AN8120-1 Power Management of a NiMH or NiCd Cell ADVANCED COMMUNICATIONS & SENSING PRELIMINARY APPLICATION NOTE AN8120-1 APPLICATION NOTE Power Management of a NiMH or NiCd Cell ACS - Revision 0.3 © Semtech Corp. September 2010 Page 1 www.semtech.com/products/ AN8120-1 Power Management of a NiMH or NiCd Cell ADVANCED COMMUNICATIONS & SENSING PRELIMINARY APPLICATION NOTE TABLE OF CONTENT Section Page INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1 2 2.1 2.2 2.3 3 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Device operating modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Safe Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Low Batt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Active or Sleep mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NiMH pin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 3 3 3 3 APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4 4.1 4.2 5 5.1 Simplest accumulator charger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Battery not charging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Battery charging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Faster accumulator charger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recharging a Deep Discharged Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ACS - Revision 0.3 © Semtech Corp. September 2010 Page 2 5 6 6 7 8 www.semtech.com/products/ AN8120-1 Power Management of a NiMH or NiCd Cell ADVANCED COMMUNICATIONS & SENSING PRELIMINARY APPLICATION NOTE INTRODUCTION 1 Abstract This document discusses the power management of a single NiMH or NiCd battery for common system application usage scenarios with the SX8120/21/22 1V Motor/LED timer series. Power cell power management is highly dependent on the individual user’s application; however, this document focuses on providing several application-usage case scenarios and the environment settings. This document applies for the following devices: • • • SX8120ISTRT SX8121ISTRT SX8122ISTRT For additional details about the SX8120/21/22 1V Motor/LED timers, see the SX8120/21 and SX8122 Datasheets. 2 Device operating modes 2.1 Safe Mode The internal system goes in Safe mode as soon as the battery voltage is above 0.6-0.7V. In this mode all pins are set in high impedance and are inactive. This guarantees a safe start-up when a battery is inserted. The power consumption is 10uA max and prevents the battery from leaking when it is empty. 2.2 Low Batt mode The device is in Low Batt mode as soon as the battery voltage reaches the 0.9V POR threshold. In this mode, each output pin is in a static state. The chip internally checks the VDD voltage each seconds and wakes up to Sleep mode when VDD > VON ( 1.05V typical). NIMH pin is forced high until the VDD voltage goes below 0.9V (back to Safe mode), or above 1.41V (battery full). 2.3 Active or Sleep mode NIMH pin is set according to VDD voltage (see 3. NiMH pin). 3 NiMH pin Pin NIMH goes high when VDD is below the NIMH threshold of 1.41V and low when VDD is above the same threshold. Pin NIMH low means that if the power is coming from an NiMH cell or an NiCd, this cell is full. A 30mV hysteresis allow to remove NiMH pin oscillation when the VDD voltage is near the threshold. The supply voltage on VDD pin is checked at a 1Hz rate, so the NiMH pin can switch with a delay of 1 seconds max. ACS - Revision 0.3 © Semtech Corp. September 2010 Page 3 www.semtech.com/products/ AN8120-1 Power Management of a NiMH or NiCd Cell ADVANCED COMMUNICATIONS & SENSING PRELIMINARY APPLICATION NOTE . VDD 1.4V NiMH full Hysteresis 1.05V Switch ON 0.95V Switch OFF 0.9V POR Hysteresis LOW BATT SAFE 0.6V SX812x ACTIVE or SLEEP LOW BATT SAFE time NIMH pin voltage 1.4V 0.9V 0.0V High impedance High impedance Figure 1. Operating modes and NiMH pin vs battery voltage 1 1. These threshold values are typical. ACS - Revision 0.3 © Semtech Corp. September 2010 Page 4 www.semtech.com/products/ AN8120-1 Power Management of