Shutdown Mode

Pulling the SHDN pin low places the MAX1678 in shut-

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

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 5

Ω

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:

fail comparator. Since PFI leakage is 10nA max, select

feedback resistor R4 in the 100k

Ω 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.

R3 = R4 x

V

V

TH

PFI

−













1

1-Cell to 2-Cell, Low-Noise,

High-Efficiency, Step-Up DC-DC Converter

_______________________________________________________________________________________

9

MAX1678

PDRV

DAMP

NDRV

TIMING

CIRCUIT

OUT

BATT

LX

DAMPING

SWITCH

GND

P

P

N

Figure 4. Simplified Diagram of Damping Switch

2

µs/div

L1 = 47

µH

Figure 5. LX Ringing Without Damping Switch (example only)

Figure 6. LX Ringing With Damping Switch

2

µs/div

L1 = 47

µH