Datasheet

LTM8052/LTM8052A
17
8052fc
For more information www.linear.com/LTM8052
applicaTions inForMaTion
In situations where the output of the LTM8052/LTM8052A
is required to sink current (that is, the load is driving cur-
rent into
the LTM8052/LTM8052A output), the μModule
regulator
will maintain voltage regulation as long as the
negative current limit is not exceeded. If the current
limit is exceeded, the output voltage may begin to rise.
If the output voltage rises more than 125% of the target
regulation voltage, the output overvoltage protection will
engage, and the LTM8052/LTM8052A will stop switch
-
ing. In this situation, the load will be free to pull up the
LTM8052/LTM8052A
output. The LTM8052A does not
have the output overvoltage protection, making it ideal
for applications where the current must remain in regula
-
tion even if the output rises above the voltage regulation
target.
For example, in a thermo-electric cooling (TEC)
application, the voltage is not particularly important and
an output overvoltage cut-off function could be more of
a nuisance than a benefit, so the LTM8052A is a good
choice. In a voltage regulation application, output OVP is
a nice feature to have to protect the load. In a LED drive,
even
though it is primarily a current regulation applica-
tion, choose the LTM8052 in order to use the OVP as an
open LED protection.
If
the output voltage of either the LTM8052 or the LTM8052A
rises above the input, current will flow through an internal
power diode. The output will be clamped to a diode drop
above the input, and current regulation will be lost.
Thermal Shutdown
If the part is too hot, the LTM8052/LTM8052A engages its
thermal shutdown, terminates switching and discharges
the soft-start capacitor. When the part has cooled, the
part automatically restarts. 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 operat
-
ing range
. This means that internal temperatures will
exceed
the 125°C absolute maximum rating when the
overtemperature protection is active, possibly impairing
the device’s reliability.
Figure 4. UVLO Configuration
UVLO and Shutdown
The LTM8052/LTM8052A has an internal UVLO that
terminates switching, resets all logic, and discharges
the soft-start capacitor when the input voltage is below
6V. The LTM8052/LTM8052A also has a precision RUN
function that
enables switching
when the voltage at the
RUN pin rises to 1.68V and shuts down the LTM8052/
LTM8052A 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 the current source 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 4.
R1=
1.55V •R2
UVLO– 1.55V
R2 =
V
ENA
–1.084•UVLO
5.5µA
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.
LTM8052
V
IN
R2
V
IN
R1
8052 F04
RUN