Datasheet MAX1682, MAX1683 (Maxim) - 5

Switched-Capacitor Voltage Doublers MAX1682/MAX1683 _____________________Pin Description Efficiency Considerations
The power efficiency of a switched-capacitor voltage converter is affected by three factors: the internal losses
PIN NAME FUNCTION
in the converter IC, the resistive losses of the capacitors, 1 GND Ground and the conversion losses during charge transfer between the capacitors. The total power loss is: Doubled Output Voltage. Connect C2 2 OUT between OUT and GND. ΣP = P LOSS INTERNAL LOSSES Negative Terminal of the Flying + PPUMP CAPACITOR LOSSES 3 C1- Capacitor + PCONVERSION LOSSES 4 IN Input Supply The internal losses are associated with the IC’s internal Positive Terminal of the Flying 5 C1+ functions, such as driving the switches, oscillator, etc. Capacitor These losses are affected by operating conditions such
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as input voltage, temperature, and frequency. The next two losses are associated with the voltage The MAX1682/MAX1683 capacitive charge pumps converter circuit’s output resistance. Switch losses double the voltage applied to their input. Figure 1 occur because of the on-resistance of the MOSFET shows a simplified functional diagram of an ideal volt- switches in the IC. Charge-pump capacitor losses age doubler. During the first half-cycle, switches S1 occur because of their ESR. The relationship between and S2 close, and capacitor C1 charges to VIN. During these losses and the output resistance is as follows: the second half cycle, S1 and S2 open, S3 and S4 close, and C1 is level shifted upward by V P + P = IN volts. This PUMP CAPACITOR LOSSES SWITCH LOSSES connects C1 to the reservoir capacitor C2, allowing 2 energy to be delivered to the output as necessary. The I x R OUT OUT actual voltage is slightly lower than 2 x VIN, since 1 R ≅ R 2 4ESR switches S1–S4 have resistance and the load drains OUT SWITCHES C1 f x C ( OSC) + + 1 charge from C2. + ESRC2
Charge-Pump Output
The MAX1682/MAX1683 have a finite output resistance where fOSC is the oscillator frequency. The first term is of about 20Ω (Table 2). As the load current increases, the effective resistance from an ideal switched- the devices’ output voltage (VOUT) droops. The droop capacitor circuit (Figures 2a and 2b). equals the current drawn from VOUT times the circuit’s output impedance (RS), as follows: f VDROOP = IOUT x RS V+ VOUT VOUT = 2 x VIN - VDROOP C2 RL C1 S1 S3 VIN Figure 2a. Switched-Capacitor Model C1 VOUT REQUIV C2 S2 S4 V+ VOUT 1 REQUIV = f × C1 C2 RL VIN Figure 1. Simplified Functional Diagram of Ideal Voltage Figure 2b. Equivalent Circuit Doubler
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