Datasheet

LTM8028
12
8028fa
For more information www.linear.com/LTM8028
applicaTions inForMaTion
Output Voltage Margining
The LTM8028’s analog margining pin, MARGA, provides a
continuous output voltage adjustment range of ±10%. It
margins V
OUT
by adjusting the internal 600mV reference
voltage up and down. Driving MARGA with 600mV to
1.2V provides 0% to 10% of adjustment. Driving MARGA
with 600mV to 0V provides 0% to –10% of adjustment.
If unused, allow MARGA to float or bypass this pin with
a 1nF capacitor to GND. Note that the analog margining
function does not adjust the PGOOD threshold. Therefore,
negative analog margining may trip the PGOOD comparator
and toggle the PGOOD flag.
Power Good
PGOOD pin is an open-drain NMOS digital output that ac
-
tively pulls low if any one of these fault modes is detected:
•
V
OUT
is less than 90% of V
OUT(NOMINAL)
on the rising
edge of V
OUT
.
• V
OUT
drops below 85% of V
OUT(NOMINAL)
for more than
25μs.
• Internal faults such as loss of internal housekeeping
voltage regulation, reverse-current on the power switch
and excessive temperature.
SENSEP and Load Regulation
The LTM8028 provides a Kelvin sense pin for V
OUT
, allowing
the application to correct for parasitic package and PCB
IR drops. If the load is far from the LTM8028, running a
separate line from SENSEP to the remote load will correct
for IR voltage drops and improve load regulation. SENSEP
is the only voltage feedback that the LTM8028 uses to
regulate the output, so it must be connected to V
OUT
, either
locally or at the load. In some systems, a loss of feedback
signal equates to a loss of output control, potentially
damaging the load. If the SENSEP signal is inadvertently
disconnected from the load, internal safety circuits in the
LTM8028 prevent the output from running away. This also
limits the amount of correction to about 0.2V.
Bear in mind that the linear regulator of the LTM8028
is a high bandwidth power device. If the load is very far
from the LTM8028, the parasitic impedance of the remote
connection may interfere with the internal control loop
and adversely affect stability. If SENSEP is connected to
a remote load, the user must evaluate the load regulation
and dynamic load response of the LTM8028.
Short-Circuit and Overload Recovery
Like many IC power regulators, the internal linear regulator
has safe operating area (SOA) protection. The safe area
protection decreases current limit as input-to-output volt
-
age increases and keeps the power transistor inside a safe
operating region for all values of input-to-output voltage
up to the absolute maximum voltage rating.
Under maximum I
LOAD
and maximum V
IN
-V
OUT
conditions,
the internal linear regulator’s power dissipation peaks at
about 1.5W. If ambient temperature is high enough, die
junction temperature will exceed the 125°C maximum
operating temperature. If this occurs, the LTM8028 relies
on two additional thermal safety features. At about 145°C,
the device is designed to make the PGOOD output pull
low providing an early warning of an impending thermal
shutdown condition. At 165°C typically, the LTM8028 is
designed to engage its thermal shutdown and the output
is turned off until the IC temperature falls below the
thermal hysteresis limit. The SOA protection decreases
current limit as the in-to-out voltage increases and keeps
the power dissipation at safe levels for all values of input-
to-output voltage.