MAX1720 Просмотр технического описания (PDF) - ON Semiconductor

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MAX1720

Switched Capacitor Voltage Inverter with Shutdown

ON Semiconductor 

MAX1720

1

6

OSC

Vin

+2

5

C3

3

4

−Vout

+C2

RL

+

C1

C1 = C2 = C3 = 10 mF

Figure 14. Test Setup/Voltage Inverter

DETAILED OPERATING DESCRIPTION

The MAX1720 charge pump converter inverts the voltage

applied to the Vin pin. Conversion consists of a two−phase

operation (Figure 15). 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.

S1

S2

Vin

C1

S3

From Osc

C2

S4

−Vout

Figure 15. Ideal Switched Capacitor Charge Pump

APPLICATIONS INFORMATION

Output Voltage Considerations

The MAX1720 performs voltage conversion but does 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 26 W nominal

at 25°C with Vin = 5.0 V. Vout is approximately −5.0 V at light

loads, and drops according to the equation below:

VDROP + Iout Rout

Vout + * (Vin * VDROP)

Charge Pump Efficiency

The overall power conversion 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 Equivalent

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.

These losses are given by Equation 1.

PLOSS(2,3,4) + Iout 2 Rout ^ Iout 2

ƪ ƫ 1

(fOSC)C1

)

8RSWITCH

)

4ESRC1

)

ESRC2

(eq. 1)

The 1/(fOSC)(C1) term in Equation 1 is the effective output

resistance of an ideal switched capacitor circuit (Figures 16

and 17).

The losses due to charge transfer above are also shown in

Equation 2. The output voltage ripple is given by Equation 3.

PLOSS + [ 0.5C 1 (Vin2 * Vout2)

) 0.5C2 (VRIPPLE2 * 2VoutVRIPPLE)]

fOSC

(eq. 2)

VRIPPLE

+

Iout

(fOSC)(C2)

)

2(Iout)(ESRC2)

(eq.

3)

f

Vin

Vout

C1

C2

RL

Figure 16. Ideal Switched Capacitor Model

REQUIV

Vin

REQUIV + f

1

C1

Vout

C2

RL

Figure 17. Equivalent Output Resistance

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