TPS54561_15 Datasheet, PDF (12/53 Pages) Texas Instruments – TPS54561 4.5-V to 60-V Input, 5-A, Step-Down DC-DC Converter with Soft-Start and Eco-mode™

TPS54561
SLVSBO1D – JULY 2013 – REVISED JULY 2015
7 Detailed Description
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7.1 Overview
The TPS54561 is a 60-V, 5-A, step-down (buck) regulator with an integrated high side n-channel MOSFET. The
device implements constant frequency, current mode control which reduces output capacitance and simplifies
external frequency compensation. The wide switching frequency range of 100 kHz to 2500 kHz allows either
efficiency or size optimization when selecting the output filter components. The switching frequency is adjusted
using a resistor to ground connected to the RT/CLK pin. The device has an internal phase-locked loop (PLL)
connected to the RT/CLK pin that will synchronize the power switch turn on to a falling edge of an external clock
signal.
The TPS54561 has a default input start-up voltage of 4.3 V typical. The EN pin can be used to adjust the input
voltage undervoltage lockout (UVLO) threshold with two external resistors. An internal pull up current source
enables operation when the EN pin is floating. The operating current is 152 μA under no load condition when not
switching. When the device is disabled, the supply current is 2 μA.
The integrated 87-mΩ high side MOSFET supports high efficiency power supply designs capable of delivering 5
A of continuous current to a load. The gate drive bias voltage for the integrated high side MOSFET is supplied by
a bootstrap capacitor connected from the BOOT to SW pins. The TPS54561 reduces the external component
count by integrating the bootstrap recharge diode. The BOOT pin capacitor voltage is monitored by a UVLO
circuit which turns off the high side MOSFET when the BOOT to SW voltage falls below a preset threshold. An
automatic BOOT capacitor recharge circuit allows the TPS54561 to operate at high duty cycles approaching
100%. Therefore, the maximum output voltage is near the minimum input supply voltage of the application. The
minimum output voltage is the internal 0.8-V feedback reference.
Output overvoltage transients are minimized by an Overvoltage Protection (OVP) comparator. When the OVP
comparator is activated, the high side MOSFET is turned off and remains off until the output voltage is less than
106% of the desired output voltage.
The SS/TR (soft start/tracking) pin is used to minimize inrush currents or provide power supply sequencing
during power up. A small value capacitor should be connected to the pin to adjust the soft start time. A resistor
divider can be connected to the pin for critical power supply sequencing requirements. The SS/TR pin is
discharged before the output powers up. This discharging ensures a repeatable restart after an over-temperature
fault, UVLO fault or a disabled condition. When the overload condition is removed, the soft start circuit controls
the recovery from the fault output level to the nominal regulation voltage. A frequency foldback circuit reduces the
switching frequency during start up and overcurrent fault conditions to help maintain control of the inductor
current.
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