Datasheet

LTM8028
16
8028fa
For more information www.linear.com/LTM8028
Load Sharing
Each LTM8028 features an accurate current limit that en-
ables the use of multiple devices to power a load heavier
than
5A. This is
accomplished by simply tying the V
OUT
terminals of the LTM8028s together, and set the outputs of
the parallel units to the same voltage. There is no need to
power the μModule regulators from the same power supply.
That is, the application can use multiple LTM8028s, each
powered from separate input voltage rails and contribute
a different amount of current to the load as dictated by the
programmed current limit. Keep in mind that the paralleled
LTM8028s will not share current equally. In most cases, one
LTM8028 will provide almost all the load until its current
limit is reached, and then the other unit or units will start
to provide current. This might be an unacceptable operat
-
ing condition in other power regulators, but the accurate
current loop of the LTM8028 controls the electrical and
thermal performance of each individual μModule regulator.
This prevents the oscillations, thermal runaway and other
issues that other regulators might suffer. An example of
two LTM8028s connected in parallel to deliver 1.8V at
10A, while powered from two disparate power sources,
is given in the Typical Applications section. A graph of the
output current delivered from each μModule regulator is
given below in Figure 5.
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8028. However, these capacitors
can cause problems if the LTM8028 is plugged into a live
input supply (see Application Note 88 for a complete dis
-
cussion). The low loss ceramic capacitor combined with
stray inductance in series with the power source forms an
underdamped tank circuit, and the voltage at the V
IN
pin
of the LTM8028 can ring to more than twice the nominal
input voltage, possibly exceeding the LTM8028’s rating
and damaging the part. If the input supply is poorly con
-
trolled or the user will be plugging the LTM8028 into an
energized supply, the input network should be designed
to prevent this
overshoot. This can be accomplished by
installing a small resistor in series to V
IN
, but the most
popular method of controlling input voltage overshoot is
to add an electrolytic bulk capacitor to the V
IN
net. This
capacitor’s relatively high equivalent series resistance
damps the circuit and eliminates the voltage overshoot.
The extra capacitor improves low frequency ripple filter
-
ing and can slightly improve the efficiency of the circuit,
though it is physically large.
applicaTions inForMaTion
TOTAL LOAD CURRENT (A)
1
0
CURRENT DELIVERED BY LTM8028s (A)
1
2
3
4
5
6
2 4 6 8
8028 F05
10
Figure 5. In Most Cases Where Paralleled LTM8028s are
Used, One µModule Will Deliver All of The Load Current Until
Its Current Limit Is Reached, Then The Other Unit(s) Will
Provide Current. The Tightly Controlled Output Current Prevents
Oscillations and Thermal Runaway Observed In Other Types of
Regulators