LTM8025

13

8025f

APPLICATIONS INFORMATION

Figure 1. To Soft-Start the LTM8025, Add a Resistor and

Capacitor to the RUN/SS Pin

on is higher than that required to run, and depends upon

whether the RUN/SS is used. As shown in the Typical

Performance Characteristics section, the minimum input

voltage to run a 3.3V output at light load is only about 3.6V,

start. If the LTM8025 is enabled with the RUN/SS pin, the

minimum voltage to start at light loads is lower, about 4.3V.

Similar curves detailing this behavior of the LTM8025 for

other outputs are also included in the Typical Performance

Characteristics section.

Soft-Start

The RUN/SS pin can be used to soft-start the LTM8025,

reducing the maximum input current during start-up. The

RUN/SS pin is driven through an external RC filter to cre-

ate a voltage ramp at this pin. Figure 1 shows the start-up

and shutdown waveforms with the soft-start circuit. By

choosing an appropriate RC time constant, the peak start-

up current can be reduced to the current that is required to

regulate the output, with no overshoot. Choose the value

of the resistor so that it can supply at least 20μA when

the RUN/SS pin reaches 2.5V.

1A/DIV

2V/DIV

2V/DIV

8025 F01

2ms/DIV

RUN/SS

RUN

RUN

15k

0.22μF

Frequency Foldback

The LTM8025 is equipped with frequency foldback which

acts to reduce the thermal and energy stress on the internal

power elements during a short circuit or output overload

condition. If the LTM8025 detects that the output has fallen

out of regulation, the switching frequency is reduced as a

function of how far the output is below the target voltage.

This in turn limits the amount of energy that can be delivered

to the load under fault. During the start-up time, frequency

foldback is also active to limit the energy delivered to the

potentially large output capacitance of the load.

Synchronization

The internal oscillator of the LTM8025 can be synchronized

by applying an external 250kHz to 2MHz clock to the SYNC

pin. Do not leave this pin floating. When synchronizing

to an operating frequency 20% lower than the intended

synchronization frequency (see the Frequency Selection

section).

In addition to synchronization, the SYNC pin controls Burst

Mode behavior. If the SYNC pin is driven by an external

clock, or pulled up above 0.7V, the LTM8025 will not en-

ter Burst Mode operation, but will instead skip pulses to

maintain regulation instead.

Shorted Input Protection

Care needs to be taken in systems where the output will be

held high when the input to the LTM8025 is absent. This

may occur in battery charging applications or in battery

backup systems where a battery or some other supply is

allowed to float and the SHDN pin is held high (either by a

internal circuitry will pull its quiescent current through

its internal power switch. This is fine if your system can

tolerate a few milliamps in this state. If you ground the

SHDN pin, the SW pin current will drop to essentially zero.

held high, then parasitic diodes inside the LTM8025 can

Figure 2 shows a circuit that will run only when the input

voltage is present and that protects against a shorted or

reversed input.

PCB Layout

Most of the headaches associated with PCB layout have

been alleviated or even eliminated by the high level of

integration of the LTM8025. The LTM8025 is neverthe-

less a switching power supply, and care must be taken to

minimize EMI and ensure proper operation. Even with the