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LTM8001

8001f

For more information www.linear.com/8001

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-

startcapacitor.Whentheparthascooled,thepartautomati-

cally restarts. This thermal shutdown is set to engage at

temperaturesabovethe125°Cabsolutemaximuminternal

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, possibly impairing the device’s reliability.

UVLO and Shutdown

The LTM8001 VOUT0 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 LTM8001 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 (VENA) as configured in Figure 4.

VENA –1.084 UVLO

5.5µA

1.55 R2

UVLO–1.55

APPLICATIONS INFORMATION

LTM8001

VREF

RNTC

(OPTION A TO D)

8001 F02

ILIM

RNTC

Figure 2. Load Current Derating vs

Temperature Using an NTC Resistor

VOUT0 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

voltagelevel(1.5VattheFB0pin),theLTM8001terminates

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:

VOUT =1.5V 1+

10k

RFB0









where RFB0 is shown in Figure 3.

Iftheoutputovervoltageprotectionengages,theLTM8001

will stop switching. If this is due to some external power

source connected to VOUT0, this source will be free to pull

up VOUT0. If the VOUT0 voltage exceeds the VIN0 input, an

internal power diode will clamp the output to a diode drop

above the input.

LTM8001

VOUT

RFB0

8001 F03

FB0

Figure 3. Voltage Regulation and Overvoltage

Protection Feedback Connections

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