9.3.1 Current Limit Operation

The TPS6125x device employs a valley current limit sensing scheme. Current limit detection occurs during the off-time by sensing of the voltage drop across the synchronous rectifier.

The output voltage is reduced as the power stage of the device operates in a constant current mode. The maximum continuous output current (I_OUT(CL)), before entering current limit (CL) operation, can be defined by Equation 1.

Equation 1.

The duty cycle (D) can be estimated by Equation 2

Equation 2.

and the peak-to-peak current ripple (ΔI_L) is calculated by Equation 3

Equation 3.

The output current, I_OUT(DC), is the average of the rectifier ripple current waveform. When the load current is increased such that the lower peak is above the current limit threshold, the off-time is increased to allow the current to decrease to this threshold before the next on-time begins (so called frequency fold-back mechanism). When the current limit is reached the output voltage decreases during further load increase.

Figure 26 illustrates the inductor and rectifier current waveforms during current limit operation.

Figure 26. Inductor/Rectifier Currents in Current Limit Operation

9.3.2 Enable

The TPS6125x device starts operation when EN is set high and starts up with the soft-start sequence. For proper operation, the EN pin must be terminated and must not be left floating.

Pulling the EN and BP pins low forces the device in shutdown, with a shutdown current of typically 1 µA. In this mode, true load disconnect between the battery and load prevents current flow from V_IN to V_OUT, as well as reverse flow from V_OUT to V_IN.

Pulling the EN pin low and the BP pin high forces the device in standby mode, refer to the Standby Mode section for more details.

9.3.3 Load Disconnect and Reverse Current Protection

Regular boost converters do not disconnect the load from the input supply and therefore a connected battery will be discharged during shutdown. The advantage of TPS6125x is that this converter disconnects the output from the input of the power supply when it is disabled (so called true shutdown mode). In case of a connected battery it prevents it from being discharged during shutdown of the converter.

9.3.4 Softstart

The TPS6125x device has an internal softstart circuit that limits the inrush current during start-up. The first step in the start-up cycle is the pre-charge phase. During pre-charge, the rectifying switch is turned on until the output capacitor is charged to a value close to the input voltage. The rectifying switch is current limited (approximately 200 mA) during this phase. This mechanism is used to limit the output current under short-circuit condition.

Once the output capacitor has been biased to the input voltage, the converter starts switching. The soft-start system progressively increases the on-time as a function of the input-to-output voltage ratio. As soon as the output voltage is reached, the regulation loop takes control and full current operation is permitted.

9.3.5 Undervoltage Lockout

The under voltage lockout circuit prevents the device from malfunctioning at low input voltages and the battery from excessive discharge. It disables the output stage of the converter once the falling V_IN trips the under-voltage lockout threshold V_UVLO which is typically 2.0V. The device starts operation once the rising V_IN trips V_UVLO threshold plus its hysteresis of 100 mV at typically 2.1 V.

9.3.6 Thermal Regulation

The TPS6125x device contains a thermal regulation loop that monitors the die temperature during the pre-charge phase. If the die temperature rises to high values of about 110 °C, the device automatically reduces the current to prevent the die temperature from increasing further. Once the die temperature drops about 10 °C below the threshold, the device will automatically increase the current to the target value. This function also reduces the current during a short-circuit condition.

9.3.7 Thermal Shutdown

As soon as the junction temperature, T_J, exceeds 140°C (typ.) the device goes into thermal shutdown. In this mode, the high-side and low-side MOSFETs are turned-off. When the junction temperature falls below the thermal shutdown minus its hysteresis, the device continuous the operation.

9.4.1 Power Save Mode

The TPS6125x integrates a power save mode to improve efficiency at light load. In power save mode the converter only operates when the output voltage trips below a set threshold voltage.

It ramps up the output voltage with several pulses and goes into power save mode once the output voltage exceeds the set threshold voltage.

The PFM mode is left and PWM mode entered in case the output current can not longer be supported in PFM mode.

Figure 27. Power Save

9.4.2 Standby Mode

The TPS6125x device is able to maintain its output biased at the input voltage level. In so called standby mode (EN = 0, BP = 1), the synchronous rectifier is current limited to ca. 150mA allowing an external load (e.g. audio amplifier) to be powered with a restricted supply. The output voltage is slightly reduced due to voltage drop across the rectifier MOSFET and the inductor DC resistance. The device consumes only a standby current of 21 µA (typ).