7.1 Overview

The TPS54218 device is a 6-V, 2-A, synchronous step-down (buck) converter with two integrated n-channel MOSFETs. To improve performance during line and load transients the device implements a constant frequency, peak current mode control which reduces output capacitance and simplifies external frequency compensation design. The wide supported switching frequency range of 200 kHz to 2000 kHz allows for efficiency and size optimization when selecting the output filter components. The switching frequency is adjusted using a resistor to ground on the RT/CLK pin. The device has an internal phase lock loop (PLL) on the RT/CLK pin that is used to synchronize the power switch turn on to a falling edge of an external system clock.

The TPS54218 device has a typical default start up voltage of 2.6 V. The EN pin has an internal pull-up current source that can be used to adjust the input voltage under voltage lockout (UVLO) with two external resistors. In addition, the pull up current provides a default condition when the EN pin is floating for the device to operate. The total operating current for the TPS54218 device is 350 μA when not switching and under no load. When the device is disabled, the supply current is less than 5 μA.

The integrated, 30-mΩ MOSFETs allow for high-efficiency power supply designs with continuous output currents up to 2 amperes.

The TPS54218 device reduces the external component count by integrating the boot recharge diode. The bias voltage for the integrated high-side MOSFET is supplied by a capacitor on the BOOT to PH pin. The boot capacitor voltage is monitored by an UVLO circuit and turns off the high-side MOSFET when the voltage falls below a preset threshold. This BOOT circuit allows the TPS54218 device to operate approaching 100%. The output voltage can be stepped down to as low as the 0.8 V reference.

The TPS54218 device has a power good comparator (PWRGD) with 2% hysteresis.

The TPS54218 device minimizes excessive output overvoltage transients by taking advantage of the overvoltage power good comparator. When the regulated output voltage is greater than 109% of the nominal voltage, the overvoltage comparator is activated, and the high-side MOSFET is turned off and masked from turning on until the output voltage is lower than 105%.

The SS (soft-start) pin is used to minimize inrush currents or provide power supply sequencing during power up. A small value capacitor should be coupled to the pin for soft-start. The SS pin is discharged before the output power up to ensure a repeatable re-start after an over-temperature fault, UVLO fault or disabled condition.

The use of a frequency-foldback circuit reduces the switching frequency during startup and over current fault conditions to help limit the inductor current.