Datasheet

LMZ31704

SLVSBV8A –JUNE 2013–REVISED AUGUST 2013

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Power-Up Characteristics

When configured as shown in the front page schematic, the LMZ31704 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 25 shows the start-up waveforms for a LMZ31704, operating from a 5-V input (PVIN=VIN) and

with the output voltage adjusted to 1.8 V. Figure 26 shows the start-up waveforms for a LMZ31704 starting up

into a pre-biased output voltage.

Figure 25. Start-Up Waveforms Figure 26. Start-up into Pre-bias

Pre-Biased Start-Up

The LMZ31704 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 26.

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.

Thermal Shutdown

The internal thermal shutdown circuitry forces the device to stop switching if the junction temperature exceeds

175°C typically. The device reinitiates the power up sequence when the junction temperature drops below 165°C

typically.

Product Folder Links: LMZ31704