7.3.12 Overcurrent Protection and Frequency Shift

The TPS57140-EP implements current-mode control, which uses the COMP pin voltage to turn off the high-side MOSFET on a cycle-by-cycle basis. During each cycle, the device compares the switch current and COMP pin voltage. When the peak switch current intersects the COMP voltage, the high-side switch turns off. During overcurrent conditions that pull the output voltage low, the error amplifier responds by driving the COMP pin high, increasing the switch current. The internal clamping of the error amplifier output functions as a switch current limit.

To increase the maximum operating switching frequency at high input voltages, the TPS57140-EP implements a frequency shift. The divisor of the switching frequency goes to 8, 4, 2, and 1 as the voltage ramps from 0 to 0.8 V on the VSENSE pin.

The device implements a digital frequency shift to enable synchronizing to an external clock during normal start-up and fault conditions. Because the device can only divide the switching frequency by 8, there is a maximum input voltage limit in which the device operates and still has frequency-shift protection.

During short-circuit events (particularly with high-input-voltage applications), the control loop has a finite minimum controllable on-time and the output has a very-low voltage. During the switch on-time, the inductor current ramps to the peak current limit because of the high input voltage and minimum on-time. During the switch off time, the inductor would normally not have enough off-time and output voltage for the inductor to ramp down by the ramp-up amount. The frequency shift effectively increases the off-time, allowing the current to ramp down.