Circuit enables single nimh cell to simulate a lithium battery - reference design - maxim
Keywords: NiMH battery, power management, lithium-battery simulation
REFERENCE DESIGN 3744 INCLUDES: Tested Circuit Schematic Description Test Data
Circuit Enables Single NiMH Cell to Simulate aLithium Battery
Mar 08, 2006 Abstract: This circuit uses a NiMH cell to replace a lithium battery. Its output simulates the dischargecharacteristic of a lithium (Li+) battery.
Lithium-based (Li+) batteries are becoming common in portable equipment. They have desirablecharacteristics, but they are often in short supply. Lead times can be long unless you have a preferred-customer status with the battery manufacturer. A backup alternative to Li+ is therefore desirable,especially for smaller companies. This note describes a circuit that allows a nickel-metal hydride (NiMH)cell to be used in a circuit designed for a Li+ battery. It provides the same performance, size, and costas a Li+ battery.
Design Considerations for a NiMH Interface
A circuit that interfaces a NiMH battery with a lithium-optimized circuit should mimic the battery's terminalvoltage, which declines as the battery discharges. The nominal terminal voltage for Li+ batteries (3.6V) isabout three times that of NiMH batteries (1.2V). As a simple approach, therefore, you can force theoutput of an efficent step-up converter to equal battery voltage times 3: 6V/1.2V = 3.
Allowing the circuit to run constantly during shutdown, however, consumes unnecessary power. TheNiMH circuit's equivalent leakage (its quiescent current) can be as a high as 200µA during shutdown,which is unacceptable. In fact, only a power-control capability is needed during shutdown.
Instead of maintaining 3x the NiMH voltage while in shutdown, you can run the proposed circuit in burstmode, which activates the step-up converter only when the circuit output drops below a definedthreshold. When the output reaches the upper threshold, the step-up shuts down, allowing the outputcapacitor to discharge through the output load plus the NiMH circuit. Thus, the output voltage forms asawtooth wave. If battery voltage falls below a lower limit, however, the circuit remains deactivated toprotect the battery from depletion.
The circuit of Figure 1 implements the above ideas by providing an interface between a NiMH battery and a lithium-optimized power-management circuit. The MODE input controls the circuit; HIGH gives the sawtooth, and LOW gives 3*VBATT. Figure 1. Powered by a single NiMH cell, this circuit output simulates the discharge characteristic of a Li+battery.
The integrator-connected op amp multiplies the battery voltage by driving U1's feedback node to produce an output three times that voltage (Figure 2). A large integrator time constant is necessary both to avoid interaction with U1's internal error comparator, and to provide noise filtering. In low-power mode, a microprocessor-supervisor IC (U2) monitors the output voltage and controls U1. The resistor string associated with U2 sets approximate 2.4V and 4V thresholds for that device. Figure 2. For normal operation of the circuit in Figure 1, i.e., VIN ≥ 0.9V, VOUT = 3VIN.
Finally, the step-up converter must always be shut down when battery voltage drops below a threshold,which is usually 0.9V. That shutdown is accomplished by the converter's own internal comparator; theTiny 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 1.0Vwhen the output is 0V, the logic network must be supplied directly from the battery. The logic familyshown (ULP) functions down to 0.9V; below 0.9V, the battery is nearing depletion.
Using the components as shown, the sawtooth duty cycle of this circuit is 10% (Figure 3). Note that the circuit has a potential lockup state. When running in sawtooth mode, U2's upper threshold must always be less than U1's maximum output. Otherwise, U2 does not 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 does not pull current from the node). Figure 3. In sawtooth mode, the circuit in Figure 1 exhibits a 10% duty cycle.
This proposed circuit does not include an interface for Li+ battery charging. That function can beimplemented with a p-channel MOSFET in series with the charger and the battery, plus the addition ofan op amp to servo the charger side of the MOSFET to 3*VBATT.
The authors wish to thank Magnus Thulesius of Anoto for the original idea for this circuit.
A similar article appeared in the April 14, 2005 edition of Electronic Design.
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Application Note 3744: REFERENCE DESIGN 3744, AN3744, AN 3744, APP3744, Appnote3744, Appnote 3744 Copyright by Maxim Integrated ProductsAdditional Legal Notices:
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