Three Main Specifications to Understand

When choosing the right buck converter, note the different terms in the datasheet and what those terms represent in a real-life scenario. For battery-powered wearable applications, differentiating between the following current specifications is crucial:

• Shutdown current Isd:
  • This represents the tiny leakage current flowing when the device is disabled or operating in a shutdown mode while connected to the input supply. Generally, this is the lowest possible consumption contributor of the buck converter. All buck devices exhibit a small shutdown current when turned off and no load is present.
• Non-switching IQ:
  • This is the quiescent current drawn when the buck converter is enabled but not actively switching. During this state, the internal control logic is in idle mode, therefore the current consumption is very low. An example is a pulse frequency mode (PFM) device that is turned on but under no load.
• Switching IQ:
  • This current is consumed while the integrated circuit (IC) is actively switching to regulate the output voltage. Most of the internal circuitry for example, gate drivers, oscillators, control loops, are operating and yielding a higher quiescent current than in the non-switching case. A common example is a Pulse Width Modulation (PWM) buck converter that is enabled under no load, yet continuously switching.

How to Select the Right Low IQ Buck Converter for Your Applications?

A wide range of low IQ buck converters optimized for low input voltage wearable applications are available on ti.com. Table 1 lists the latest low IQ devices with current ratings, package options and recommended Bill of Material (BOM) highlighted:

* TI has low IQ power modules such as TPSM82843 with an integrated inductor in a QFN package and TPSM82740 with 360nA IQ available in MicroSIP™ package.

All mentioned devices are designed following an RF‑friendly DCS‑Control™ topology.

Additional Feature Sets Provided by Low IQ Buck Converters:

MODE pin:

To maintain high efficiency over a wide load current range, our converters can automatically transition to PFM operation, also referred to as pulse-skipping mode or power save mode (PSM) as the load current decreases. In this mode, the conversion efficiency is increased by skipping switching pulses and reducing the device current consumption (quiescent current). Pulse-skipping eliminates the negative inductor current (current flowing from the output towards the input) that otherwise occurs at light loads with fixed-frequency, PWM operation. The trade-off of this mode of operation is a higher V_out ripple.

If a lower V_out ripple and cleaner noise profile is required by the application at the expense of light-load efficiency, a forced-PWM mode can be favored, where the device always operates in continuous conduction mode.

A MODE pin is available in many TI buck converters, that enables the selection between automatic transition to PFM or FPWM operation depending on the application needs. The resulting efficiency curves are shown in Figure 2.

STOP function/pin:

This STOP mode is analog to telling the buck converter stay quiet for a moment. A normal switching regulator is efficient, but inherently creates tiny switching noise and ripple that can disturb applications relying on distortion-free data-acquisition or RF communication procedures. When the STOP pin is set to logic high, the converter fully stops switching after the current switching cycle, and the output load is powered only from the output capacitor C_out for a short time. This gives the ADC, sensor or RF block a clean, quiet window to take a measurement or transmit without power-supply noise getting in the way. As soon as a logic-low level is applied to the STOP pin, the regulator immediately resumes switching operation, without any startup and/or soft-start delay.

In STOP mode, the output voltage behavior as a function of time can be estimated by:

Where V_out, set is the set output voltage and I_out is the constant output current discharging C_out.

The longer STOP mode is kept active, or the more current the load takes, the more the output voltage falls. A bigger output capacitor yields a slower voltage decrease. This feature is mostly important for very light-load applications where the current is in the range of 100µA to 20 mA.

Dynamic Voltage Scaling (DVS)

This is a control technique where the output voltage of the switching regulator is dynamically adjusted depending on the application system demands and the changing load conditions. DVS aims to improve energy efficiency, reduce power consumption while maintaining flexibility to adapt to rapidly changing performance requirements. During light load or idle modes, the voltage is automatically decreased in order to save power and is instantly increased during high-performance tasks as depicted in Figure 3.

TI implements dynamic voltage scaling through several standard approaches depending on the user's requirements:

I²C Interface:

This allows a processor to digitally change voltage presets, ramp speed, and other internal features by sending commands over a bus. This enables software-controlled, real-time adjustment of the output voltage in exact, small step sizes, for example in TPS628600.

Hardware Select / VSEL Pins:

If the voltage only needs to be selected from two or up to four options using external hardware pins on the switching regulator, this can be done by changing the logic levels of these pins during operation. Examples include TPS62740, TPS62743, and TPS62860.

TPS62860/1 offers the ability to dynamically control V_out via the I2C interface or a dedicated VSEL pin with different default V_out settings as shown in Table 3 below:

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