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MAX1678 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1678 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page Shutdown Mode Pulling the SHDN pin low places the MAX1678 in shut- down mode (ISHDN = 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 (VBATT). 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: where VTH is the desired threshold of the power-fail detector, and VPFI is the 614mV threshold of the power- 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 VOUT VIN BATT LX DAMPING SWITCH GND P P N Figure 4. Simplified Diagram of Damping Switch 2 µs/div VBATT = 2.5V VOUT = 3.3V L1 = 47 µH Figure 5. LX Ringing Without Damping Switch (example only) Figure 6. LX Ringing With Damping Switch 2 µs/div VBATT = 1.8V VOUT = 3.3V L1 = 47 µH |
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