Rainbow-electronics MAX1708 Manual do Utilizador descarregar pdf (Página 11)

MAX1708

High-Frequency, High-Power, Low-Noise,

Step-Up DC-DC Converter

______________________________________________________________________________________ 11

__________________Design Procedure

Inductor Selection (L1)

The MAX1708’s high switching frequency allows the

use of a small-size inductor. Use a 2.2µH inductor for

600kHz operation. If the MAX1708 is synchronized at a

different frequency, scale the inductor value with the

inverse of frequency (L

= 2.2µH

600kHz / f

SYNC

The PWM design tolerates inductor values within ±25%

of this calculated value, so choose the closest standard

inductor value. For example, use 3.3µH for 350kHz and

1.5µH for 1MHz).

Inductors with a ferrite core or equivalent are recom-

mended; powder iron cores are not recommended for

use at high switching frequencies. Ensure the induc-

tor’s saturation rating (the current at which the core

begins to saturate and inductance falls) exceeds the

internal current limit. Note that this current may be

reduced through SS/LIM if less than the MAX1708’s full

load current is needed (see Electrical Characteristics

for ratings). For highest efficiency, use a coil with low

DC resistance, preferably under 20mΩ. To minimize

radiated noise, use a toroid, pot core, or shielded

inductor. See Tables 3 and 4 for a list of recommended

components and component suppliers. To calculate

the maximum output current (in amperes), use the fol-

lowing equation:

where:

= input voltage

= forward voltage drop of the Schottky diode

at I

LIM

OUT

= output voltage

D' = (V

) / (V

OUT

+ V

), neglecting switch voltage

drop

f = switching frequency

L1 = inductor value

LIM

= minimum value of switch current limit from

Electrical Characteristics or set by R1 of

Figure 1.

IDID

VVV

OUT MAX LIM

OUT D IN

()

''=−

+−

×ƒ×

























SUPPLIER PHONE FAX

Central 631-435-1110 631-435-1824

Coilcraft 847-639-6400 847-639-1489

Coiltronics 561-241-7876 561-241-9339

Motorola 602-303-5454 602-994-6430

Panasonic 714-373-7939 714-373-7183

Table 4. Component Suppliers

Table 3. Component Selection Guide

PRODUCTION INDUCTORS CAPACITORS DIODES

Coiltronics UP2B-2R2 Sanyo 6TPC100M Motorola MBRD1035CTL

Surface mount

Coilcraft DO3316P-222HC Panasonic EEFUE0J151R Central CMSH5-20

Figure 3. Simplified PWM Controller Functional Diagram

PGND

11mΩ

REF

SLOPE

COMP

SS/LIM

12.5

OSCILLATOR

(LIMITED TO 100mV)

Figure 4. Adjustable Output Voltage

KEEP SHORT

OUT

MAX1708

1 2 ... 6 7 8 9 10 11 12 13

Step-Up DC-DC Converter 1

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