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 currents at which the mode changes can be found in the Application Curves.
In PWM, the converter operates as a constant frequency, current mode, full synchronous 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. When in PWM the converter synchronizes to any valid clock signal on the SYNC input (seeDropout and Input Voltage Frequency Foldback ).
In PFM the high side FET is turned on in a burst of one or more cycles to provide energy to the load. The frequency of these bursts is adjusted to regulate the output, while diode emulation is used to maximize efficiency (see the Glossary). This mode provides high light-load efficiency by reducing the amount of input supply current required to regulate the output voltage at small loads. This trades off very good light load efficiency for larger output voltage ripple and variable switching frequency. Also, a small increase in the output voltage occurs in PFM. The actual switching frequency and output voltage ripple will depend on the input voltage, output voltage, and load. Typical switching waveforms for PFM are shown in Figure 16. See the Application Curves for output voltage variation in AUTO mode. The SYNC input is ignored during PFM operation.
A unique feature of this device is that a minimum input voltage is required for the regulator to switch from PWM to PFM at light load. This feature is a consequence of the advanced architecture employed to provide high efficiency at light loads. Figure 17 and Figure 18 indicates typical values of input voltage required to switch modes at no load. Also, once the regulator switches to PFM, at light load, it remains in that mode if the input voltage is reduced.
IQ_VIN is the current consumed by a converter utilizing a LM53635-Q1 or LM53625-Q1 device while regulating without a load. While operating without a load, the LM53635-Q1 or LM53625-Q1 is only powering itself. The device draws power from two sources, its VIN pin, IQ, and either its FB pin for fixed versions or BIAS pin for adjustable versions, IB. Since BIAS or FB is connected to the output of the circuit, the power consumed is converted from input power with an effective efficiency, ηeff, of approximately 80 %. Here, effective efficiency is the added input power needed when lightly loading the converter of the LM53625-Q1 and LM53635- Q1 devices and is divided by the corresponding additional load. This allows unloaded current to be calculated as follows:
The EN pin consumes a few micro-amperes when tied to high; see IEN. Add IEN to IQ as shown in Equation 2 if EN is tied to VIN. If EN is tied to a voltage less than 5 V, virtually no current is consumed allowing EN to be used as an UVLO pin once a voltage divider is added.