LTM8027

13

8027f

APPLICATIONS INFORMATION

in Table 2 for the input and output operating condition.

System level or other considerations, however, may ne-

cessitate another operating frequency. While the LTM8027

is flexible enough to accommodate a wide range of oper-

ating frequencies, a haphazardly chosen one may result

in undesirable operation under certain operating or fault

conditions. A frequency that is too high can damage the

LTM8027 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.

should be allowed to switch and the minimum frequency

set resistor value that should be used for a given set of

input and output operating condition is given in Table 2

satisfied if the synchronization function is used. Please

refer to the Synchronization section for details.

Output Voltage Programming

The LTM8027 regulates its ADJ pin to 1.23V. Connect the

RUN Control

The LTM8027 RUN pin uses a reference threshold of

1.4V. This precision threshold allows use of the RUN pin

for both logic-level controlled applications and analog

monitoring applications such as power supply sequencing.

The LTM8027 operational status is primarily controlled

by a UVLO circuit on internal power source. When the

tor is enabled. Switching remains disabled until the UVLO

of the LTM8027 is enabled and switching commences.

Because the LTM8027 high power converter is a power

transfer device, a voltage that is lower than expected on

the input supply could require currents that exceed the

sourcing capabilities of that supply, causing the system

to lock up in an undervoltage state. Input supply start-

up protection can be achieved by enabling the RUN pin

Setting the divider output to 1.4V when that supply is at

an adequate voltage prevents an LTM8027 converter from

drawing large currents until the input supply is able to

provide the required power. 200mV of input hysteresis on

the RUN pin allows for about 15% of input supply droop

before disabling the converter.

Input UVLO and RUN

The RUN pin has a precision voltage threshold with hys-

teresis which can be used as an undervoltage lockout

threshold (UVLO) for the power supply. Undervoltage

lockout keeps the LTM8027 in shutdown until the supply

input voltage is above a certain voltage programmed by

the user. The hysteresis voltage prevents noise from falsely

(refer to Figure 2). Then:

R

V

IN(ON)

1.4V

–1

⎛

⎝⎜

⎞

⎠⎟

lockout is disabled and the supply turns on.

Figure 2. Undervoltage Lockout Resistive Divider

RUN PIN

8027 F02