Rainbow-electronics MAX865 Manual do Utilizador descarregar pdf (Página 6)

converter (RS+), where I

LOAD+

is the combination of I

V-

and the external load on V+ (I

V+

Determine V+ and V- as follows:

The output resistance for the positive and negative

charge pumps are tested and specified separately. The

positive charge pump is tested with V- unloaded. The

negative charge pump is tested with V+ supplied from

an external source, isolating the negative charge

pump.

Current draw from either V+ or V- is supplied by the

reservoir capacitor alone during one half cycle of the

clock. Calculate the resulting ripple voltage on either

output as follows:

where I

LOAD

is the load on either V+ or V-. For the typi-

cal f

PUMP

of 30kHz with 3.3µF reservoir capacitors, the

ripple is 25mV when I

LOAD

is 5mA. Remember that, in

most applications, the total load on V+ is the V+ load

current (I

V+

) and the current taken by the negative

charge pump (I

V-

Efficiency Considerations

Theoretically, a charge-pump voltage multiplier can

approach 100% power efficiency under the following

conditions:

• The charge-pump switches have virtually no offset

and extremely low on-resistance.

• The drive circuitry consumes minimal power.

• The impedances of the reservoir and pump capaci-

tors are negligible.

For the MAX865, the energy loss per clock cycle is the

sum of the energy loss in the positive and negative

converters, as follows:

The average power loss is simply:

Resulting in an efficiency of:

MAX865

C1+

C1-

GND

3.3µF

3.3µF 3.3µF

OUT+

OUT-

C2-

V-

V+

GND

MAX865

C1+

C1-

C2+

C2-

V-

V+

GND

Figure 3. Paralleling MAX865s

MAX865

Compact, Dual-Output Charge Pump

6 _______________________________________________________________________________________

V = I x RS+= I + I x RS+

DROOP+ LOAD+ V+ V-

( )

P = LOSS x f

LOSS CYCLE PUMP

η = −

( )

Total Output Power Total Output Power P

LOSS

V = I (1 / f ) (1 / C )

RIPPLE

LOAD PUMP RESERVOIR

V+ = 2V - V

V- = (V+ - V )=-(2V - V - V )

IN DROOP+

DROOP IN DROOP+ DROOP-

LOSS = LOSS + LOSS

= C1

CYCLE POS NEG

V V V

C V V

(

)

− +

(

)

(

)













+ +

(

)

− −

(

)













2 2

1 2 3 4 5 6 7 8

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