Datasheet

TPS2492
TPS2493
www.ti.com SLUSA65C –JULY 2010–REVISED JANUARY 2013
Action of the Constant Power Engine (Figure 14)
The calculated power dissipated in the external FET, V
DS
x I
D
, is computed under the same startup conditions as
Figure 13. 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 25 W / 48 V = 0.52 A. The 25-W value is programmed into the engine by setting the
PROG voltage using R4 and R5. V
DS
of the external FET, which is calculated as V
(SENSE-OUT)
, falls as C
O
charges, thus allowing 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 14, labeled POWER, is seen to be reasonably 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
while still protecting the device.
Figure 14. Computation of the External FET Stress During Startup
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