Rainbow-electronics MAX1842 Manual do Utilizador descarregar pdf (Página 10)

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

10 ______________________________________________________________________________________

Synchronous Rectification

In a step-down regulator without synchronous rectifica-

tion, an external Schottky diode provides a path for cur-

rent to flow when the inductor is discharging. Replacing

the Schottky diode with a low-resistance NMOS syn-

chronous switch reduces conduction losses and

improves efficiency.

The NMOS synchronous-rectifier switch turns on follow-

ing a short delay after the PMOS power switch turns off,

thus preventing cross conduction or “shoot through.” In

470pF

2.2µF

1µF

10µF

10Ω

FBSEL

0.01µF

FEEDBACK

SELECTION

CURRENT

SENSE

PWM LOGIC

AND

DRIVERS

3.0V TO 5.5V

PGNDTOFF

TOFF

GND

NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.

REF

SUMMING

COMPARATOR

REF

COMP

SKIP

SHDN

TIMER

CURRENT

SENSE

OUT

MAX1742

MAX1842

Figure 2. Functional Diagram

TOFF

COMP

FBSEL

SHDN

PGND

GND

REF

MAX1742

TOFF

1µF

0.01µF

2.2µF

470pF

10Ω

INPUT

= 10µF (MAX1742)

= 33µF (MAX1842)

OUTPUT

OUT

= 47µF (MAX1742)

OUT

= 150µF (MAX1842)

OUT

= 2.5V, FBSEL = V

OUT

= 1.8V, FBSEL = REF

OUT

= 1.5V, FBSEL = FLOATING

Figure 1. Typical Circuit

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 16

MAX1742/MAX1842 1

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