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

Step-Up DC-DC Converter

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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

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 20m

Ω. 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:

drop

f = switching frequency

L1 = inductor value

Electrical Characteristics or set by R1 of

Figure 1.

ID I

D

VV

V

L

OUT MAX

LIM

OUT

D

IN

()

''

=−

+−

׃ ×

















21

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

N

R

S

Q

LX

PGND

11m

Ω

REF

SLOPE

COMP

FB

SS/LIM

12.5

OSCILLATOR

(LIMITED TO 100mV)

Figure 4. Adjustable Output Voltage

R4

R3

KEEP SHORT

FB

LX

MAX1708