MAX1678EUA Maxim Integrated Products, MAX1678EUA Datasheet - Page 9

• Manufacturer Part Number: MAX1678EUA
• Description: DC/DC Switching Converters
• Manufacturer: Maxim Integrated Products

Specifications of MAX1678EUA

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Pulling the SHDN pin low places the MAX1678 in shut-
down mode (I
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
To disable the shutdown feature, connect SHDN (a
logic input) to BATT or OUT.
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.
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:
where V
detector, and V
fail comparator. Since PFI leakage is 10nA max, select
feedback resistor R4 in the 100kΩ to 1MΩ range.
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.
BATT
).
TH
is the desired threshold of the power-fail
SHDN
High-Efficiency, Step-Up DC-DC Converter
PFI
R3 = R4 x
_______________________________________________________________________________________
is the 614mV threshold of the power-
Reverse-Battery Protection
= 2µA typical). In shutdown, the
Power-Fail Comparator
V
V
PFI
TH
Damping Switch
Shutdown Mode
1
1-Cell to 2-Cell, Low-Noise,
Figure 4. Simplified Diagram of Damping Switch
Figure 5. LX Ringing Without Damping Switch (example only)
Figure 6. LX Ringing With Damping Switch
CIRCUIT
MAX1678
TIMING
DAMP
NDRV
PDRV
OUT
N
P
GND
2 s/div
2 s/div
P
DAMPING
SWITCH
V
V
L1 = 47 H
V
V
L1 = 47 H
BATT
BATT
OUT
BATT
OUT
LX
= 3.3V
= 3.3V
= 1.8V
= 2.5V
V
OUT
V
IN
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