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TC1225EUA Arkusz danych(PDF) 4 Page - Microchip Technology |
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TC1225EUA Arkusz danych(HTML) 4 Page - Microchip Technology |
4 / 9 page 4 Inverting Dual (–VIN, –2VIN) Charge Pump Voltage Converters TC1225 TC1226 TC1227 TC1225/6/7-1 3/24/00 © 2001 Microchip Technology Inc. DS21369A The internal switch resistance for the first stage (i.e. RSWITCH1) is approximately 3Ω and the switch resistance for the second stage (i.e. RSWITCH2) is approximately 7Ω. The losses in the circuit due to factor (4) above are also shown in Equation 2a for stage 1 and Equation 2b for stage 2. The output voltage ripple for stage 1 is given by Equation 3a and the output voltage ripple for stage 2 is given by Equation 3b. PLOSS1 (4) = [ (0.5)(C1)(VIN 2 – VOUT12 ) + (0.5) (COUT1) (VRIPPLE1 2 - 2VOUT1 VRIPPLE1) ] x fOSC Equation 2a. PLOSS2 (4) = [ (0.5) (C2) (VIN 2 – VOUT22 ) + (0.5) (COUT2) (VRIPPLE2 2 - 2VOUT2 VRIPPLE2) ] x fOSC Equation 2b. VRIPPLE1 = [ IOUT1 / (fOSC) (COUT1) ] + 2 (IOUT1) (ESRCOUT1) Equation 3a. VRIPPLE2 = [ IOUT2 / (fOSC) (COUT2) ] + 2 (IOUT2) (ESRCOUT2) Equation 3b. Capacitor Selection In order to maintain the lowest output resistance and output ripple voltage, it is recommended that low ESR capacitors be used. Additionally, larger values of C1 and C2 will lower the output resistance and larger values of COUT1 and COUT2 will reduce output ripple. (See Equations 1a, 1b, 3a, and 3b). NOTE: For proper charge pump operation, C1 and COUT1 must have a voltage rating greater than or equal to VIN, while C2 and COUT2 must have a voltage rating greater than or equal to 2VIN. Table 1a shows various values of C1 and the corre- sponding output resistance values for VIN=5V @ +25°C for stage 1 and Table 1b shows various values of C2 and the corresponding output resistance values for VIN=5V @ +25°C for stage 2. It assumes a 0.1 Ω ESRC1, a 0.1Ω ESRC2, a 3Ω RSWITCH1, and a 7Ω RSWITCH2. Table 2a shows the output voltage ripple for various values of COUT1 and Table 2b shows the output voltage ripple for various values of COUT2 (again assuming VIN=5V @ +25 oC). The V RIPPLE1 values assume a 3mA output load current for stage 1 and a 0.1 Ω ESRCOUT1. The VRIPPLE2 values assume a 200uA output load current for stage 2 and a 0.1 Ω ESRCOUT1. Input Supply Bypassing TheVIN input should be capacitively bypassed to reduce AC impedance and minimize noise effects due to the switch- ing internal to the device. It is recommended that a large value capacitor (at least equal to C1) be connected from VIN to GND for optimal circuit performance. Table 1a. Output Resistance vs. C1 (ESR = 0.1 Ω). For Stage 1 C1 ( µF) TC1225 R OUT ( Ω) TC1226 R OUT ( Ω) TC1227 R OUT ( Ω) 0.47 202 85 42 1 108 53 33 3.3 50 33 27 Table 1b. Output Resistance vs. C2 (ESR = 0.1 Ω). For Stage 2 C2 ( µF) TC1225 ROUT (Ω) TC1226 ROUT (Ω) TC1227 ROUT (Ω) 0.1 890 342 137 0.47 239 117 74 1 140 85 65 Table 2a. Output Voltage Ripple vs. COUT1 (ESR = 0.1Ω) For Stage 1 (IOUT1 = 3mA) COUT1 TC1225 VRIPPLE1 TC1226 VRIPPLE1 TC1227 VRIPPLE1 ( µF) (mV) (mV) (mV) 0.47 533 183 52 1 251 86 25 3.3 76 27 8 Table 2b. Output Voltage Ripple vs. COUT2 (ESR = 0.1Ω) For Stage 2 (IOUT2 = 200 µA) COUT2 TC1225 VRIPPLE2 TC1226 VRIPPLE2 TC1227 VRIPPLE2 ( µF) (mV) (mV) (mV) 0.1 167 57 16 0.47 36 12 3.4 1 17 5.8 1.6 |
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