Design Ideas December 5, 1996

Single NiCd cells drives op amp  

Alexander Belousov, Rego Park, NYLow-supply-voltage op amps, such as the LMC6582 and LMC6681 families, feature rail-to-rail I/O with a gain-bandwidth product greater than 1 MHz operating from just 1.8V. Although impressive, this voltage is not low enough to power these amps using a single alkaline or NiCd/NiMH cell, which have typical voltages of 1.25 to 1.5V. It is rather tempting to bridge this gap by designing a dc/dc converter (preferably inductorless) that converts 1.25V or less into at least 1.8V. Unfortunately, there is no off-the-shelf inductorless IC that can operate at such low input voltages. Typically, the charge pumps start operating from 1.5V and higher voltages. Inductor-based converter ICs can operate from a single cell, but they cost more than $2.50 (1000). Also, the inductor increases the cost and may cause EMI problems.

Figure 1a shows an ultra-low-voltage dc/dc converter that drives op amps from a single alkaline or NiCd cell. The circuit produces the regulated 1.8 to 1.9V dc from inputs as low as 1.0V dc and costs approximately $0.50 (without the LED). The converter actually starts operating around 0.7V. You can cascade two of these converters to produce higher voltages. To minimize the cost, the converter uses general-purpose npn and pnp transistors and small, inexpensive ceramic capacitors. You can obtain better efficiency by using transistors with low collector-emitter saturation voltages, but this change may increase the cost.

The principle of operation is simple. The dc/dc converter consists of a symmetrical, self-running multivibrator (Q₃ and Q₄) with an active collector load (Q₁ and Q₂) and synchronous rectifiers (Q₅ and Q₆). The R₁C₂ and R₂C₁ networks determine the 50-kHz operating frequency, which can vary. C₃ and C₄ improve the efficiency of the synchronous rectifiers. C₅ to C₇ perform output filtering. The LED is optional; it works as a power-on indicator and at the same time as a simple but effective low-voltage parallel regulator. The green LED was chosen for this design because it has typical 1.8 to 1.9V forward voltage. Tables 1 and 2summarize the circuit's performance with the LED in place.

Without the LED and with a V_IN of 1.25V, the circuit exhibits the following performance: Output short-circuit current is approximately 5 mA, input current is about 12 mA; output voltage for open circuit is approximately 2.4V, input current is 4.5 mA; when the load current is 1.4 mA, V_OUT is approximately 2.0V, and the input current is about 8 mA.

The circuit in Figure 1b is an example of the type of circuit this converter can drive. This stereo-base expander processes audio signals with nearly professional quality. The circuit enables you to improve the channel separation by partially subtracting the parasitic signals from each channel. Because of the op amp's rail-to-rail performance, the maximum peak-to-peak I/O signal is almost 1.8V. (DI #1967)

| EDN Access | feedback | subscribe to EDN! |