ltm8027 Linear Technology Corporation, ltm8027 Datasheet - Page 13

• Manufacturer Part Number: ltm8027
• Description: 60v, 4a Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
It is recommended that the user apply the R
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 fl exible 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 fi nal design
that has too much output ripple or too large of an output
capacitor.
The maximum frequency (f
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
as R
satisfi ed 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
adjust resistor from this pin to ground. The value of R
is given by the equation R
where R
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
LTM8027 is enabled via the RUN pin, only the V
tor is enabled. Switching remains disabled until the UVLO
threshold is achieved at the V
of the LTM8027 is enabled and switching commences.
T(MIN)
ADJ
. There are additional conditions that must be
is in kΩ.
ADJ
MAX
CC
= 613.77/(V
) at which the LTM8027
pin, when the remainder
T
OUT
value given
CC
– 1.23),
regula-
ADJ
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
using a resistive divider from the V
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
tripping UVLO. Resistors are chosen by fi rst selecting R
(refer to Figure 2). Then:
where V
lockout is disabled and the supply turns on.
R
A
= R
Figure 2. Undervoltage Lockout Resistive Divider
IN(ON)
B
•
⎛
⎜
⎝
is the input voltage at which the undervoltage
V
1.4V
IN(ON)
RUN PIN
V
SUPPLY
– 1
⎞
⎟
⎠
R
R
IN
8027 F02
A
B
supply to ground.
LTM8027
13
8027f
B