TPS62902-Q1 datasheet(21/54 Pages) TI

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8.4 Device Functional Modes

8.4.1 Forced Pulse Width Modulation (FPWM) Operation

The TPS62902-Q1 has two operating modes: Forced PWM (FPWM) mode discussed in this section and auto

PFM and PWM as discussed in Section 8.4.2.

With the MODE/S-CONF pin configured for FPWM mode, the TPS62902-Q1 operates with pulse width

modulation in continuous conduction mode (CCM) with a nominal switching frequency of either 2.5 MHz or

1.0 MHz. The frequency variation in PWM is controlled and depends on VIN, VOUT, and the inductance. The on

time in forced PWM mode is given by Equation 4:

TON=VOUTVIN× 1fSW

(4)

For very small output voltages, a minimum on time of approximately 30 ns is kept to limit switching losses. The

operating frequency is thereby reduced from its nominal value, which keeps efficiency high.

8.4.2 Power Save Mode Operation (Auto PFM and PWM)

When the MODE/S-CONF pin is configured for power save mode (auto PFM and PWM). The device operates

in PWM mode as long the output current is higher than half of the ripple current of the inductor, and seamlessly

transitions to PSM operation as the load decreases. In power save mode, the device operates in PFM mode

by reducing the switching frequency linearly with the load current to maintain high efficiency under light load

operation. The device remains in power save mode as long as the inductor current remains discontinuous, and

the transition out of PSM also occurs seamlessly when the load current increases above the DCM boundary.

In addition to operating in PFM under light load, when the 2.5 MHz FSW option is selected, the TPS62902-Q1

further adjusts the on time (TON), depending on the input voltage and the output voltage to maintain highest

efficiency using the AEE function as described in Section 8.4.3 .

In power save mode, the TON time can be estimated using Equation 4 for 1 MHz and Equation 8 for 2.5 MHz.

For higher input voltages and small output voltages, an absolute minimum on time of approximately 30 ns is

kept to limit switching losses. The operating frequency is thereby reduced from its nominal value, which keeps

efficiency high. Using TON, the typical peak inductor current in power save mode is approximated by Equation 5:

ILPSMpeak=VIN −VOUTL × TON

(5)

The output voltage ripple in power save mode is given by Equation 6:

∆V=L×VIN2

200 ×C + 1

VIN −VOUT + 1VOUT

(6)

where

• L is the effective inductance.

• C is the output effective capacitance.

Note

When VIN decreases to typically 15% above VOUT, the TPS62902-Q1 does not enter power save

mode, regardless of the load current. The device maintains output regulation in PWM mode.

8.4.3 AEE (Automatic Efficiency Enhancement)

When the MODE/S-CONF pin is configured for auto PFM/PWM with AEE mode, the TPS62902-Q1 provides the

highest efficiency over the entire input voltage and output voltage range by automatically adjusting the switching

frequency of the converter (see Equation 7). To keep the efficiency high over the entire duty cycle range, the

switching frequency is adjusted while maintaining the ripple current amplitudes. This feature compensates for the

very small duty cycles of high VIN to low VOUT conversions, which can limit the control range in other topologies.

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TPS62902-Q1

SLVSG70 – APRIL 2023

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