MAX1678 Datasheet, PDF (9/12 Pages) Maxim Integrated Products – 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter

1-Cell to 2-Cell, Low-Noise,
High-Efficiency, Step-Up DC-DC Converter
Shutdown Mode
Pulling the SHDN pin low places the MAX1678 in shut-
down mode (I_SHDN = 2µA typical). In shutdown, the
internal switching MOSFET turns off, PFO goes high
impedance, and the synchronous rectifier turns off to
prevent the flow of reverse current from the output back
to the input. However, there is still a forward current
path through the synchronous-rectifier body diode from
the input to the output. Thus, in shutdown, the output
remains one diode drop below the battery voltage
(V_BATT).
To disable the shutdown feature, connect SHDN (a
logic input) to BATT or OUT.
Reverse-Battery Protection
The MAX1678 can sustain/survive battery reversal up to
the package power-dissipation limit. An internal 5Ω
resistor in series with a diode limits reverse current to
less than 220mA, preventing damage. Prolonged oper-
ation above 220mA reverse-battery current can
degrade the device’s performance.
Power-Fail Comparator
The MAX1678 has an on-chip comparator for power-fail
detection. This comparator can detect a loss of power
at the input or output (Figures 7 and 8). If the voltage at
the power-fail input (PFI) falls below 614mV, the PFO
output sinks current to GND. Hysteresis at PFI is 2%.
The power-fail monitor threshold is set by two resistors,
R3 and R4, using the following equation:
R3
=
R4
x


V_TH
V_PFI
−

1_
where V_TH is the desired threshold of the power-fail
detector, and V_PFI is the 614mV threshold of the power-
fail comparator. Since PFI leakage is 10nA max, select
feedback resistor R4 in the 100kΩ to 1MΩ range.
Damping Switch
The MAX1678 is designed with an internal damping
switch to minimize ringing at the LX node. The damping
switch (Figure 4) connects the LX node to BATT, effec-
tively depleting the inductor’s remaining energy. When
the energy in the inductor is insufficient to supply cur-
rent to the output, the capacitance and inductance at
LX form a resonant circuit that causes ringing. The
damping switch supplies a path to quickly dissipate
this energy, suppressing the ringing at LX. This does
not reduce the output ripple, but does reduce EMI.
Figures 5 and 6 show the LX node voltage waveform
without and with the damping switch.
MAX1678
PDRV
TIMING
CIRCUIT
DAMP
NDRV
V_OUT
V_IN
OUT
BATT
P
DAMPING
SWITCH
P
LX
N
GND
Figure 4. Simplified Diagram of Damping Switch
V_BATT = 2.5V
V_OUT = 3.3V
L1 = 47µH
2µs/div
Figure 5. LX Ringing Without Damping Switch (example only)
V_BATT = 1.8V
V_OUT = 3.3V
L1 = 47µH
2µs/div
Figure 6. LX Ringing With Damping Switch
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