Every circuit needs some sort of power source. Frequently, the success of a circuit will be made or broken by the convenience and flexibility of its power supply. This is particularly true of portable instrumentation, which often must combine line-powered and battery-powered operation with management of rechargeable batteries. The circuit illustrated in Figures 1 and 2 is a power-supply design that accommodates line, battery, and fast-recharge modes of operation with a maximum degree of transparency and flexibility. In addition, the fast-recharge mode is compatible with the long service life of popular NiCd and NiMH battery chemistries.
The Micro-UPS consists of two subcircuits. Figure 1 comprises the battery and recharge control. Quad op-amp U1 and its associated discrete components act to manage battery operation in three different operating modes. Recharge mode is entered automatically when ac power is restored after a period of battery-powered operation and consequent partial or complete battery discharge.
During recharge mode, approximately 0.5 A is delivered to the battery, achieving full recharge in two to three hours. As recharging progresses, the battery voltage will initially ramp up. For as long as this continues, comparator U1A enables the battery current regulator U1D and Q1. Once the battery becomes fully charged, however, some means must be provided to turn off the recharge circuit or the battery will be destroyed.
The recommended method for termination of rapid (less than 10 hours) recharge of NiCd and NiMH batteries is the so-called "delta-V" technique. The Delta-V method takes advantage of the reversal in the slope of the battery voltage versus time that occurs at end-of-charge due to rising cell temperature. The cell heats up when the battery is fully replenished and the subsequent surplus energy input has nowhere to go except into heat production. Delta-V is included in the power supply to detect arrival of the battery at full charge and to terminate recharge mode.
To accomplish this, U1B and Q3 act as a very "low-droop" peak-detector that continuously tracks the maximum battery voltage developed in the course of the recharge cycle, as well as stores the detected voltage level on analog memory capacitor C1. U1A then compares C1's stored voltage to the instantaneous battery voltage. When the slope of battery voltage versus time reverses and drops to approximately 15 mV below the maximum recorded on C1, U1A toggles and shuts off U1D and Q1, thus ending the recharge mode.
Once the recharge mode ends, a very small "trickle" current of about 5 mA is applied, via R10, to offset the self-discharge mechanisms present in the battery. Q4 resets C1 and thereby prevents the possibility of spurious-noise-induced reentry into recharge mode.
Loss of the 5-V input supply causes the positive rail (raw dc input to the LT1372 regulator) to drop to the level sustainable by the battery. As a result, the negative battery terminal actually drops below ground, to about 0.3 V. U1C detects this onset of discharge mode and, via Q2, charges C1 in preparation for the eventual battery recharge cycle that will occur when ac power is restored. This action also toggles U1A's output state, enabling the U1D/Q1 current-driver to provide a lower impedance. Consequently, there's a more efficient ground path for the negative battery terminal.
In Figure 2, U2 forms the core of an efficient regulator (greater than 80%) that produces a stable +5-V output from input voltages in the range of 2.5 V to 5.5 V. High-frequency (500-kHz) PWM operation is used for efficiency and to keep the inductor small.
Achieved performance results include endurance of greater than 3 watt-hours from three "high-capacity" NiCd AA cells, and greater than 5 watt-hours from NiMH. Recharge time was less than three hours from a fully-discharged state. The external power supply is 5 V at 1 A. Also, ac/battery changeover is automatic upon loss of external power, thus the use of the term "Micro-UPS." The output capacity is a regulated 5 V at 1 A maximum peak, 0.5 A sustained.