a NiMH or NiCd Cell ADVANCED COMMUNICATIONS & SENSING PRELIMINARY APPLICATION NOTE APPLICATIONS 4 Simplest accumulator charger For a bare-bone charger (slow but cheap), one resistor is sufficient. It shall limit the peak current in the cell. When the cell is full, the SX8120/21/22 will short the NIMH pin to ground and disconnect the charge voltage from the cell. If using a NiMH cell, as there is no temperature control, the maximal load current (measured in mA) must be set below one tenth of the total capacity (measured in mAh) and below the absolute max rating for the pin (IAMRISS) which is -100mA to 100mA. 0.9 – 2.0 V SX812x VDD MOTOR Motor LED Single cell NiMH or NiCd VSS 5V for recharge NIMH LED Figure 2. Example of a USB lowest cost rechargeable cell controlled by the SX8120/21 ACS - Revision 0.3 © Semtech Corp. September 2010 Page 5 www.semtech.com/products/ AN8120-1 Power Management of a NiMH or NiCd Cell ADVANCED COMMUNICATIONS & SENSING 4.1 PRELIMINARY APPLICATION NOTE Battery not charging When VDD is below 0.9V (deep discharge) or above 1.4V (cell full): 5V for recharge NIMH VDD VSS Single cell NiMH or NiCd Figure 3. Equivalent circuit if NIMH pin is low When VDD is higher than the maximal cell voltage, then NIMH pin is internally connected to VSS, shorting the external supply to ground through the current limting resistor. 4.2 Battery charging When VDD is between 0.9V and 1.4V: 5V for recharge NIMH VDD VSS Single cell NiMH or NiCd Figure 4. Equivalent circuit if NIMH pin is high When VDD is lower than the maximal cell voltage, then NIMH pin is internally connected to VDD, connecting the external supply to the cell through the current limting resistor ACS - Revision 0.3 © Semtech Corp. September 2010 Page 6 www.semtech.com/products/ AN8120-1 Power Management of a NiMH or NiCd Cell ADVANCED COMMUNICATIONS & SENSING 5 PRELIMINARY APPLICATION NOTE Faster accumulator charger For a faster charge, the following 2 transistors circuit increases the top charge current while limiting the idle current when the cell is full. If using a NiMH cell, as there is no temperature control, the maximal load current (measured in mA) must be set below on tenth of the total capacity (measured in mAh). 500 Ohm 2N3904 2N3904 1 – 1.6 V USB 5.0 V 400 Ohm VDD AA NiMH cell NIMH VSS SX812x Figure 5. Example of a USB recharged NiMH single cell controlled by the SX8120/21 !! !! !! !"! Figure 6. Current seen by the NiMH battery and other voltages ACS - Revision 0.3 © Semtech Corp. September 2010 Page 7 www.semtech.com/products/ AN8120-1 Power Management of a NiMH or NiCd Cell ADVANCED COMMUNICATIONS & SENSING 5.1 PRELIMINARY APPLICATION NOTE Recharging a Deep Discharged Battery In the case of a deep discharged NiMH cell, the battery must not be charged with a fast charge current rate until the battery voltage is at least 0.9V. This can be done by adding a resistor between VUSB the chip VDD (see Figure 7). The resistor is dimensionned to supply a 50-100uA current to the discharged battery while the SX8120/21 is in Safe mode. In this case the NIMH pin is set to high impedance. Then, if a deep-discharged battery is placed in the charger, the charger will apply a small 'conditioning' current to gently bring it up to 0.9V. Once 0.9V is reached, the SX8120/21/22 switches to Low Batt mode and provides a fast charge. 500 Ohm 68 KOhm 2N3904 2N3904 1 – 1.6 V USB 5.0 V 400 Ohm VDD AA NiMH cell NIMH VSS SX812x Figure 7. Adding a resostir for deep discharge battery ACS - Revision 0.3 © Semtech Corp. September 2010 Page 8 www.semtech.com/products/ AN8120-1 Power Management of a NiMH or NiCd Cell ADVANCED COMMUNICATIONS & SENSING PRELIMINARY APPLICATION NOTE © Semtech 2010 All rights reserved. Reproduction in whole or in part is prohibited without the prior written consent of the copyright owner. The information presented in this document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice. 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