Electronic Design

A complete battery backup solution using a rechargable NiCd cell

Battery-powered systems, including notebook computers, personal digital assistants (PDAs), and portable instruments, require backup systems to keep the memory alive while the main battery is being replaced. The most common solution is to use an expensive, nonrechargeable lithium battery. This solution requires low-battery detection, necessitates battery access, and invites inadvertent battery removal.

The circuit presented here eliminates these problems by permitting the use of a single, low-cost 1.2-V rechargeable nickel-cadmium (NiCd) cell (Fig. 1). U1, an LTC1558 battery backup controller, has a built-in fast/tricklemode charger that charges the NiCd cell when main power is present. It provides backup power to U2, an LTC1435 synchronous step-down switching regulator. The backup circuit components consist of the NiCd cell, R11-R14, C11-C12, L11, and Q11. SW11 and R15 provide a soft or hard reset function.

During normal operation, U2 is powered from the main battery, which can range from 4.8 V to 10 V (for example, a 2-series or 2-series times 2-parallel Li-Ion battery pack, or the like) and generates the 3.3-V system output. U1 operates in standby mode. In standby mode, U1’s BKUP (backup) pin is pulled low and p-channel MOSFET is turned on. The NiCd cell is fast charged by a 15-mA current source connected between the U1’s VCC and SW pins. Once the NiCd cell is fully charged (according to U1’s gas-gauge counter), U1 trickle charges the NiCd cell. R14 sets the trickle-charge current according to the formula I(TRICKLE) = 10 × (VNiCd − 0.5)/R14. The trickle-charge current is set to overcome the NiCd cells’ self-discharge current, thereby maintaining the cells’ full charge.

The main battery voltage is scaled down through resistor-divider R11- R12 and monitored by U1 via the FB pin. If the voltage on the FB pin drops 7.5% below the internal 1.272-V reference voltage (due to discharging or exchanging the main battery), the system enters backup mode.

In backup mode, U1’s internal switches and L11 form a synchronous boost converter that generates a regulated 4 V at VBAK. U2 operates from this supply voltage to generate the 3.3-V output voltage. The BKUP pin is pulled high by R13 and Q11 turns off, leaving its body diode reverse-biased. The BKUP pin also signals the system microprocessor. C11, a 47-µF capacitor, provides a low impedance bypass to handle the boost converter’s transient load current. Otherwise, the voltage drop across the NiCd cells’ internal resistance would activate the U1’s undervoltage-lockout function. Figure 2 shows the maximum output power available at the 3.3-V output versus the NiCd cell voltage.

Over 100 mW of output power is achieved for a NiCd cell voltage greater than 1 V. Figure 3 shows the backup time versus the 3.3-V load current using a Sanyo Cadnica N-110AA cell (standard series with a capacity of 110 mAh). Over one hour of backup time is realized for less than 80 mW of 3.3-V output power.

When a new battery pack is inserted, Q11’s body diode forward-biases. Once the voltage at the FB pin increases to more than 6% below VREF, the boost converter is disabled and the system returns to normal mode. The BKUP pin pulls low and turns Q11 back on, allowing the new battery pack to supply input power to U2. U1 now replenishes the charge removed from the NiCd cell through the internal charger and gas-gauge counter.

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