Rainbow-electronics MAX1885 Manual do Utilizador descarregar pdf (Página 27)

MAX1778/MAX1880–MAX1885

Quad-Output TFT LCD DC-DC

Converters with Buffer

______________________________________________________________________________________ 27

Rectifier Diode

Use a Schottky diode with an average current rating

equal to or greater than the peak inductor current, and

a voltage rating at least 1.5 times the main output volt-

age (V

MAIN

Charge Pumps (MAX1778/ MAX1880/

MAX1881/MAX1882 Only)

Selecting the Number of Charge-Pump Stages

The number of charge-pump stages required to regu-

late the output voltage depends on the supply voltage,

output voltage, load current, switching frequency, the

diode’s forward voltage drop, and ceramic capacitor

values.

For positive charge-pump outputs, the number of

required stages may be determined by:

where V

SUPD

is the positive charge-pump diode supply

(Figure 4), V

DIODE

is the diode’s forward voltage drop,

and R

is the charge pump’s output impedance. The

charge pump’s output impedance may be approximat-

ed using the following equation:

where the charge pump’s switching frequency (f

CHP

) is

equal to 0.5 x f

OSC

, the P-channel MOSFET’s on-resis-

tance (R

PCH(ON)

) is 10Ω, and the N-channel MOSFET’s

on-resistance (R

NCH(ON

)) is 4Ω (see Electrical

Characteristics).

For negative charge pump outputs, the number of

required stages may be determined by:

where N

NEG

is rounded up to the nearest integer.

VVRI

NEG

SUPN DROP TX LOAD

.( )

≥













-112

RR R

TX PCH ON NCH ON

X CHP

OUT CHP

( )

() ()

=++

























VVRI

POS

POS SUPD

SUPP DIODE TX LOAD

.( )

≥













-112

CIRCUIT #1 CIRCUIT #2 CIRCUIT #3 CIRCUIT #4 CIRCUIT #5

3.3V 3.3V 3.3V 5V 5V

MAIN

9V 9V 9V 12V 12V

MAIN(MAX)

100mA 200mA 200mA 220mA 220mA

NEG

-5V -5V -5V -5V -5V

NEG

2mA 5mA 5mA 5mA 5mA

POS

24V 24V 24V 24V 24V

POS

2mA 5mA 5mA 5mA 5mA

L2.2µH4.7µH4.7µH6.8µH6.8µH

PEAK

>1A >1A >1A >1A >1A

OUT

4.7µF10µF20µF10µF20µF

R1 309kΩ 309kΩ 309kΩ 429kΩ 429kΩ

R2 49.9kΩ 49.9kΩ 49.9kΩ 49.9kΩ 49.9kΩ

COMP

None None 39kΩ* None 20kΩ*

COMP

None None 100pF* None 200pF*

Table 1. MAX1778/MAX1880/MAX1883 Component Values (f

OSC

= 1MHz)

COMP

and C

COMP

are connected between the step-up converter’s output (V

MAIN

) and FB.

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 37 38

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