How to use a boost bypass as the pre-regulator in a smartphone

A typical power source for a portable device like a smartphone is a single lithium-ion battery. With the development of silicon anode material and maximizing the battery energy as much as possible, the minimum operating voltage in a smartphone application for example, is usually lower, for instance 2.7V.

Some load configurations, such as Wi-Fi® modules, embedded multimedia cards (eMMC) and Secure Digital (SD) cards, require a regulated voltage greater than 2.7V. The low-dropout regulators (LDOs) integrated into the power-management integrated circuit (PMIC) supply these power rails. The LDOs’ input (VLDO_IN) must be slightly higher than the highest LDO’s output voltage. Therefore, if VLDO_IN ends up in the middle of the lithium-ion battery’s operating range, a boost regulator is necessary between the battery and the LDOs’ input end to guarantee that the LDOs’ input is higher than their highest regulated voltage.

If the battery is in the well state of charge and the voltage is higher than the required minimum system voltage, the load configurations don’t need a boost function, but just directly bypass the battery voltage to the LDO’s input.

Figure 1 illustrates the power system using the boost bypass as a pre-regulator in a smartphone application.

Figure 1: Boost with bypass for the PMIC pre-regulator

TI developed the TPS61280A, TPS61281A and TPS61282A PMIC family especially for the pre-regulator of the smartphone.  It operates in a low-ohmic, high-efficient bypass mode when the battery voltage is higher than the minimum required voltage of the LDOs’ input. If the battery’s voltage becomes lower than the required minimum voltage, the device seamlessly transitions into boost mode, as shown in Figures 2 and 3.

Figure 2: TPS618xA boost/bypass connection

Figure 3: TPS618xA output voltage regulation

Sweeping the input voltage of the TPS6128xA, with the bench measurement of TPS6128xA conditions at VOUT_Boost = 3.4V, the output voltage follows the input voltage, with a gap around 70mV at a 1.5A load in bypass mode, which is caused by the current flowing through the bypass field-effect transistor (FET) (M3).

During input voltage ramp down, when the input voltage crosses the boost/bypass threshold (3.4V in this case), the TPS61280A enters into boost mode, with around 100mV of undershoot at the output of the TPS61280A.

When ramping up the input voltage, the TPS61280A enters into bypass mode as long as the output voltage is 2% higher than the 3.4V threshold, and the boost-to-bypass has no undershoot (or overshoot) with very smooth transition.

As an example, consider bench measurements with the conditions at load = 1.5A, output capacitance = 16µF (effective), and sweeping the input voltage from 3.3V to 3.7V. In this case, the input voltage is larger than the desired target voltage and the output of the TPS61280A follows the input voltage with a drop voltage of the bypass FET. The output voltage keeps at the target value when the input voltage sweeps below the target value as shown in Figure 4.

Figure 4: TPS6180A output voltage regulation

With a smooth transition between boost and bypass as well as the high efficiency in either boost or bypass mode, the TPS6128xA enables the use of the full battery capacity. You can overcome a high battery cut-off voltage originated by powered components with a high minimum input voltage and silicon anode discharge battery chemistries. The device buffers high current pulses forcing the system into shutdown, seamlessly transitioning between boost and bypass mode.

The pre-regulator benefits with extending the battery on-time and this has a significant impact on battery on-time and translates into either a longer use time or a better user experience at an equal battery capacity, or into reduced battery costs at similar use times.

The TPS61281A and TPS61282A have a fixed current-limit threshold as well as a default voltage value where the transition between boost and bypass takes place. The TPS61280A is a fully programmable device via I2C (two-wire interface). The TPS61280A gives you a high level of flexibility to tailor the device to your own system needs. Table 1 summarizes the key parameters of the TPS6128xA device family.

Part number

DC/DC boost/bypass voltage threshold

Valley inductor current limit

I2C interface


VSEL = L à 3.15V

VSEL = H à 3.35V



valley inductor current limit

DC/DC boost/bypass threshold voltage


VSEL = L à 3.15V

VSEL = H à 3.35V




VSEL = L à 3.30V

VSEL = H à 3.50V



Table 1: TPS6128x family device overview

TI’s TPS61280A, TPS61281A or TPS61282A PMIC used as a pre-regulator extends the battery run time and overcomes input current and input voltage limitations of the system being powered. Offered in a 16-pin chip-scale package (CSP), the PMIC provides a very small total solution footprint (<20mm2) with minimal external inductor and input capacitors. Get more information on TI’s family of boost converters with integrated switches for Li-ion battery-powered applications.