Datasheet

TPS54360

SLVSBB4E –AUGUST 2012 –REVISED MARCH 2014

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Feature Description (continued)

8.3.3 Pulse Skip Eco-mode

The TPS54360 operates in a pulse skipping Eco-mode at light load currents to improve efficiency by reducing

switching and gate drive losses. If the output voltage is within regulation and the peak switch current at the end

of any switching cycle is below the pulse skipping current threshold, the device enters Eco-mode. The pulse

skipping current threshold is the peak switch current level corresponding to a nominal COMP voltage of 600 mV.

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

Since 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 terminal 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 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 35 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.

8.3.4 Low Dropout Operation and Bootstrap Voltage (BOOT)

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

terminals 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 will

operate at 100% duty cycle as long as the BOOT to SW terminal voltage is greater than 2.1V. 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.

Since 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 needed 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.

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Product Folder Links: TPS54360