LTM8025

12

8025f

APPLICATIONS INFORMATION

SWITCHING FREQUENCY

0.2MHz

215kΩ

0.3MHz

137kΩ

0.4MHz

100kΩ

0.5MHz

76.8kΩ

0.6MHz

63.4kΩ

0.7MHz

52.3kΩ

0.8MHz

44.2kΩ

0.9MHz

38.3kΩ

1MHz

34.0kΩ

1.2MHz

26.7kΩ

1.4MHz

21.5kΩ

1.6MHz

17.8kΩ

1.8MHz

14.7kΩ

2MHz

12.1kΩ

2.2MHz

9.76kΩ

2.4MHz

8.25kΩ

Operating Frequency Tradeoffs

value given in Table 1 for the input and output operating

condition. System level or other considerations, however,

may necessitate another operating frequency. While the

LTM8025 is flexible enough to accommodate a wide range

of operating frequencies, a haphazardly chosen one may

result in undesirable operation under certain operating or

fault conditions. A frequency that is too high can reduce

efficiency, generate excessive heat or even damage the

LTM8025 if the output is overloaded or short circuited.

A frequency that is too low can result in a final design

that has too much output ripple or too large of an output

capacitor.

BIAS Pin Considerations

The BIAS pin is used to provide drive power for the in-

ternal power switching stage and operate other internal

circuitry. For proper operation, it must be powered by at

least 2.8V. If the output voltage is programmed to 2.8V

source. If the BIAS pin voltage is too high, the efficiency

of the LTM8025 may suffer. The optimum BIAS voltage is

dependent upon many factors, such as load current, input

voltage, output voltage and switching frequency, but 4V to

5V works well in many applications. In all cases, ensure

that the maximum voltage at the BIAS pin is less than 25V

power is applied from a remote or noisy voltage source, it

may be necessary to apply a decoupling capacitor locally

to the pin.

Load Sharing

Two or more LTM8025’s may be paralleled to produce higher

pins of all the paralleled LTM8025’s together. To ensure

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

may be tied together, as well. If the RUN/SS pins are not

tied together, make sure that the same valued soft-start

capacitors are used for each module. Current sharing can

be improved by synchronizing the LTM8025s. An example

of two LTM8025s configured for load sharing is given in

the Typical Applications section.

Burst Mode Operation

To enhance efficiency at light loads, the LTM8025 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 LTM8025 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

each reduced to microamps during the sleep time. As the

load current decreases towards a no load condition, the

percentage of time that the LTM8025 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 LTM8025 is a step-down converter, so a minimum

amount of headroom is required to keep the output in

regulation. In addition, the input voltage required to turn