Datasheet

LM3102
SNVS515G –SEPTEMBER 2007–REVISED JANUARY 2012 www.ti.com
Figure 25. Alternate Shutdown Implementation
THERMAL PROTECTION
The junction temperature of the LM3102 should not exceed the maximum limit. Thermal protection is
implemented by an internal Thermal Shutdown circuit, which activates (typically) at 165°C to make the controller
enter a low power reset state by disabling the main MOSFET, disabling the on-timer, and grounding the SS pin.
Thermal protection helps prevent catastrophic failures from accidental device overheating. When the junction
temperature falls back below 145°C (typical hysteresis = 20°C), the SS pin is released and normal operation
resumes.
THERMAL DERATING
The LM3102 is capable of supplying 2.5A below an ambient temperature of 100°C. Under worst case operation,
with either input voltage up to 42V, operating frequency up to 1 MHz, or voltage of the RON pin below the
absolute maximum of 7V, the LM3102 can deliver a minimum of 1.9A output current without thermal shutdown
with a PCB ground plane copper area of 40cm
2
, 2 oz/Cu. Figure 26 shows a thermal derating curve for the
minimum output current without thermal shutdown against ambient temperature up to 125°C. Obtaining 2.5A
output current is possible by increasing the PCB ground plane area, or reducing the input voltage or operating
frequency.
Figure 26. Thermal Derating Curve
APPLICATIONS INFORMATION
EXTERNAL COMPONENTS
The following guidelines can be used to select external components.
R
FB1
and R
FB2
: These resistors should be chosen from standard values in the range of 1.0 kΩ to 10 kΩ,
satisfying the following ratio:
R
FB1
/R
FB2
= (V
OUT
/0.8V) - 1 (7)
For V
OUT
= 0.8V, the FB pin can be connected to the output directly with a pre-load resistor drawing more than
20 µA. It is because the converter operation needs a minimum inductor current ripple to maintain good regulation
when no load is connected.
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