Datasheet

V ac
OUT
I
L
V
PH
TPS5401
www.ti.com SLVSAB0 –DECEMBER 2010
DETAILED DESCRIPTION (continued)
Figure 25. Pulse-Skip Mode Operation
Low-Dropout Operation and Bootstrap Voltage (BOOT)
The TPS5401 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 0.1 mF.
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 dropout, the TPS5401 is designed to operate at 100% duty cycle as long as the BOOT-to-PH pin
voltage is greater than 2.1 V. When the voltage from BOOT to PH drops below 2.1 V, 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. Because 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 2.1 V.
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 2.1-V 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 2.1 V. The output capacitor then decays until the difference in the input
Copyright © 2010, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Link(s): TPS5401