MAX1795 Просмотр технического описания (PDF) - Maxim Integrated

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MAX1795

Low-Supply Current, Step-Up DC-DC Converters with True Shutdown

Maxim Integrated 

MAX1795/MAX1796/

MAX1797

Low-Supply Current, Step-Up DC-DC Converters

with True Shutdown

BATT

R1

1M

SHDN

LBO

LBI

+

_

0.85V

MAX1795

MAX1796

MAX1797

TIMER BLOCK

START

OUT

TON MAX TOFF

MAX

R2

FB

FB SELECT

R3

REFERENCE

ERROR

AMPLIFIER

S

Q

R

ZERO-

CROSSING

AMPLIFIER

S

Q

R

S

Q

R

CURRENT-LIMIT

AMPLIFIER

OUT

OUT

BODY

DIODE

CONTROL

47µF

BATT

22µH

BATT

47F

LX

GND

Figure 2. Functional Diagram

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 Characteristics section.

Shutdown

The device enters shutdown when VSHDN 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

shutdown mode. In shutdown, OUT becomes a high-

impedance node. The LBI/LBO comparator remains

active in shutdown.

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

can be automatically shut down when the input voltage

drops below a preset threshold by connecting LBO to

SHDN (see the Low-Battery Detection section).

BATT/Damping Switch

The MAX1795/MAX1796/MAX1797 each contain an inter-

nal damping switch to minimize ringing at LX. The damp-

ing 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 dissipated through the series

resistance of the inductor. The damping switch supplies a

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