Datasheet

Table Of Contents

LMZ31530

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SLVSBC7B –OCTOBER 2013–REVISED DECEMBER 2013

Auto-Skip Eco-Mode ™ / Forced Continuous Conduction Mode

Auto-skip Eco-mode or Forced Continuous Conduction Mode (FCCM) can be selected using the SS_SEL pin

(pin 3). Connect the SS_SEL pin to PGND to select Auto-skip Eco-mode or to the PWRGD pin to select FCCM.

In Auto-skip Eco-mode, the LMZ31530 automatically reduces the switching frequency at light load conditions to

maintain high efficiency. In FCCM, the controller keeps continuous conduction mode in light load condition and

the switching frequency is kept almost constant over the entire load range. Transient performance is best in

FCCM.

Power-Up Characteristics

When configured as shown in the front page schematic, the LMZ31530 produces a regulated output voltage

following the application of a valid input voltage. During the power-up, internal soft-start circuitry slows the rate

that the output voltage rises, thereby limiting the amount of in-rush current that can be drawn from the input

source. Figure 21 shows the start-up waveforms for a LMZ31530, operating from a 12-V input (PVIN=VIN) and

with the output voltage adjusted to 1.8 V. Figure 22 shows the start-up waveforms for a LMZ31530 starting up

into a pre-biased output voltage. The waveforms were measured with a 15-A constant current load.

Figure 21. Start-Up Waveforms Figure 22. Start-up into Pre-bias

Pre-Biased Start-Up

The LMZ31530 has been designed to prevent the low-side MOSFET from discharging a pre-biased output.

During pre-biased startup, the low-side MOSFET does not turn on until the high-side MOSFET has started

switching. The high-side MOSFET does not start switching until the slow start voltage exceeds the voltage on the

VADJ pin. Refer to Figure 22.

Remote Sense

The SENSE+ pin must be connected to V

OUT

at the load, or at the device pins.

Connecting the SENSE+ pin to V

OUT

at the load improves the load regulation performance of the device by

allowing it to compensate for any I-R voltage drop between its output pins and the load. An I-R drop is caused by

the high output current flowing through the small amount of pin and trace resistance. This should be limited to a

maximum of 300 mV.

NOTE

The remote sense feature is not designed to compensate for the forward drop of nonlinear

or frequency dependent components that may be placed in series with the converter

output. Examples include OR-ing diodes, filter inductors, ferrite beads, and fuses. When

these components are enclosed by the SENSE+ connection, they are effectively placed

inside the regulation control loop, which can adversely affect the stability of the regulator.

Product Folder Links: LMZ31530