Electronic Design

Simulate Lithium Batteries With A NiMH Cell

Lithium-based batteries are becoming standard in portable equipment. They have desirable characteristics, but lead times may be long unless you have a preferred-customer status with the battery manager. A backup alternative is desirable, especially for smaller companies. It should (ideally) provide the same performance, size, and cost as a lithium-type battery.

Nickel-metal-hydride (NiMH) batteries are still in wide use. They're much cheaper than lithium batteries, and they come in very small sizes (AAAA is now available). A proposed interface circuit should mimic the batteries' terminal voltage, which declines as the battery discharges. The nominal terminal voltage for lithium batteries (3.6 V) is about three times that of NiMH batteries (1.2 V). As a simple approach, therefore, you can force the output of an efficient step-up converter to equal battery voltage times 3.6 V/1.2 V = 3.

However, allowing the circuit to run constantly during shutdown consumes unnecessary power. The NiMH circuit's equivalent leakage (its quiescent current) can be as a high as 200 mA, which is unacceptable. Only a power-control capability must be provided while in shutdown.

Instead of maintaining a three-times NiMH voltage while in shutdown, you can run the circuit in burst mode, activating the step-up converter only when the circuit output drops below a certain threshold. When it reaches the upper threshold, the step-up shuts down, allowing the output capacitor to discharge through the output load plus the NiMH circuit. Thus, the output voltage forms a sawtooth wave. On the other hand, if battery voltage falls below a lower limit, the circuit remains deactivated to protect the batteries from depletion.

The circuit of Figure 1 implements these ideas by providing an interface between a NiMH battery and a lithium-optimized power-management circuit. The state of the circuit is controlled by the MODE input (HIGH gives the sawtooth, and LOW gives 3 × VBATT). The integrator-connected op amp multiplies the battery voltage by driving U1's feedback node to produce an output three times that voltage (Fig. 2).

A large integrator time constant is necessary to avoid interaction with U1's internal error comparator, as well as to provide noise filtering. In low-power mode, a mP-supervisor IC (U2) monitors the output voltage and controls U1. The resistor string associated with U2 sets approximate 2.4- and 4-V thresholds for that device.

Finally, the step-up converter must always be shut down when battery voltage drops below a threshold, which is usually 0.9 V. That shutdown is accomplished by the converter's own internal comparator, and the Tiny Logic network selects the correct operating mode according to the state of the MODE control input, the ramp threshold detector (U2), and U1's internal battery comparator. To ensure startup from 1.0 V when the output is 0 V, the logic network must be supplied directly from the battery. The logic family shown (ULP) functions down to 0.9 V; below that, the battery is nearing depletion.

With the components as shown, the sawtooth duty cycle is 10% (Fig. 3). Note that the circuit has a potential lock-up state. When running in sawtooth mode, the upper threshold of U2 must always be less than U1's maximum output. Otherwise, U2 doesn't shut down U1. U1's maximum output is set by the R9-R10 resistor string at its FB node. (Because the op-amp output is at high impedance in low-power mode, it doesn't pull current from the node.)

An interface for lithium-battery charging isn't included. However, that function can be implemented with a p-channel MOSFET in series with the charger and the battery, plus the addition of an op amp to servo the charger side of the MOSFET to 3 3 VBATT.

The authors wish to thank Magnus Thulesius of Anoto for the original idea.

CAPS 2. For normal operation of the Figure 1 circuit (i.e., V IN>= 0.9 V), V OUT = 3 × V IN.
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