SW,

5V/Div

VOUT,

10mV/Div

Inductor

Current,

0.5A/Div

50µs/Div

2µs/Div

Inductor

Current,

2A/Div

VOUT,

10mV/Div

SW,

5V/Div

14

LM60440, LM60430

SNVSBN5A – FEBRUARY 2020 – REVISED JUNE 2020

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Copyright © 2020, Texas Instruments Incorporated

8.4 Device Functional Modes

8.4.1 Auto Mode

In auto mode, the device moves between PWM and PFM as the load changes. At light loads, the regulator

operates in PFM. At higher loads, the mode changes to PWM. The load current for which the device moves from

PFM to PWM can be found in the Application Curves. The output current at which the device changes modes

depends on the input voltage, inductor value, and the nominal switching frequency. For output currents above the

curve, the device is in PWM mode. For currents below the curve, the device is in PFM. The curves apply for a

nominal switching frequency of 400 kHz. At higher switching frequencies, the load at which the mode change

occurs is greater. For applications where the switching frequency must be known for a given condition, the

transition between PFM and PWM must be carefully tested before the design is finalized.

In PWM mode, the regulator operates as a constant frequency converter using PWM to regulate the output

voltage. While operating in this mode, 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

low output voltage ripple.

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

bursts is adjusted to regulate the output, while diode emulation (DEM) is used to maximize efficiency (see

Glossary). This mode provides high light-load efficiency by reducing the amount of input supply current required

to regulate the output voltage at light loads. PFM results in very good light-load efficiency, but also yields larger

output voltage ripple and variable switching frequency. Also, a small increase in output voltage occurs at light

loads. The actual switching frequency and output voltage ripple depends on the input voltage, output voltage,

and load. Typical switching waveforms in PFM and PWM are shown in Figure 7 and Figure 8.

See the Application Curves for output voltage variation with load in auto mode.

Figure 7. LM60430 Typical PFM Switching Waveforms

Figure 8. LM60430 Typical PWM Switching Waveforms

8.4.2 Dropout

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 (see

the Timing Characteristics). Beyond this point, the switching can become erratic, and the output voltage falls out

of regulation. To avoid this problem, the LM604x0 automatically reduces the switching frequency to increase the

effective duty cycle and maintain regulation. In this data sheet, the dropout voltage is defined as the difference

between the input and output voltage when the output has dropped by 1% of its nominal value. Under this

condition, the switching frequency has dropped to its minimum value of about 140 kHz. Note that the 0.4 V short

circuit detection threshold is not activated when in dropout mode.