Datasheet

TPS2480
TPS2481
SLUS939B –APRIL 2010–REVISED DECEMBER 2010 www.ti.com
Action of the Constant Power Engine (Figure 13)
The calculated power dissipated in the external FET, V
DS
x I
D
, is computed under the same startup conditions as
Figure 12. The current of the external FET, labeled I
IN
, initially rises to the value that satisfies the constant power
engine; in this case it is 54 W / 48 V = 1.1 A. The 54-W value is programmed into the engine by setting the
PROG voltage using Equation 1. V
DS
of the external FET, which is calculated as V
(SENSE-OUT)
, falls as C
O
charges, thus allowing the the external FET drain current to increase. This is the result of the internal constant
power engine adjusting the current limit reference to the GATE amplifier as C
O
charges and V
DS
falls. The
calculated device power in Figure 13, labeled MOSFET POWER, is seen to be flat-topped and constant within
the limitations of circuit tolerance and acquisition noise. A fixed current limit is implemented by clamping the
constant power engine output to 50 mV when V
DS
is low. This protection technique can be viewed as a
specialized form of foldback limiting; the benefit over linear foldback is that it yields the maximum output current
from a device over the full range of V
DS
and still protects the device.
Figure 13. Computation of the External FET Stress During Startup
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