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MAX828 Datasheet(PDF) 3 Page  ON Semiconductor 

MAX828 Datasheet(HTML) 3 Page  ON Semiconductor 
3 / 12 page MAX828 MAX829 http://onsemi.com 3 DETAILED OPERATING DESCRIPTION The MAX828/829 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. Figure 1. Ideal Switched Capacitor Charge Pump S3 S4 C2 C1 S1 S2 IN Vout = –(Vin) MAX828/829 APPLICATIONS INFORMATION Output Voltage Considerations The MAX828/829 perform voltage conversion but do not provide regulation. The output voltage will drop in a linear manner with respect to load current. The value of this equivalent output resistance is approximately 25 W nominal at +25 °C and V IN = +5V. VOUT is approximately  5V at light loads, and droops according to the equation below: VDROP = IOUT x ROUT VOUT = – (VIN – VDROP) 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 given by Equation 1. P LOSS(2,3,4) + I OUT 2 R OUT ^ I OUT 2 Equation 1. 1 (f OSC )C1 ) 8R SWITCH ) 4ESR C1 ) ESR C2 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. P LOSS(4) + (0.5)(C1)( V IN 2 * V OUT 2) ) (0.5)(C2) Equation 2. (V RIPPLE 2 * 2V OUT V RIPPLE ) f OSC V RIPPLE + I OUT (f OSC )(C2) ) 2(I OUT )(ESR C2 ) Equation 3. Figure 2a. Ideal Switched Capacitor Model RL C2 C1 V+ Vout f Figure 2b. Equivalent Output Resistance RL C2 V+ Vout REQUIV R EQUIV + 1 f C1 
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