Electronic Design

Boost Circuit Charges Capacitor At Constant Power

Newly available, high-energy capacitors have extended RF communications capabilities to lowpower devices such as pagers and remote instruments. The power source for these devices, such as an AA alkaline cell in a pager or a longlife lithium battery in a wireless power meter, is unable to supply the peak power required to transmit data.

These new capacitors, which provide impressive capacitance values such as 1F at 5 V in a mere cubic centimeter, can provide high power to an RF amplifier to transmit a short burst of data. The capacitor must first be charged, usually from a battery whose voltage is lower than the voltage on the charged capacitor. This must be done in a controlled manner; the low series resistance of these capacitors allows them to sink or source surprisingly large currents. The circuit in Figure 1 will charge a capacitor at constant power from a single alkaline cell.

U1 is an integrated current-mode boost regulator. Its internal power switch, between the SW pin and the PGND pin of U1, turns on to energize inductor L1. It then turns off, allowing L1’s stored energy to pass through diode D1 to the output. U1 monitors the voltage on CBIG through a feedback divider (R1, R2, and R3), and tries to regulate this point to 4.5 V.

The output current of this boost circuit passes through a 20-W sense resistor (R4). If the voltage drop across this resistor is high enough to turn on pnp transistor Q1, its collector current will add to the current in the feedback divider, throttling back U1 and limiting the current into CBIG.

The base-emitter voltage necessary to turn on Q1 determines the current threshold. Note that part of this voltage appears across R1. This has two important benefits. First, the voltage drop on the current sense resistor is lower. Without R1, this resistor would drop 0.6 V. This decreases to −0.2 V as CBIG approaches full charge, raising the efficiency of the circuit. Second, the voltage across R1 depends on the output voltage. Near the beginning of the charge, VOUT is low and most of the base-emitter voltage appears across R4, resulting in a relatively large output current. As the voltage on CBIG rises, the increased drop across R1 results in a smaller output current. In fact, the output power stays nearly constant over the charging range (Fig. 2).

This constant-power operation allows the boost circuit's components to be optimally chosen for efficiency and size. Combined with the high (1.7 MHz) oscillator frequency of U1, this results in a tiny circuit. Each of the power components (C1, C2, L1 and D1) fits in a 1210 footprint.

The circuit can be shut down without draining the capacitor. When shut down, the circuit draws less than 1 mA from the battery, and only 3 mA from CBIG, conserving the stored energy for the next burst of data.

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