4.7

0

0.1

0.15

0.35

0.5

Load Current - A

Start

Stop

4.8

4.9

5

5.1

5.3

4.6

5.4

5.5

5.6

5.2

0.05

0.25

0.2

0.3

0.4

0.45

Dropout

Voltage

Dropout

Voltage

TPS54360-Q1

www.ti.com

SLVSBZ2 – SEPTEMBER 2013

When in Eco-mode, the COMP-pin voltage is clamped at 600 mV and the high-side MOSFET is inhibited.

Because the device is not switching, the output voltage begins to decay. The voltage control loop responds to the

falling output voltage by increasing the COMP-pin voltage. The high-side MOSFET is enabled and switching

resumes when the error amplifier lifts COMP above the pulse skipping threshold. The output voltage recovers to

the regulated value, and COMP eventually falls below the Eco-mode pulse skipping threshold at which time the

device again enters Eco-mode. The internal PLL remains operational when in Eco-mode. When operating at light

load currents in Eco-mode, the switching transitions occur synchronously with the external clock signal.

During Eco-mode operation, the TPS54360-Q1 senses and controls peak switch current, not the average load

current. Therefore the load current at which the device enters Eco-mode is dependent on the output inductor

value. The circuit in Figure 34 enters Eco-mode at about 24 mA output current. As the load current approaches

zero, the device enters a pulse skip mode during which it draws only 146

μA input quiescent current.

Low Dropout Operation and Bootstrap Voltage (BOOT)

The TPS54360-Q1 provides an integrated bootstrap voltage regulator. A small capacitor between the BOOT and

SW pins provides the gate drive voltage for the high-side MOSFET. The BOOT capacitor is refreshed when the

high-side MOSFET is off and the external low-side diode conducts. The recommended value of the BOOT

capacitor is 0.1

μF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or

higher is recommended for stable performance over temperature and voltage.

When operating with a low voltage difference from input to output, the high-side MOSFET of the TPS54360-Q1

operates at 100% duty cycle as long as the BOOT to SW-pin voltage is greater than 2.1 V. When the voltage

from BOOT to SW drops below 2.1V, the high-side MOSFET is turned off and an integrated low-side MOSFET

pulls SW low to recharge the BOOT capacitor. To reduce the losses of the small low-side MOSFET at high

output voltages, it is disabled at 24 V output and re-enabled when the output reaches 21.5 V.

Because the gate drive current sourced from the BOOT capacitor is small, the high-side MOSFET can remain on

for many switching cycles before the MOSFET is turned off to refresh the capacitor. Thus the effective duty cycle

of the switching regulator can be high, approaching 100%. The effective duty cycle of the converter during

dropout is mainly influenced by the voltage drops across the power MOSFET, the inductor resistance, the low-

side diode voltage and the printed circuit board resistance.

The start and stop voltage for a typical 5 V output application is shown in Figure 22 where the Vin voltage is

plotted versus load current. The start voltage is defined as the input voltage required to regulate the output within

1% of nominal. The stop voltage is defined as the input voltage at which the output drops by 5% or where

switching stops.

During high duty cycle (low dropout) conditions, inductor current ripple increases when the BOOT capacitor is

being recharged resulting in an increase in output voltage ripple. Increased ripple occurs when the off time

required to recharge the BOOT capacitor is longer than the high-side off time associated with cycle by cycle

PWM control.

spacer

Figure 22. 5V Start/Stop Voltage

Copyright © 2013, Texas Instruments Incorporated

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