Rainbow Electronics MAX1678 - Manual (Page 7)

MAX1678
1-Cell to 2-Cell, Low-Noise,
High-Efficiency, Step-Up DC-DC Converter
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Detailed Description
The MAX1678 consists of an internal 1
Ω
, N-channel
MOSFET power switch, a built-in synchronous rectifier
that acts as the catch diode, a reference, PFM control
circuitry, and an inductor damping switch (Figure 1).
The device is optimized for applications that are pow-
ered by 1 to 3-cell alkaline, NiMH, or NiCd batteries, or
a 1-cell lithium battery such as pagers, remote controls,
and battery-powered instruments. They are designed to
meet the specific demands of the operating states
characteristic of such systems:
1) Primary battery is good and load is active: In this
state the load draws tens of milliamperes and the
MAX1678 typically offers 80% to 90% efficiency.
2) Primary battery is good and load is sleeping: In this
state the load draws hundreds of microamperes and
the DC-DC converter IC draws very low quiescent
current. Many applications maintain the load in this
state most of the time.
3) Primary battery is dead and DC-DC converter is
shut down:
In this state the load is sleeping or sup-
plied by the backup battery, and the MAX1678
draws 0.1µA current from the OUT pin.
4) Primary and backup battery dead: The DC-DC con-
verter can restart from this condition.
BATT
PFI
0.5REF
PFO
GND
OUT
REF
N
P
OUT 1.7V
FB
SHDN
LX
RFRDY
1.23V REF
REF
START-UP COMPARATOR
0.5REF
START-UP
OSCILLATOR
EN
DAMP TON TOFF PDRV
CONTROL LOGIC
MAX1678
t ON = K/V BATT
BACKUP t
OFF
TIMER
DAMPING
SWITCH
ZERO-CROSSING
DETECTION
NDRV
Figure 1. Functional Diagram