TPS54060A Datasheet, PDF (13/50 Pages) Texas Instruments – 0.5A, 60V Step Down SWIFT™ DC/DC Converter with ECO-MODE™

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DETAILED DESCRIPTION (continued)
VOUT_(ac)
I_L
TPS54060A
SLVSB57 – MARCH 2012
PH
Figure 25. Pulse Skip Mode Operation
Low Dropout Operation and Bootstrap Voltage (BOOT)
The TPS54060A has an integrated boot regulator, and requires a small ceramic capacitor between the BOOT
and PH pins to provide the gate drive voltage for the high side MOSFET. The BOOT capacitor is refreshed when
the high side MOSFET is off and the low side diode conducts. The value of this ceramic capacitor should be
0.1μF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10V or higher is
recommended because of the stable characteristics overtemperature and voltage.
To improve drop out, the TPS54060A is designed to operate at 100% duty cycle as long as the BOOT to PH pin
voltage is greater than 2.1V. When the voltage from BOOT to PH drops below 2.1V, the high side MOSFET is
turned off using an UVLO circuit which allows the low side diode to conduct and refresh the charge on the BOOT
capacitor. Since the supply current sourced from the BOOT capacitor is 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 high.
The effective duty cycle during dropout of the regulator is mainly influenced by the voltage drops across the
power MOSFET, inductor resistance, low side diode and printed circuit board resistance. During operating
conditions in which the input voltage drops and the regulator is operating in continuous conduction mode, the
high side MOSFET can remain on for 100% of the duty cycle to maintain output regulation, until the BOOT to PH
voltage falls below 2.1V.
Attention must be taken in maximum duty cycle applications which experience extended time periods with light
loads or no load. When the voltage across the BOOT capacitor falls below the 2.1V UVLO threshold, the high
side MOSFET is turned off, but there may not be enough inductor current to pull the PH pin down to recharge the
BOOT capacitor. The high side MOSFET of the regulator stops switching because the voltage across the BOOT
capacitor is less than 2.1V. The output capacitor then decays until the difference in the input voltage and output
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