Rainbow-electronics MAX1885 Manual do Utilizador descarregar pdf (Página 21)

MAX1778/MAX1880–MAX1885

Quad-Output TFT LCD DC-DC

Converters with Buffer

______________________________________________________________________________________ 21

The switching waveforms will appear noisy and asyn-

chronous when light loading causes pulse-skipping

operation; this is a normal operating condition that

improves light-load efficiency.

Dual Charge-Pump Regulator (MAX1778/

MAX1880/MAX1881/MAX1882 Only)

The MAX1778/MAX1880/MAX1881/MAX1882 con-

trollers contain two independent low-power charge

pumps (Figure 4). One charge pump inverts the input

voltage and provides a regulated negative output volt-

age. The second charge pump doubles the input volt-

age and provides a regulated positive output voltage.

The controllers contain internal P-channel and N-chan-

nel MOSFETs to control the power transfer. The

internal MOSFETs switch at a constant frequency

(fCHP = fOSC/2).

Positive Charge Pump

During the first half-cycle, the N-channel MOSFET turns

on and charges flying capacitor C

X(POS)

(Figure 4).

This initial charge is controlled by the variable N-chan-

nel on-resistance. During the second half-cycle, the N-

channel MOSFET turns off and the P-channel MOSFET

turns on, level shifting C

X(POS)

by V

SUPP

volts. This

connects C

X(POS)

in parallel with the reservoir capaci-

tor C

OUT(POS)

. If the voltage across C

OUT(POS)

plus a

diode drop (V

POS

+ V

DIODE

) is smaller than the level-

shifted flying capacitor voltage (V

CX(POS)

+ V

SUPP

charge flows from C

X(POS)

to C

OUT(POS)

until the diode

(D3) turns off.

INDUCTOR CURRENT

LOAD

OFF

TIME

PEAK

Figure 3. Discontinuous-to-Continuous Conduction Crossover

Point

MAX1778

MAX1880

MAX1881

MAX1882

NEG

= - V

REF

= 1.25V

( )

OUT(NEG)

X(NEG)

SUPN

2.7V TO 13V

OSC

REF

PGNDGND

SUPN

DRVN

FBN

SUPP

DRVP

FBP

POS

= 1 + V

REF

= 1.25V

( )

SUPP

2.7V TO 13V

SUPD

REF

0.22µF

REF

1.25V

X(POS)

OUT(POS)

POS

NEG

Figure 4. Low-Power Charge Pump Block Diagram

1 2 ... 16 17 18 19 20 21 22 23 24 25 26 ... 37 38

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