Datasheet

LTM8028
14
8028fa
For more information www.linear.com/LTM8028
section. There is a 10k resistor internally connected from
a 2V reference to the I
MAX
pin, so the current limit can
be set as shown in Figure 2 with the following equation:
R
IMAX
=
10 •I
MAX
7.467 –I
MAX
kΩ
applicaTions inForMaTion
LTM8028 when the RUN pin voltage falls to 1.55V. There
is also an internal current source that provides 5.5μA of
pull-down current to program additional UVLO hysteresis.
For RUN rising, the current source is sinking 5.5µA until
RUN = 1.68V, after which it turns off. For RUN falling, the
current source is off until the RUN = 1.55V, after which it
sinks 5.5µA. The following equations determine the voltage
divider resistors for programming the falling UVLO voltage
and rising enable voltage (V
ENA
) as configured in Figure 3.
R1=
1.55 •R2
UVLO–1.55






R2=
V
ENA
– 1.084 •UVLO
5.5µA
Figure 2. Setting The Output Current Limit, I
MAX
I
MAX
R
IMAX
8028 F02
LTM8028
Thermal Shutdown
At about 145°C, the LTM8028 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, discharge the soft-start capacitor and turn off
the output until the internal temperature falls below the
thermal hysteresis limit. When the part has cooled, the part
automatically restarts. Note that this thermal shutdown is
set to engage at temperatures above the 125°C absolute
maximum internal operating rating to ensure that it does
not interfere with functionality in the specified operating
range. This means that internal temperatures will exceed the
125°C absolute maximum rating when the overtemperature
protection is active, so repeated or prolonged operation
under these conditions may impair the device’s reliability.
UVLO and Shutdown
The LTM8028 has an internal UVLO that terminates switch
-
ing, resets all logic, and discharges the soft-start capacitor
for input voltages below 4.2V. The LTM8028 also has a
precision RUN function that enables switching when the
voltage at the RUN pin rises to 1.68V and shuts down the
V
IN
V
IN
RUN
R2
R1
8028 F03
LTM8028
Figure 3. UVLO Configuration
The RUN pin has an absolute maximum voltage of 6V.
To accommodate the largest range of applications, there
is an internal Zener diode that clamps this pin, so that it
can be pulled up to a voltage higher than 6V through a
resistor that limits the current to less than 100µA. For
applications where the supply range is greater than 4:1,
size R2 greater than 375k.
PCB Layout
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8028. The LTM8028 is neverthe
-
less a switching power supply, and care must be taken to