dados(PDF) 5 Page - Microchip Technology |
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TC1220ECH Folha de dados(HTML) 5 Page - Microchip Technology |
5 / 16 page 2002 Microchip Technology Inc. DS21366B-page 5 TC1219/TC1220 4.0 APPLICATIONS INFORMATION 4.1 Output Voltage Considerations The TC1219/TC1220 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 approxi- mately 25 Ω nominal at +25°C and VIN = +5V. VOUT is approximately -5V at light loads, and droops according to the equation below: VDROP = IOUT x ROUT VOUT = – (VIN – VDROP) 4.2 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 the 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 4-1(b). EQUATION 4-1: The 1/(fOSC)(C1) term in Equation 4-1(b) is the effective output resistance of an ideal switched capacitor circuit (Figure 4-1 and Figure 4-2). The value of RSWITCH can be approximated at 0.5Ω for the TC1219/TC1220. The remaining losses in the circuit are due to factor (4) above, and are shown in Equation 4-2. The output voltage ripple is given by Equation 4-3. EQUATION 4-2: EQUATION 4-3: FIGURE 4-1: IDEAL SWITCHED CAPACITOR MODEL FIGURE 4-2: EQUIVALENT OUTPUT RESISTANCE a) PLOSS (2, 3) = IOUT2 x ROUT b) where ROUT = [ 1 / [fOSC(C1) ] + 8RSWITCH + 4ESRC1 + ESRC2 ] PLOSS(4) = [(0.5)(C1)(VIN2 – VOUT2)+(0.5) (C2)(VRIPPLE2 – 2VOUT VRIPPLE) ] xfOSC VRIPPLE =[ IOUT /2 x ( fOSC)(C2)] + 2 ( IOUT) (ESRC2) V+ VOUT RL C2 C1 f V+ VOUT REQUIV REQUIV = RL C2 f x C1 1 |
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