DESIGN IDEAS A Complete Battery Backup Solution Using a Rechargeable NiCd Cell by L.Y. Lin and S.H. Lim 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 lowbattery detection, necessitates battery access and invites inadvertent battery removal. The LTC1558 battery backup controller eliminates these problems by permitting the use of a single, low cost 1.2V rechargeable Nickel-Cadmium (NiCd) cell. The LTC1558 has a built-in fast-/tricklemode charger that charges the NiCd cell when main power is present. Figure 1 shows a typical application circuit with an LTC1558-3.3 providing backup power to 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. FROM µP OPEN DRAIN SOFT RESET L11† 22µH BACKUP BATTERY NiCd†† 1.2V + R14 10k 1 C11 47µF 6.3V SW VCC VBAK 3 7 + C12 8 1µF CTL LTC1558-3.3 5 2 RESET GND PUSH-BUTTON RESET 4 SW11 BKUP FB R15 12k TO µP 6 R13 100k Q11 Si4431DY R11 51k 1% MAIN BATTERY 4.5V–10V R12 21.2k (20.0k 1% + 1.21k 1%) C2 0.1µF 13 9 CSS 0.1µF CC2 51pF C1 100pF RC 10k 16 TG SW 14 C4 15 BOOST 0.1µF D1*** LTC1435 6 12 INTVCC VOSENSE 3 8 ITH SENSE+ C5 2 7 1000pF RUN/SS SENSE– Q2 1 C 11 BG OSC Si4412DY + C3 SGND PGND 4.7µF 5 10 16V COSC 68pF 4 CC 330pF VIN EXTVCC Q1 Si4412DY SFB * ** *** † †† + CIN 100µF 16V ×2 L1* 10µH RSENSE** 0.033Ω + D2 MBRS140T3 SUMIDA CDRH125-100 IRC LR2010-01-R033-F CENTRAL CMDSH-3 SUMIDA CDRH73-220 SANYO CADNICA N-110AA COUT 100µF 10V ×2 VOUT 3.3V LOAD CURRENT 3A IN NORMAL MODE 30mA IN BACKUP MODE R1 35.7k 1% R5 20k 1% C6 100pF 1558 01.eps Figure 1. LTC1558 backup system with LTC1435 as main system regulator 26 Linear Technology Magazine • February 1998 DESIGN IDEAS OUTPUT POWER (mW) VBAK = 4V VOUT = 3.3V 140 120 100 80 60 40 20 0 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 BACKUP CELL VOLTAGE (V) 1558_02 Figure 2. 3.3V output power vs backup cell voltage Normal Mode (Operation from the Main Battery) During nor mal operation, the LTC1435 is powered from the main battery, which can range from 4.5V to 10V (for example, a 2-series or 2series × 2-parallel Li-Ion battery pack, or the like) and generates the 3.3V system output. The LTC1558 operates in standby mode. In standby mode, the LTC1558 BKUP (backup) pin is pulled low and P-channel MOSFET Q11 is on. The NiCd cell is fast charged by a 15mA current source connected between the LTC1558’s VCC and SW pins. Once the NiCd cell is fully charged (according to the LTC1558’s gas-gauge counter), the LTC1558 trickle charges the NiCd cell. R14 sets the trickle-charge current according to the formula I(TRICKLE) = 10 • (VNiCd – 0.5)/R14. The tricklecharge current is set to overcome the NiCd cell’s self-discharge current, thereby maintaining the cell’s full charge. Backup Mode (Operation from the Backup Battery) The main battery voltage is scaled down through resistor divider R11– R12 and monitored by the LTC1558 Authors can be contacted at (408) 432-1900 Linear Technology Magazine • February 1998 via the FB pin. If the voltage on the FB pin drops 7.5% below the internal 1.272V reference voltage (due to discharging or exchanging the main battery), the system enters backup mode. In backup mode, the LTC1558’s internal switches and L11 form a synchronous boost converter that generates a regulated 4V at VBAK. The LTC1435 operates from this supply voltage to generate the 3.3V output voltage. The BKUP pin is pulled high by R13 and Q11 turns off , leaving its 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. body diode reverse biased. The BKUP pin also alerts the system microprocessor. C11, a 47µ F capacitor, provides a low impedance bypass to handle the boost converter’s transient load current; otherwise, the voltage drop across the NiCd cell’s internal resistance would activate the Table 1. VFB and VBAK voltages Relative % Below VR E F % of VR E F VF B VB A K –0% 100% 1.272V 4.332V –6% 94% 1.196V 4.073V –7.5% 92.5% 1.177V 4.008V 300 VBAK = 4V VOUT = 3.3V 250 BACKUP TIME (MINS) 180 160 200 150 100 50 0 0 5 10 15 20 LOAD CURRENT (mA) 25 30 1558_03 Figure 3. Backup time vs 3.3V output load current LTC1558’s undervoltage-lockout function. Table 1 shows several values of VFB vs the VBAK voltage. Figure 2 shows the maximum output power available at the 3.3V output vs the NiCd cell voltage. Over 100mW of output power is achieved for a NiCd cell voltage greater than 1V. Figure 3 shows the backup time vs the 3.3V load current using a Sanyo Cadnica N-110AA cell (standard series with a capacity of 110mAhrs). Over one hour of backup time is realized for less than 80mW of 3.3V output power. Recovery from Backup Mode to Normal Mode When a new main battery pack is inserted into the system, 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. This allows the new battery pack to supply input power to the LTC1435. The LTC1558 now accurately replenishes the amount of charge removed from the NiCd cell through the internal charger and gas-gauge counter. for the latest information on LTC products, visit www.linear-tech.com 27