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Buck converter handles battery-backup system

-April 24, 2003

A synchronous buck converter is inherently bidirectional. That is, it transfers energy from input to output as a buck regulator when the output voltage is low, but, when the output voltage is high, the converter acts as a boost regulator, transferring power from output to input. This Design Idea shows how to use this bidirectional energy transfer to automatically recharge a battery when the main 5V supply is available in a battery-backed 5V system. The circuit in Figure 1 provides as much as 7A current at 5V output set at 4.8V and recharges a 12V sealed lead-acid battery with a current as high as 2A. The basic concept is that the ITH-pin voltage of the LTC3778 controls the L1 inductor current, or the valley level. Above approximately 0.7V at the ITH pin, the net inductor current is positive from input to output. Below that level, the inductor current becomes increasingly negative, resulting in a boost function that transfers energy from output to input. When the FCB pin of the LTC3778 is high, the IC inhibits negative inductor current and the boost function by turning off the bottom MOSFET.

Figure 2 shows a 5V power supply backed up by a battery-powered, LTC3778-controlled power supply. The synchronous, bidirectional LTC3778 buck circuit acts as a battery-to-5V converter if the main 5V supply is off and as a battery charger when the 5V supply is alive. As Figure 1 shows, in the charging mode, the circuit regulates the battery current by sensing the charge current through R1 by means of an LT1787 current-sense amplifier, IC2. An error amplifier, IC3B and IC3C, compares the current-sense signal with a reference voltage from the LT1460GCZ, IC4, and drives the ITH pin of IC1. When the ITH-pin voltage falls lower than approximately 0.7V, the circuit forces the average inductor current to a negative value, causing reverse power flow from the output to the input of the LTC3778, thereby charging the battery. The lower the voltage at the ITH pin, the higher the charge current.

At the beginning of the charge cycle, a constant current charges the battery. When the battery voltage reaches 13.8V, IC3D pulls the FCB pin of IC1 high, thereby not allowing Q2 to turn on. So, the circuit inhibits boost mode regardless of the level at the ITH pin. The interruption in charging current causes the battery voltage to drop below 13.2V to restart charging. This action results in pulse charging with the pulse frequency gradually decreasing until the battery fully charges. In the backup mode, IC3A senses the output-voltage drop to 4.8V and drives the ITH pin to maintain the output voltage at 4.8V. The recharging resumes when the system's 5V power returns and the 5V bus goes higher than 4.8V. In this scheme, the main supply voltage must be slightly higher than the backup-supply voltage for proper switchover. Approximately 100 to 200 mV should be adequate to prevent unnecessary mode switching attributable to ripple.

If the lower voltage in the backup mode is objectionable, then you can use a power-good signal from the main supply to change the reference voltage to the desired value when the power-good signal is low. Q3 through Q6 prevent low-battery discharge by shutting down IC1 when the battery voltage is low and the power-good signal from the main supply is inactive. You could implement more sophisticated charging algorithms using a system microcontroller or analog circuitry that sets the charge current as a function of battery voltage. You can implement float charging by reducing the charge current to approximately 100 mA when the battery is nearly fully charged. A gradually tapering charge current can mimic constant-voltage charging as a alternative to pulse charging. The circuit can use a three-cell (in series) lithium-ion battery if you set the maximum voltage to 12.6V.

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