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FP6711 Datasheet(PDF) 10 Page - Fitipower Integrated Technology Inc. |
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FP6711 Datasheet(HTML) 10 Page - Fitipower Integrated Technology Inc. |
10 / 13 page ![]() 10 FP6711-1.4-DEC-2011 FP6711 85T fitipower integrated technology lnc. Application Information (Continued) Auto-Discharge The auto-discharge function is useful for applications where the supply voltage of a µC, µP, or memory has to be removed during shutdown in order to ensure a defined state of the system. The auto-discharge function will be enabled when the ADEN is set high; and it will be disabled when the ADEN is set to GND. When the auto-discharge function is enabled, the output capacitor will be discharged after the device is shut down by setting EN to GND. The capacitors connected to the output are discharged by an integrated switch of 300Ω, hence the discharge time depends on the total output capacitance. The residual voltage on VOUT is less than 0.4V after auto-discharge. The resistive divider scales down the battery voltage to a voltage level of 500mV, which is then compared to the LBI threshold voltage. The LBI pin has a built-in hysteresis of 10mV. See the application section for more details about the programming of the LBI threshold. If the low-battery detection circuit is not used, the LBI pin should be connected to GND (or to VBAT) and the LBO pin can be left unconnected. Do not let the LBI pin float. Low-Battery Detector Circuit (LBI and LBO) The low-battery detector circuit is typically used to supervise the battery voltage and generate an error flag when the battery voltage drops below user-set threshold voltage. The function is active only when the device is enabled. When the device is disabled, the LBO pin will be high impedance. The LBO pin goes active low when the voltage on the LBI pin decreases below the set threshold voltage of 500 mV ±15 mV, which is equal to the internal reference voltage. The battery voltage, at the detection circuit switches, can be programmed with a resistive divider connected to the LBI pin. Anti-Ringing Switch The device integrates a circuit which removes the ringing that typically appears on the SW node when the converter enters the discontinuous current mode. In this case, the current through the inductor ramps to zero and the integrated PMOS switch turns off to prevent a reverse current from the output capacitors back to the battery. Due to remaining energy that is stored in parasitic components of the semiconductors and the inductor, a ringing on the SW pin is induced. The integrated anti-ringing switch clamps this voltage internally to VBAT; therefore, dampens this ringing. Adjustable Output Voltage The accuracy of the output voltage is determined by the accuracy of the internal voltage reference, the controller topology, and the accuracy of the external resistor. The reference voltage has an accuracy of ± 4%. The controller switches between fixed frequency and PFM mode, depending on load current. The tolerance of the resistors in the feedback divider determines the total system accuracy. Design Procedure The FP6711 boost converter family is intended for systems that are powered by a single-cell NiCd or NiMH battery with a typical terminal voltage between 0.9V to 1.6V. It can also be used in systems that are powered by two-cell NiCd or NiMH batteries with a typical stack voltage between 1.8V to 3.2V. Additionally, single or dual-cell, primary and secondary alkaline battery cells can be the power source in systems where the FP6711 is used. (1) Programming the Output Voltage The output voltage of the FP6711 can be adjusted with an external resistor divider. The typical value of the voltage on the FB pin is 500mV in fixed frequency operation. The maximum allowed value for the output voltage is 3.3V. The current through the resistive divider should be about 100 times greater than the current into the FB pin. The typical current into the FB pin is 0.01µA, and the voltage across R4 is typically 500mV. Based on those two values, the recommended value for R4 is in the range of 500kΩ in order to set the divider current at 1µA. From that, the value of resistor R3, depending on the needed output voltage (VO), can be calculated using Equation 1. 1) - mV 500 V ( 500k ) 1 - V V ( R4 R3 O FB O .....(1) |
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