Datasheet

VSENSE
V
O
+
–
TPS54618
R1
R2
0.799 V
O
0.799 V
R2 = R1
V 0.799 V
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TPS54618
SLVSAE9D –NOVEMBER 2010–REVISED DECEMBER 2013 www.ti.com
BOOTSTRAP VOLTAGE (BOOT) AND LOW DROPOUT OPERATION
The TPS54618 has an integrated boot regulator and requires a small ceramic capacitor between the BOOT and
PH pin to provide the gate drive voltage for the high side MOSFET. The value of the ceramic capacitor should be
0.1 μF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher is
recommended because of the stable characteristics over temperature and voltage.
To improve drop out, the TPS54618 is designed to operate at 100% duty cycle as long as the BOOT to PH pin
voltage is greater than 2.2 V. The high side MOSFET is turned off using an UVLO circuit, allowing for the low
side MOSFET to conduct when the voltage from BOOT to PH drops below 2.2 V. Since the supply current
sourced from the BOOT pin is very low, the high side MOSFET can remain on for more switching cycles than are
required to refresh the capacitor, thus the effective duty cycle of the switching regulator is very high.
ERROR AMPLIFIER
The TPS54618 has a transconductance amplifier. The error amplifier compares the VSENSE voltage to the lower
of the SS/TR pin voltage or the internal 0.799 V voltage reference. The transconductance of the error amplifier is
245μA/V during normal operation. When the voltage of VSENSE pin is below 0.799 V and the device is
regulating using the SS/TR voltage, the gm is typically greater than 79 μA/V, but less than 245 μA/V. The
frequency compensation components are placed between the COMP pin and ground.
VOLTAGE REFERENCE
The voltage reference system produces a precise ±1% voltage reference over temperature by scaling the output
of a temperature-stable bandgap circuit. The bandgap and scaling circuits produce 0.799 V at the non-inverting
input of the error amplifier.
ADJUSTING THE OUTPUT VOLTAGE
The output voltage is set with a resistor divider from the output node to the VSENSE pin. It is recommended to
use divider resistors with 1% tolerance or better. Start with a 100 kΩ for the R1 resistor and use the Equation 1
to calculate R2. To improve efficiency at very light loads consider using larger value resistors. If the values are
too high the regulator is more susceptible to noise and voltage errors from the VSENSE input current are
noticeable.
vertical spacer
vertical spacer
(1)
Figure 25. Voltage Divider Circuit
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