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TCM828ECT Arkusz danych(PDF) 13 Page - Microchip Technology |
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TCM828ECT Arkusz danych(HTML) 13 Page - Microchip Technology |
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13 / 26 page ![]() © 2010 Microchip Technology Inc. DS21488B-page 13 TCM828/TCM829 5.0 APPLICATIONS INFORMATION 5.0.1 OUTPUT VOLTAGE CONSIDERATIONS The TCM828/TCM829 devices 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 ×ROUT VOUT =–(VIN –VDROOP) 5.0.2 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, 3 and 4 above. These losses are shown in Equation 5-1. EQUATION 5-1: The 1/(fOSC)(C1) term in Equation 5-1 is the effective output resistance of an ideal switched capacitor circuit (Figures 5-1 and 5-2). The losses in the circuit due to factor 4 above are also shown in Equation 5-2. The output voltage ripple is shown in Equation 5-3. EQUATION 5-2: EQUATION 5-3: FIGURE 5-1: Ideal Switched Capacitor Model. FIGURE 5-2: Equivalent Output Resistance. I OUT 2 1 f OS C ()C1 -------------------------- 8R SWITCH 4ES R C1 ESR C2 ++ + × ≅ P LOSS 234 ,, () I OUT 2 R OUT × = 2V OUT V RIP PLE ] – f × OSC P LOSS 4 () 0.5 () C1 () V ( IN 2 V OUT 2 ) 0.5 () C2 () V RIPPLE 2 – ( ++ [ = V RIPPLE I OUT f OSC () C2 () ---------------------------- 2 I OUT () ESR C2 () + = V+ VOUT RL C1 C2 f V+ VOUT REQUIV RL C2 R EQUIV 1 fC1 × --------------- = |
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