Datasheet

TPS54618
www.ti.com SLVSAE9D –NOVEMBER 2010–REVISED DECEMBER 2013
OVERVIEW
The TPS54618 is a 6-V, 6-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 switching frequency 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 TPS54618 has a typical default start up voltage of 2.45 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 TPS54618 is typically 515 μA when not switching and under no load. When the device
is disabled, the supply current is less than 5.5 μA.
The integrated 12 mΩ MOSFETs allow for high efficiency power supply designs with continuous output currents
up to 6 amperes.
The TPS54618 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 between the BOOT and PH pins. 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 TPS54618 to operate approaching 100%. The output
voltage can be stepped down to as low as the 0.799 V reference.
The TPS54618 has a power good comparator (PWRGD) with 2% hysteresis.
The TPS54618 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 107%.
The SS/TR (slow start/tracking) 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 slow start. The SS/TR pin is
discharged before the output power up to ensure a repeatable restart after an overtemperature fault, UVLO fault
or disabled condition.
The use of a frequency fold-back circuit reduces the switching frequency during startup and over current fault
conditions to help limit the inductor current.
DETAILED DESCRIPTION
FIXED FREQUENCY PWM CONTROL
The TPS54618 uses an adjustable fixed frequency, peak current mode control. The output voltage is compared
through external resistors on the VSENSE pin to an internal voltage reference by an error amplifier which drives
the COMP pin. An internal oscillator initiates the turn on of the high side power switch. The error amplifier output
is compared to the high side power switch current. When the power switch current reaches the COMP voltage
level the high side power switch is turned off and the low side power switch is turned on. The COMP pin voltage
increases and decreases as the output current increases and decreases. The device implements a current limit
by clamping the COMP pin voltage to a maximum level and also implements a minimum clamp for improved
transient response performance.
SLOPE COMPENSATION AND OUTPUT CURRENT
The TPS54618 adds a compensating ramp to the switch current signal. This slope compensation prevents sub-
harmonic oscillations as duty cycle increases. The available peak inductor current remains constant over the full
duty cycle range.
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