LTM8031

11

8031fa

APPLICATIONS INFORMATION

than 56V. If BIAS power is applied from a remote or noisy

voltage source, it may be necessary to apply a decoupling

capacitor locally to the LTM8031.

Load Sharing

Two or more LTM8031s may be paralleled to produce higher

currents. This may, however, alter the EMI performance of

pins of all the paralleled LTM8031s together. To ensure that

paralleled modules start up together, the RUN/SS pins may

be tied together, as well. Synchronize the LTM8031s to an

external clock to eliminate beat frequencies, if required.

If the RUN/SS pins are not tied together, make sure that

the same valued soft-start capacitors are used for each

module. An example of two LTM8031 modules configured

for load sharing is given in the Typical Applications sec-

tion. For 2A applications also see the LTM8032, 2A EMC

DC/DC μModule regulator

Burst Mode Operation

To enhance efficiency at light loads, the LTM8031 auto-

matically switches to Burst Mode operation which keeps

the output capacitor charged to the proper voltage while

minimizing the input quiescent current. During Burst Mode

operation, the LTM8031 delivers single cycle bursts of

current to the output capacitor followed by sleep periods

where the output power is delivered to the load by the output

reduced to typically 20μA and 50μA respectively during

the sleep time. As the load current decreases towards a

no-load condition, the percentage of time that the LTM8031

operates in sleep mode increases and the average input

current is greatly reduced, resulting in higher efficiency.

Burst Mode operation is enabled by tying SYNC to GND. To

disable Burst Mode operation, tie SYNC to a stable voltage

above 0.7V. Do not leave the SYNC pin floating.

Minimum Input Voltage

The LTM8031 is a step-down converter, so a minimum

amount of headroom is required to keep the output in regu-

lation. In addition, the input voltage required to turn on is

higher than that required to run, and depends upon whether

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

LTM8031 to run a 3.3V output at light load. If RUN/SS is

is enabled via the RUN/SS pin, the minimum voltage to

start at light loads is lower, about 4.4V. Similar curves for

the Typical Performance Characteristics section.

Soft-Start

The RUN/SS pin can be used to soft-start the LTM8031,

reducing the maximum input current during start-up. The

RUN/SS pin is driven through an external RC network to

create 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.

8031 F01

0.5A/DIV

2V/DIV

2V/DIV

RUN/SS

GND

0.22μF

RUN

15k

2ms/DIV

Figure 1. To Soft-Start the LTM8031, Add a Resistor

and Capacitor to the RUN/SS Pin

Synchronization

The internal oscillator of the LTM8031 can be synchro-

nized by applying an external 250kHz to 2MHz clock to

the SYNC pin. Do not leave this pin floating. The resistor

tied from the RT pin to ground should be chosen such

that the LTM8031 oscillates 20% lower than the intended

synchronization frequency (see the Frequency Selection

section). The LTM8031 will not enter Burst Mode operation

while synchronized to an external clock, but will instead

skip pulses to maintain regulation.