Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
BATT
R1
1M
SHDN
ZERO-
CROSSING
MAX1795
MAX1796
MAX1797
LBO
LBI
AMPLIFIER
-
+
S
OUT
+
_
Q
OUT
BATT
LX
R
+
47µF
BODY
DIODE
CONTROL
0.85V
_
S
R
BATT
Q
Q
22µH
TIMER BLOCK
47µF
S
R
TOFF
MAX
TON MAX
START
OUT
R2
R3
+
FB
FB SELECT
REFERENCE
+
_
_
CURRENT-LIMIT
AMPLIFIER
ERROR
AMPLIFIER
+
_
GND
Figure 2. Functional Diagram
ance node. The LBI/LBO comparator remains active in
shutdown.
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.
As shown in Figure ±, the MAX±795/MAX±796/MAX±797
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 MAX±795/MAX±796/MAX±797 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-
Shutdown
The device enters shutdown when V
is high,
SHDN
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-
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9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
