Datasheet

3
3.2
3.4
3.6
3.8
4
0 0.05 0.10 0.15 0.20
I -OutputCurrent- A
O
V -InputVoltage-V
I
V =3.3V
O
Start
Stop
4.6
4.8
5
5.2
5.4
5.6
0 0.05 0.10 0.15 0.20
I -OutputCurrent- A
O
V -InputVoltage-V
I
V =5V
O
Start
Stop
TPS54140
www.ti.com SLVS889B –OCTOBER 2008–REVISED SEPTEMBER 2013
DETAILED DESCRIPTION (continued)
Bootstrap Voltage (BOOT)
The TPS54140 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
0.1μF. A ceramic capacitor with an X7R or X5R grade dielectric is recommended because of the stable
characteristics over temperature and voltage. To improve drop out, the TPS54140 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 allowing for the low side diode
to conduct which allows refreshing of 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 it refreshes, thus, the
effective duty cycle limitation that is attributed to the boot regulator system is high.
Low Dropout Operation
The duty cycle during dropout of the regulator will be mainly determined by the voltage drops across the power
MOSFET, inductor, low side diode and printed circuit board resistance. During operating conditions in which the
input voltage drops, the high side MOSFET can remain on for 100% of the duty cycle to maintain output
regulation or until the BOOT to PH voltage falls below 2.1V.
Once the high side is off, the low side diode will conduct and the BOOT capacitor will be recharged. During this
boot capacitor recharge time, the inductor current will ramp down until the high side MOSFET turns on. The
recharge time is longer than the typical high side off time of previous switching cycles, and thus, the inductor
current ripple is larger resulting in more ripple voltage on the output. The recharge time is a function of the input
voltage, boot capacitor value, and the impedance of the internal boot recharge diode.
Attention needs to be taken in maximum duty cycle applications which experience extended time periods without
a load current. When the voltage across the BOOT capacitors falls below the 2.1V threshold in applications that
have a difference in the input voltage and output voltage that is less than 3V, the high side MOSFET will be
turned off but there is not enough current in the inductor to pull the PH pin down to recharge the boot capacitor.
The regulator will not switch because the boot capacitor is less than 2.1V and the output capacitor will decay until
the difference in the input voltage and output voltage is 2.1V. At this time the boot under voltage lockout is
exceeded and the device will switch until the desired output voltage is reached.
The start and stop voltages are shown in Figure 26 and Figure 27 for 3.3V and 5V applications. The voltages are
plotted versus the load current. The start voltage is defined as the input voltage needed to regulate within 1%.
The stop voltage is defined as the input voltage at which the output drops by 5% or stops switching.
Figure 26. 3.3V Start/Stop Voltage Figure 27. 5.0V Start/Stop Voltage
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