Extending Battery Life with LDOs, Voltage Supervisors, and FETs

Extending battery life is a common design requirement in a variety of applications. Whether it is a toy or a water meter, designers have a variety of techniques to improve battery life. In this blog post, I will explain a technique that can strategically bypass low dropout linear regulators (LDOs). Generating the Rail Using an LDO is a common way to generate a regulated voltage from a battery. This is especially true for a single-cell lithium-ion (Li-ion) battery that outputs 4.2V when fully charged. Suppose you want to generate 3.3V for a microcontroller (MCU) with a supply voltage range of 3V to 3.6V, and choose the TPS706 to generate the rail. Figure 1 illustrates the circuit.

Figure 1: TPS706 regulates 3.3V from battery

Although this circuit is simple, it has some limitations. The first of these is brownout, which causes the LDO to stop regulating and can cause the MCU's supply voltage to go out of regulation. What brownout means As the battery discharges, the voltage of a Li-ion battery drops. Figure 2 shows an example of a discharge curve.

Figure 2: Li-ion battery voltage drop over time

This can be disconcerting when you remember that the LDO runs the risk of going into dropout as the input voltage approaches the regulated output voltage. At some point, the battery voltage will drop so low that the TPS706 will no longer be able to regulate 3.3 V. Instead, the output voltage will begin to track the battery voltage by a difference equal to the dropout voltage.

The TPS706 specifies a typical dropout voltage of 295mV at an output current of 50mA and an output voltage of 3.3V . Therefore, once the battery voltage drops below 3.6V, the LDO may enter brownout. Figure 3 provides an example of this type of behavior.

Figure 3: TPS706 enters power-down mode

As shown, once VIN drops to around 3.6V, VOUT starts to drop. Since the lower limit of the MCU supply range is 3V, this is disturbing - a brownout could cause VOUT to drop below 3V very quickly. Avoiding Brownouts One way to circumvent this problem is to bypass the LDO before or when it goes into brownout. Figure 4 illustrates this workaround.

Figure 4: Using a P-channel MOSFET to bypass the LDO

In this circuit, the TPS3780 is a dual-channel voltage detector that monitors the battery voltage through SENSE1. If the battery voltage should fall below 3.4V, OUT1 drives the gate of the P-channel MOSFET low. This causes the current (blue arrow) to flow through the drain-source terminals of the MOSFET instead of through the input-output terminals of the LDO (red arrow). Since the MOSFET has a lower on-resistance than the LDO, the output voltage will track the input voltage more closely.

SENSE2 monitors the output voltage. Once the output voltage falls below 3V (or the bottom of the MCU's power supply range), OUT2 will be set low. This signal can put the MCU into reset mode.

Figure 5: Falling input voltage without bypassing the MOSFET

To simulate a battery, the input voltage is ramped down at a rate of 1V/ms. You can see that once the input voltage reaches 3.4V, it takes about 100ms for the output to drop to 3V.

Now, let’s look at the behavior of the circuit using a bypassed MOSFET, as shown in Figure 6.

Figure 6: Falling input voltage bypassing MOSFET

Once the input voltage drops below 3.4V, the MOSFET turns on. The output voltage is now equal to the input voltage minus the voltage drop across the MOSFET. Therefore, it now takes nearly 320ms for the output to reach 3V. By enhancing the PMOS device, the output voltage tracks the input voltage more closely than an LDO in dropout. In other words, the low on-resistance of the external PMOS helps extend battery life.

实际上，电池电压将以较慢的转换速率下降。因此，使用旁路电路可显著延长工作时间。

电流消耗
当关闭电池时，您还必须考虑电路的电流消耗。见表1。

Table 1: Current consumption of various circuit components

It is important to consider this consumption because it contributes to the overall discharge of the battery. Fortunately, however, its consumption is extremely low and the additional circuitry allows the continued use of the battery to outweigh the increased current draw. This is especially true for applications that require higher load currents. Conclusion LDOs are an effective low current draw method for generating rails from a battery. However, when the battery voltage begins to drop, brownouts can cause voltage regulation issues. Using a MOSFET in conjunction with an LDO helps avoid this problem to achieve the longest battery life.

Reposted from: deyisupport

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