Button battery voltage level monitoring in portable device system
Monitoring the voltage levels of button cells in portable devices or ancillary services is a common and simple application for modern CMOS operational amplifiers.
Figure 1 shows an implementation using a 1.8V OPA333 zero-drift op amp. The button battery has a voltage of 3V and the circuit is powered from 3 to 5V.
Oddly enough, I heard from customers that the life of the button battery in this type of circuit is much shorter than expected; only a few days or a few hours! These customers found that the button battery remained powered after the op amp was removed. This caused my curiosity, so I started to investigate what happened.
Figure 1 - Connecting the OPA333 unity gain amplifier to monitor the button cell voltage
The lithium battery CR2032 is a common button battery with a rated capacity of 175 mAh and 200uA continuous current. The OPA333's input bias current is typically 70pA, and it won't drain the button battery for years. There may be other circuits that consume the battery. After carefully observing the internal schematic of the OPA333, a plausible discharge situation was found.
Figure 2 is a block diagram of the amplifier core and its input ESD protection. Don't forget that most ICs have ESD protection during circuit disconnection. The ESD unit is turned off under normal operating conditions.
The OPA333's ESD input protection feature has a low-leakage steering diode connected between each input and the power line. ESD clamps are connected between these lines. These diodes are usually reverse biased. However, if the supply voltage V+ is turned off and becomes high impedance, then diode D3 will become forward biased. The amplifier core and any components connected to V+ then draw current from the button battery.
The OPA333 has a quiescent current of only 17uA, so it is likely that other components connected to the +V supply line are also consuming current.
Figure 2 – Internal block diagram showing the current path from the non-inverting input through the D3 to the +V power line
Some op amps are related to the break pin. When in this mode, they consume a very small amount of supply current. However, if the ESD unit uses the design in Figure 2, the diode can still conduct current.
The solution is to use an op amp that supports different ESD cell designs.
Figure 3 shows the ESD cell design used in the TLV2450 rail-to-rail input/output op amp. It uses a fast low-leakage clamp similar to a Zener diode. Not only can it be turned on quickly during an ESD event, but it can also limit the applied voltage to a safe level. There is no internal current path for the VDD pin.
Figure 3 - TLV2450 uses internal ESD clamps. There is no internal current path between the input and the VDD pin.
It may be difficult for an engineer to determine the ESD unit used by the amplifier. However, you can find tips in the amplifier product manual. When viewing the absolute maximum ratings, if the signal input range is -0.3V to (V+) + 0.3V, then 0.3V is the limit to ensure that the ESD diode remains off. If it is high, the diode may turn on.
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