How Important are Low Iq LDOs to Long Battery Life in Wearables?

Using linear low dropout regulators (LDOs) with low quiescent current (I_q) can extend battery life in wearables and wireless Internet of Things (IoT) devices, but there are performance tradeoffs. These include transient response, noise performance, and output power range. Also, the quiescent current is sometimes confused with shutdown or disable current (I_d). They are not the same, and a balance needs to be arrived at between the two. And, of course, neither I_q nor I_d optimization is of much use if the overall system design is not optimized for low-power operation.

Here we’ll differentiate between I_q and I_d and briefly discuss the impact each has on power dissipation. We’ll then review some of those performance tradeoffs before closing with some exemplary LDOs from Microchip and Texas Instruments, including a demo board.

The difference between quiescence and shutdown

Readiness is the difference between quiescence and shutdown. In a quiescent state, the system is in a low power, active state and ready to run. During shutdown, sometimes referred to as disabled mode, the system is asleep and not ready to run immediately. The difference is especially important in battery-powered systems, such as wireless locks that spend extended periods (often >99% of the time) in standby and have large differences between standby and active current consumption. Quiescent current can be used to calculate the power at light loads, while shutdown current can be used to determine the long-term battery life.

In devices such as LDOs, there can be a significant difference between I_q and I_d. For example, one LDO has an I_q of 25 nanoamperes (nA) and an I_d of 3 nA. In another case, an LDO has an I_q of 0.6 microamperes (µA) and an I_d of 0.01 µA. Of course, it’s not that simple:

• The operating temperature can affect I_q and I_d. This can be an important consideration for devices that will be used for extended periods at higher temperatures.
• Low I_q devices can have longer response times to dynamic load changes. This factor varies considerably between LDOs.
• Low I_q devices can generate internal noise that can be an important factor in noise-sensitive applications.
• Even LDOs can generate significant heat, making it important to follow the datasheet guidelines for layout and thermal management. Otherwise, I_q and I_d performance can be compromised.
• The lowest I_q is not necessarily the best choice. If the difference between I_q and on-state current consumption is greater than two orders of magnitude, a lower-cost LDO with a higher I_q may be a good option.

150 mA low I_q LDO and demo board

Designers of systems with a single lithium-ion battery that need an LDO rated for inputs from 1.4 to 6.0 volts and delivering up to 150 milliamperes (mA) of current can consider LDOs such as the MCP1711 from Microchip Technology. This device has a typical I_q of 0.6 µA and an I_d of 0.01 µA. When shutdown mode is enabled, the output capacitor is discharged through a dedicated switch in the MCP1711 to quickly reduce the output voltage to zero. The MCP1711 has an ambient operating temperature range of -40 to +85°C.

To explore the operation of the MCP1711 over a wide input voltage and load range, designers can use the ADM00672 demo board that includes two voltage options and two package options:

• 1.8 V_out with a 3.2 to 6.0 volt input range in a five lead SOT-23
• 3.3 V_out with a 4.0 to 6.0 volt input range in a four lead 1×1 UQFN

The demo board has two isolated circuits that can be independently tested.

Fast transient response and low I_q

For systems that benefit from fast transient response and low I_q, designers can turn to the TPS7A02 from Texas Instruments. It is rated for 200 mA with an I_q of 25 nA and an I_d of 3 nA. It supports an output voltage range of 0.8 to 5.0 volts, programmable in 50 millivolt (mV) steps. This LDO has a typical transient response of under 10 microseconds (μs) settling time, with 100 mV undershoot for a step load change from 1 mA to 50 mA. Its response characteristics differ for load increases and decreases, as shown below. The TPS7A02 junction temperature is specified from -40 to +125°C.

Conclusion

I_q is an important characteristic to consider when designing for long battery life, but it’s only one of several factors to consider. Depending on the operating profile and power consumption patterns in a device, I_d is also an important consideration. There are a variety of factors, such as the operating temperature, that impact both I_q and I_d, and there is an optimal range for Iq and Id. Less is not always better.