Notebook computers and other portable equipment often use a backup battery to retain memory contents during replacement of the main battery. Such "bridge batteries" usually consist of five or six series-connected NiCd coin cells. They're expected to keep the system alive for approximately 5 minutes—plenty of time to swap batteries. The circuit in Figure 1 reduces size, weight, and cost by using one NiCd cell instead of five or six. A Saft VB4E 40-mAhr NiCd coin cell has enough capacity to keep a typical notebook computer in suspend mode for approximately 10 minutes. All components (excluding the coin cell) consume less than 1/2 in.2 of pc-board area, and the cost savings from fewer cells help pay for the added circuitry.
When it's operational, the main battery provides power to the system's dc/dc converters. The LT1304 boost converter constantly monitors the dc/dc-converter input via the feedback divider comprising R3 and R4. Once the FB pin drops below 1.24V (corresponding to approximately 6V at the dc/dc-converter input), boost converter IC1 begins switching. Current comes from B1, through L1, and into the SW pin on IC1. When IC1's internal switch turns off, the SW pin goes high, delivering current through D2 into the dc/dc converter's input capacitor, C4. C4 already exists for the dc/dc converter, so you need no additional output capacitor.
The LT1304 switches at approximately 200 kHz; thus, L1 can be small. Switching occurs automatically as needed to hold the dc/dc converter's input at approximately 6V. Power for IC1 is "bootstrapped" from the 5V dc/dc converter's output, so it never needs to operate directly from the 1.2V NiCd coin cell. Should the backup battery ever fully discharge, the 5V output drops below the LT1304's minimum operating voltage of 1.5V, and the entire system shuts down. At that point, the NiCd cell becomes unloaded, thereby preventing overdischarge damage. Backup operation is re-established only after the main power source (main battery or ac power) is restored to the system.
It is often necessary to generate a logic signal to indicate that the system is receiving its power from the backup battery. This signal can serve to shed load (the backup converter cannot support full system power) or to open a MOSFET switch that prevents current from flowing back into the main battery. When the LT1304 is operating, the flyback pulse present at the SW pin turns on Q1, thereby peak-charging C1 through D1. Once the voltage on pin 1 exceeds 1.17V (the LBI comparator threshold), the LBO open-collector driver is released and pulled high by the resistors within Q2.
You can connect the LBO signal to logic circuitry or to a microcontroller input. LBO also turns off Q2 to disable charging during backup operation. During normal nonbackup operation, LBO is low, keeping Q2 saturated. In this state, approximately 350 mA of charging current flows through R5 into the battery. You can scale R5 for different charge currents as appropriate. (DI#1985)
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