The TPSM8A28 and TPSM8A29 provide analog input pin (SS/REFIN) to accept an external reference (that is, a DC voltage source). The device always looks at the voltage on this SS/REFIN pin as the reference for the control loop. When an external voltage reference is applied between the SS/REFIN pin and VSNS– pin, it acts as the reference voltage, so the FB voltage follows this external voltage reference exactly. The same ±0.6% SS/REFIN-to-FB accuracy from a -40°C to 125°C temperature range applies here too.

In the middle of internal power-on delay, a detection circuit senses the voltage on the SS/REFIN pin to tell whether an active DC voltage source is applied. Before the detection happens, the SS/REFIN pin tries to discharge any energy on the SS/REFIN capacitors through an internal 120-Ω resistor to AGND, lasting 125 μs. Then, within a 32-μs window, the detection circuit compares the SS/REFIN pin voltage with an internal reference equal to 89% of V_INTREF. This discharge operation makes sure a SS capacitor with left-over energy is not wrongly detected as a voltage reference. If the external voltage reference failed to supply sufficient current and hold a voltage level higher than 89% of V_INTREF, the SS/REFIN detection circuit provides a wrong detection result.

If the detection result is that the SS/REFIN pin voltage holds higher than 89% of V_INTREF, which tells an active DC voltage source is used as external reference, the device always uses the SS/REFIN pin voltage instead of the internal V_INTREF as the reference for PGOOD threshold, V_OUT OVP, and V_OUT UVP threshold. On this configuration, since the SS/REFIN pin senses a DC voltage and no soft-start ramp on this pin, the internal fixed soft start is used for start-up. Once the internal soft-start ramp finishes, the power-good signal becomes high after a 1.06-ms internal delay. The whole internal soft-start ramp takes 2 ms to finish because the soft-start ramp goes beyond V_INTREF.

If the detection result is that SS/REFIN pin voltage falls below 89% of V_INTREF, which tells no external reference is connected, the device first uses the internal fixed V_INTREF as the reference for PGOOD threshold, V_OUT OVP, and V_OUT UVP threshold. On this configuration, given the SS/REFIN pin sees a soft-start ramp on this pin, the slower ramp amongst the internal fixed soft start and the external soft start determines the start-up of FB. Once both the internal and external soft-start ramp finishes, the power-good signal becomes high after a 1.06-ms internal delay. The whole internal soft-start ramp takes 2 ms to finish. The external soft-start done signal goes high when FB reaches a threshold equal to V_INTREF – 50 mV. The device waits for the PGOOD status transition from low to high, then starts using the SS/REFIN pin voltage instead of the internal V_INTREF as the reference for PGOOD threshold, V_OUT OVP, and V_OUT UVP threshold.

On this external REFIN configuration, applying a stabilized DC external reference to SS/REFIN pin before the EN high signal is recommended. During the internal power-on delay, the external reference must be capable of holding the SS/REFIN pin equal to or higher than 89% of V_INTREF, so that the device can correctly detect the external reference and choose the right thresholds for power good, V_OUT OVP, and V_OUT UVP. After the power-good status transits from low to high, the external reference can be set in a range of 0.5 V to 1.2 V. To overdrive the SS/REFIN pin during nominal operation, the external reference has to be able to sink more than 36-μA current if the external reference is lower than the internal V_INTREF, or source more than 12-μA current if the external reference is higher than the internal V_INTREF. When driving the SS/REFIN pin by an external reference through a resistor divider, the resistance of the divider must be low enough to provide the sinking or sourcing current capability.

If the external voltage source must transition up and down between any two voltage levels, the slew rate must be no more than 1 mV/μs.