TPS54418_17 Datasheet, PDF (11/42 Pages) Texas Instruments – 2.95-V to 6-V Input, 4-A Output, 2-MHz , Synchronous Step-Down Switcher With Integrated FETs (SWIFT)

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8 Detailed Description
TPS54418
SLVS946D – SEPTEMBER 2009 – REVISED DECEMBER 2014
8.1 Overview
The TPS54418 device is a 6-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 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 TPS54418 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 TPS54418 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 4 amperes.
The TPS54418 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 TPS54418 device to operate approaching 100%. The
output voltage can be stepped down to as low as the 0.8 V reference.
The TPS54418 device has a power good comparator (PWRGD) with 2% hysteresis.
The TPS54418 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.
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