TPS54560B-Q1 Datasheet, PDF (12/45 Pages) Texas Instruments – 4.5 V to 60 V Input, 5 A, Step Down DC-DC Converter with Eco-mode

TPS54560B-Q1
SLVSDP8 – FEBRUARY 2017
www.ti.com
Feature Description (continued)
7.3.3 Pulse Skip Eco-mode
The TPS54560B-Q1 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 TPS54560B-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 33 enters Eco-mode at about 25.3 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.
7.3.4 Low Dropout Operation and Bootstrap Voltage (BOOT)
The TPS54560B-Q1 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 TPS54560B-Q1
will operate at 100% duty cycle as long as the BOOT to SW terminal 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.
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.
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.
Equation 1 calculates the minimum input voltage required to regulate the output voltage and ensure proper
operation of the device. This calculation must include tolerance of the component specifications and the variation
of these specifications at their maximum operating temperature in the application
V_IN (min)
V _OUT
V_F R _dc u I _OUT
D
R _DS(on) u I _OUT
V_F
where
• V_F = Schottky diode forward voltage
• R_DC = Total DC resistance of inductor and PCB
• R_DS(on) = High-side MOSFET resistance
• D = 0.99 effective duty cycle
(1)
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