Datasheet

LTM8001
15
8001fb
For more information www.linear.com/LTM8001
Load Current Derating Using the ILIM Pin
In high current applications, derating the maximum
current based on operating temperature may prevent
damage to the load. In addition, many applications have
thermal limitations that will require the regulated current
to be reduced based on the load and/or board tempera
-
ture. To achieve this, the LTM8001 uses the ILIM pin to
reduce the effective regulated current in the load. While
ILIM programs the regulated current in the load, it may
also be configured to reduce the regulated current. The
load/board temperature derating is programmed using a
resistor divider with a temperature dependant resistance,
as shown in Figure 2. When the board/load temperature
rises, the ILIM voltage will decrease.
Thermal Shutdown
If the part is too hot, the LTM8001 engages its thermal
shutdown, terminates switching and discharges the soft-
start capacitor. When the part has cooled, the part automati
-
cally 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 inter
fere 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, possibly impairing the device’s reliability.
UVLO and Shutdown
The LTM8001 V
OUT0
step-down regulator has an internal
UVLO that terminates switching, resets all logic, and dis-
charges the soft-start capacitor for input voltages below
4.2V
. The L
TM8001 also has a precision RUN function
that enables switching when the voltage at the RUN pin
rises to 1.68V and shuts down the LTM8001 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 resis
-
tors for programming the falling UVLO voltage and rising
enable voltage (V
ENA
) as configured in Figure 4.
R2=
V
ENA
–1.084 UVLO
5.5µA
R1=
1.55 R2
UVLO–1.55
APPLICATIONS INFORMATION
LTM8001
V
REF
R
NTC
R
X
R
V
R
V
R2
R1
(OPTION A TO D)
8001 F02
ILIM
B
R
NTC
A
R
NTC
R
X
D
R
NTC
C
Figure 2. Load Current Derating vs
Temperature Using an NTC Resistor
V
OUT0
Output Overvoltage Protection
The LTM8001 switching regulator uses the FB0 pin to both
regulate the output voltage and to provide a high speed
overvoltage lockout to avoid high voltage output condi
-
tions. If the output voltage exceeds 125% of the regulated
voltage level (1.5V at the FB0 pin), the LTM8001 terminates
switching and shuts down switching for a brief period. The
output voltage at which output overvoltage protection en
-
gages must be greater than 1.5V and is set by the equation:
V
OUT
= 1.5V 1+
10k
R
FB0






where R
FB0
is shown in Figure 3.
If the output overvoltage protection engages, the LTM8001
will stop switching. If this is due to some external power
source connected to V
OUT0
, this source will be free to pull
up V
OUT0
. If the V
OUT0
voltage exceeds the V
IN0
input, an
internal power diode will clamp the output to a diode drop
above the input.
LTM8001
V
OUT
V
OUT
R
FB0
8001 F03
FB0
Figure 3. Voltage Regulation and Overvoltage
Protection Feedback Connections