SLVSDV8 July 2017 TPS54424
The TPS54424 is a 17-V, 4-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 also simplifies external frequency compensation. The wide switching frequency of 200 kHz to 1600 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 TPS54424 also has an internal phase lock loop (PLL) connected to the RT/CLK pin that can be used to synchronize the switching cycle to the falling edge of an external system clock.
The integrated MOSFETs allow for high efficiency power supply designs with continuous output currents up to 4 amperes. The MOSFETs have been sized to optimize efficiency for lower duty cycle applications. The device reduces the external component count by integrating a bootstrap recharge circuit. The bias voltage for the integrated high-side MOSFET is supplied by a capacitor between the BOOT and SW pins. The BOOT capacitor voltage is monitored by a BOOT to SW UVLO (BOOT-SW UVLO) circuit allowing SW pin to be pulled low to recharge the BOOT capacitor. The device can operate at 100% duty cycle as long as the BOOT capacitor voltage is higher than the preset BOOT-SW UVLO threshold which is typically 2.2 V.
The TPS54424 has been designed for safe monotonic startup into pre-biased loads. The default start up is when VIN is typically 4.1 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 internal pull-up current of the EN pin allows the device to operate with the EN pin floating. The operating current for the TPS54424 is typically 580 μA when not switching and under no load. When the device is disabled, the supply current is typically 3 μA.
The SS/TRK (soft start/tracking) pin is used to minimize inrush currents or provide power supply sequencing during power up. A small value capacitor or resistor divider should be coupled to the pin for soft start or critical power supply sequencing requirements. The output voltage can be stepped down to as low as the 0.6 V voltage reference (VREF). The device has a power good comparator (PGOOD) with hysteresis which monitors the output voltage through the FB pin. The PGOOD pin is an open drain MOSFET which is pulled low when the FB pin voltage is less than 89% or greater than 108% of the reference voltage VREF and asserts high when the FB pin voltage is 91% to 106% of VREF.
The device is protected from output overvoltage, overload and thermal fault conditions. The device minimizes excessive output overvoltage transients by taking advantage of the overvoltage circuit power good comparator. When the overvoltage comparator is activated, the high-side MOSFET is turned off and prevented from turning on until the FB pin voltage is lower than 106% of the VREF. The device implements both high-side MOSFET over current protection and bidirectional low-side MOSFET over current protections which help control the inductor current and avoid current runaway. The device also shuts down if the junction temperature is higher than thermal shutdown trip point. The device is restarted under control of the soft start circuit automatically when the junction temperature drops 15°C typically below the thermal shutdown trip point.