Rainbow Electronics MAX1797 - Manual (Page 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
Q
S
R
+
_
+
_
+
_
Q
S
R
Q
S
R
+
_
+
_
FB SELECT
OUT
START
TIMER BLOCK
LBO
LBI
R1
1M
TON MAX TOFF
MAX
R2
FB
ERROR
AMPLIFIER
CURRENT-LIMIT
AMPLIFIER
BATT
BATT
BODY
DIODE
CONTROL
OUT
R3 REFERENCE
22µH
47µF
47µF
GND
LX
BATT
OUT
SHDN
0.85V
-
+
ZERO-
CROSSING
AMPLIFIER