Rainbow-electronics MAX1664 Instrukcja Użytkownika Pobierz pdf (Strona 11)

MAX1664

Active-Matrix Liquid Crystal Display

(AMLCD) Supply

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Inductor Selection

The optimum inductor value for L1 is 3.3µH, as shown

in Figure 4. Inductors with less than 300mΩ DC series

resistance are recommended to achieve the highest

efficiency. Using a larger value for L1 (e.g., 4.7µH)

increases the output current capability of DC-DC 1 (by

reducing the peak ripple current) at the expense of size

and the additional output filter capacitance needed for

loop stability.

For DC-DC 2, at large input voltages (i.e., 5V) and low

switching frequencies (i.e., ≤400kHz), the value of L2

should be increased (e.g., 6.8µH or 10µH) to limit the

peak current. In some cases it may be necessary to

reduce the value of L2 to increase the output current

capability of DC-DC 2 (Table 2). The relationship between

input voltage, output voltage, switching frequency, induc-

tor value, and maximum load current for DC-DC 2 is com-

plex and nonlinear. This relationship is summarized in

Table 2. The L2 equation is as follows:

where:

Internal MOSFET on-resistance:

ON(LX2P)

= R

ON(LX2N)

= 0.9Ω typical

External inductor DC resistance:

= 0.3Ω typical

Inductor peak current:

PEAK

= 700mA (750mA absolute maximum)

Due to the MAX1664’s high switching frequency, induc-

tors with a high-frequency core material such as ferrite

are recommended. Powdered iron compounds are not

recommended due to their higher core losses. Typical

small-size, low-profile inductors include the ILS-3825

(Dale Electronics-Vishay) and the CLQ61B (Sumida).

These inductors are primarily used for DC-DC

converters with low height requirements. See Table 3

for more information on manufacturers who provide

low-profile inductors.

V - R R R

(I )

I x 2 f

INP ON(LX2P) ON(LX2N) L2

PEAK

PEAK DC-DC 1

+ +

[ ]

( )

REF

FB2-

IN INP

LX2P

LX2N

FB2+

49.9k

549k

200k

49.9k

4.7µH

PGND2

PLLC

GND

FPLL

RDY

REF

SHDN

OFF

OUT1

5.5V

2.2µF

3 x 10µF

0.22µF

22nF

100k

2.2nF

0.22µF

0.47µF

50pF

2 x 10µF

10µF

10µF2.2µF

33Ω

0.47µF

3.3µH

301k

100k

OUT2-

-5V

OUT2+

15V

BPV

BACKPLANE

DRIVER

SUPPLY

2.8V TO 5.5V

BPDRV

BPV

PGND1

FB1

LX1

BPCLK

MAX1664

Figure 4. Detailed Typical Operating Circuit

1 2 ... 6 7 8 9 10 11 12 13 14 15 16

(AMLCD) Supply 1

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