6.4.2 Heavy Load Operation

The TPSM843521 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. Regulating the output voltage provides

excellent line and load regulation and minimum output voltage ripple, and the maximum continuous output

current of 3A can be supplied by the TPSM843521.

6.4.3 Pulse Frequency Modulation

The TPSM843521 is designed to operate in pulse frequency modulation (PFM) mode at light load currents to

boost light load efficiency when MODE pin is configured to PFM mode.

When the load current is lower than half of the peak-to-peak inductor current in CCM, the devices operate in

discontinuous conduction mode (DCM). In DCM operation, the low-side switch is turned off when the inductor

current drops to ILS_ZC 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 high-side switch on time, tON_MIN, or the minimum peak inductor current

IPEAK_MIN is reached, the switching frequency decreases to maintain regulation. In PFM mode, the 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. Because the

integrated current comparator catches the peak inductor current only, the average load current entering PFM

mode varies with the applications and external output LC filters.

In PFM mode, the high-side MOSFET is turned on in a burst of one or more pulses to provide energy to the

load. The duration of the burst depends on how long the feedback voltage takes to catch VREF. The periodicity

of these bursts is adjusted to regulate the output, while zero current crossing detection turns off the low-side

MOSFET to maximize efficiency. This mode provides high light-load efficiency by reducing the amount of input

supply current required to regulate the output voltage at small loads.

6.4.4 Forced Continuous Conduction Modulation

When the TPSM843521 is configured to operate in FCCM under light load conditions, the switching frequency is

maintained at a constant level over the entire load range, which is an excellent choice for applications requiring

tight control of the switching frequency and output voltage ripple at the cost of lower efficiency under light load.

For some audio application, this mode can help avoid switching frequency drop into audible range that can

introduce some noise.

6.4.5 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 TPSM843521 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.

6.4.6 Minimum On-Time Operation

Every switching converter has a minimum controllable on time dictated by the inherent delays and blanking

times associated with the control circuits, which imposes a minimum switch duty cycle and, therefore, a minimum

conversion ratio. The constraint is encountered at high input voltages and low output voltages. To help extend

the minimum controllable duty cycle, the TPSM843521 automatically reduces the switching frequency when the

minimum on-time limit is reached. This way, the converter can regulate the lowest programmable output voltage

at the maximum input voltage. Use Equation 14 to find an estimate for the approximate input voltage for a given

output voltage before frequency foldback occurs. The values of tON_MIN and fSW can be found in Section 5.5.

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TPSM843521

SLUSG64 – APRIL 2025

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