Rainbow-electronics MAX1797 Bedienungsanleitung PDF herunterladen (Seite 9)

Synchronous Rectification

The internal synchronous rectifier eliminates the need

for an external Schottky diode, reducing cost and

board space. During the cycle off-time, the P-channel

MOSFET turns on and shunts the MOSFET body diode.

As a result, the synchronous rectifier significantly

improves efficiency without the addition of an external

component. Conversion efficiency can be as high

as 95%, as shown in the Typical Operating Char-

acteristics.

Shutdown

The device enters shutdown when V

SHDN

is high,

reducing supply current to less than 2µA. During shut-

down, the synchronous rectifier disconnects the output

from the input, eliminating the DC conduction path that

normally exists with traditional boost converters in shut-

down mode. In shutdown, OUT becomes a high-imped-

ance node. The LBI/LBO comparator remains active in

shutdown.

As shown in Figure 1, the MAX1795/MAX1796/MAX1797

may be automatically shut down when the input voltage

drops below a preset threshold by connecting LBO to

SHDN (see Low-Battery Detection section).

BATT/Damping Switch

The MAX1795/MAX1796/MAX1797 each contain an

internal damping switch to minimize ringing at LX. The

damping switch connects a resistor across the inductor

when the inductor’s energy is depleted (Figure 3).

Normally, when the energy in the inductor is insufficient

to supply current to the output, the capacitance and

inductance at LX form a resonant circuit that causes

ringing. The ringing continues until the energy is dissi-

pated through the series resistance of the inductor.

The damping switch supplies a path to quickly dissi-

MAX1795/MAX1796/MAX1797

Low Supply Current, Step-Up DC-DC Converters

with True-Shutdown

_______________________________________________________________________________________ 9

Figure 2. Functional Diagram

MAX1795

MAX1796

MAX1797

FB SELECT

OUT

START

TIMER BLOCK

LBO

LBI

TON MAX

TOFF

MAX

ERROR

AMPLIFIER

CURRENT-LIMIT

AMPLIFIER

BATT

BODY

DIODE

CONTROL

OUT

REFERENCE

22µH

47µF

GND

BATT

OUT

SHDN

0.85V

ZERO-

CROSSING

AMPLIFIER

1 2 3 4 5 6 7 8 9 10 11 12 13

MAX1795/MAX1796/MAX1797 1

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