Rainbow-electronics MAX1885 Manual do Utilizador descarregar pdf (Página 32)

MAX1778/MAX1880–MAX1885

Quad-Output TFT LCD DC-DC

Converters with Buffer

32 ______________________________________________________________________________________

regulator and charge-pump power ground plane,

and the inner layer should contain the analog

ground plane and power-ground plane/path for the

COM

buffer and LDO. Connect all three ground

planes together at one place near the PGND pin.

3) Locate all feedback resistive-dividers as close to

their respective feedback pins as possible. The

voltage-divider’s center trace should be kept short.

Avoid running any feedback trace near the LX

switching node or the charge-pump drivers. The

resistive-dividers’ ground connections should be to

analog ground (GND).

4) When using multilayer boards, separate the top sig-

nal layer and bottom signal layer with a ground

plane between to eliminate capacitive coupling

between fast-charging nodes on the top layer and

high-impedance nodes on the bottom layer. The

fast-charging nodes, such as the LX and charge-

pump driver nodes, should not have any other

traces or ground planes near by.

5) Keep the charge-pump circuitry as close to the IC

as possible, using wide traces and avoiding vias

when possible. Place 0.1µF ceramic bypass

capacitors near the charge-pump input pins (SUPP

and SUPN) to the PGND pin.

6) To maximize output power and efficiency and mini-

mize output ripple voltage, use extra wide, power

ground traces, and solder the IC’s power ground

pin directly to it.

Refer to the MAX1778/MAX1880-MAX1885 evaluation

kit for an example of proper board layout.

BUFFER OUTPUT

BUFOUT

= V

SUPB

POSITIVE

POS

= 20V

SHDN

RDY

SUPL

LDOOUT

SUPN

SUPP

DRVP

FBP

10µH

INPUT

= 5V

0.22µF

1µF

0.01µF

1.0µF

REF

0.22µF

0.1µF

BUF

1.0µF

COMP

470pF

OUT

(2) 10µF

0.1µF

(2) 4.7µF

LDOOUT

4.7µF

RDY

100kΩ

1.5kΩ

10kΩ

316kΩ

30kΩ

750kΩ

49.9kΩ

86.6kΩ

10kΩ

COMP

4.7kΩ

R10

100kΩ

49.9kΩ

16.4kΩ

NEGATIVE

NEG

= -8V

TO LOGIC

BUFOUT

BUF-

BUF+

FLTSET

GND

TGND

FBL

DRVN

FBN

REF

INTG

PGND

SUPB

MAX1778

LDO

= 3.3V

REF

MAIN

= 12V

Figure 8. 5V Input Monitor Application

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