LTM8027
applicaTions inForMaTion
Table 1 lists typical resistor values for common operating
frequencies.
RUN Control
The LTM8027 RUN pin uses a reference threshold of 1.4V.
ThisprecisionthresholdallowsuseoftheRUNpinforboth
logic-level controlled applications and analog monitor-
ing applications such as power supply sequencing. The
LTM8027 operational status is primarily controlled by a
UVLOcircuitoninternalpowersource.WhentheLTM8027
is enabled via the RUN pin, only the internal regulator
Table 1. RT Resistor Values vs Frequency
R (kΩ)
T
f
(kHz)
SW
187
118
100
150
200
250
300
350
400
450
500
82.5
63.4k
48.7k
40.2k
31.6k
27.4k
23.7k
(INTV ) is enabled. Switching remains disabled until
CC
the UVLO threshold is achieved at the BIAS1 pin, when
the remainder 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
It is recommended that the user apply the R value given
T
in Table 2 for the input and output operating condition.
Systemlevelorotherconsiderations,however,mayneces-
sitate another operating frequency. While the LTM8027 is
flexible enough to accommodate a wide range of operat-
ing 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
frequencythatistoolowcanresultinafinaldesignthathas
too much output ripple or too large of an output capacitor.
using a resistive divider from the V supply to ground.
IN
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.
The maximum frequency (f
) at which the LTM8027
MAX
Input UVLO and RUN
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
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
theuser.Thehysteresisvoltagepreventsnoisefromfalsely
as R
. There are additional conditions that must be
T(MIN)
satisfied if the synchronization function is used. Please
refer to the Synchronization section for details.
Output Voltage Programming
tripping UVLO. Resistors are chosen by first selecting R
(refer to Figure 2). Then:
B
The LTM8027 regulates its ADJ pin to 1.23V. Connect the
adjust resistor from this pin to ground. The value of R
ADJ
– 1.23),
V
IN(ON)
RA = RB •
–1
is given by the equation R
= 613.77/(V
ADJ
OUT
1.4V
where R
is in kΩ.
ADJ
where V
is the input voltage at which the undervolt-
IN(ON)
age lockout is disabled and the supply turns on.
8027fb
13
Linear [ Linear ]
