TPS62933O datasheet(22/47 Pages) TI

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8.4 Device Functional Modes

8.4.1 Modes Overview

The TPS62933 moves between CCM, DCM, and PFM mode as the load changes. Depending on the load

current, the TPS62933 will be in one of below modes:

• Continuous conduction mode (CCM) with fixed switching frequency when load current is above half of the

peak-to-peak inductor current ripple.

• Discontinuous conduction mode (DCM) with fixed switching frequency when load current is lower than half of

the peak-to-peak inductor current ripple in CCM operation.

• Pulse frequency modulation mode (PFM) when switching frequency is decreased at very light load.

8.4.2 Heavy Load Operation

The TPS62933 operates in continuous conduction mode (CCM) when the load current is higher than half of

the peak-to-peak inductor current. In CCM operation, the output voltage is regulated by switching at a constant

frequency and modulating the duty cycle to control the power to the load. This provides excellent line and

load regulation and minimum output voltage ripple, and the maximum continuous output current of 3 A can be

supplied by the TPS62933.

8.4.3 Light Load Operation (PFM Version)

For PFM version, when the load current is lower than half of the peak-to-peak inductor current in CCM, the

device operates in discontinuous conduction mode (DCM), also known as diode emulation mode (DEM). In DCM

operation, the LS switch is turned off when the inductor current drops to ILS_ZC (150 mA typical) to improve

efficiency. Both switching losses and conduction losses are reduced in DCM, compared to forced CCM operation

at light load.

At even lighter current load, pulse frequency modulation (PFM) mode is activated to maintain high efficiency

operation. When either the minimum ON time, tON_MIN, or the minimum peak inductor current, IPEAK_MIN, (750

mA typical) is reached, the switching frequency decreases to maintain regulation. In PFM mode, switching

frequency is decreased by the control loop to maintain output voltage regulation when load current reduces.

Switching loss is further reduced in PFM operation due to less frequent switching actions. The output current

for mode change depends on the input voltage, inductor value, and the programmed switching frequency. For

applications where the switching frequency must be known for a given condition, the transition between PFM

and CCM must be carefully tested before the design is finalized.

8.4.4 Dropout Operation

The dropout performance of any buck converter is affected by the RDSON of the power MOSFETs, the DC

resistance of the inductor, and the maximum duty cycle that the controller can achieve. As the input voltage level

approaches the output voltage, the off time of the high-side MOSFET starts to approach the minimum value.

Beyond this point, the switching frequency becomes erratic and the output voltage can fall out of regulation. To

avoid this problem, the TPS62933 automatically reduces the switching frequency (on-time extension function) to

increase the effective duty cycle and maintain in regulation until the switching frequency reach to the lowest limit

of about 140 kHz, the period is equal to (tON_MAX + tOFF_MIN) (7.14-μS typical). In this condition, the difference

voltage between VIN and VOUT is defined as dropout voltage. The typical overall dropout characteristics can be

found as Figure 8-9.

TPS62933

SLUSEA4A – JUNE 2021 – REVISED DECEMBER 2021

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