Datasheet

2
C1
R1
C2
3
COMP
TPS40192/3
FB
UDG−06061
V
OUT
R
COMP
0.4 V
R1
e
ǒ
*t
R1 C1
Ǔ
t 10 mA
TPS40192, TPS40193
SLUS719E –MARCH 2007–REVISED MAY 2013 www.ti.com
Selecting the Short Circuit Current
A short circuit in the TPS40192/3 is detected by sensing the voltage drop across the low-side FET when it is on,
and across the high-side FET when it is on. If the voltage drop across either FET exceeds the short circuit
threshold in any given switching cycle, a counter increments one count. If the voltage across the high-side FET
was higher that the short circuit threshold, that FET is turned off early. If the voltage drop across either FET does
not exceed the short circuit threshold during a cycle, the counter is decremented for that cycle. If the counter fills
up (a count of 7) a fault condition is declared and the drivers turn off both MOSFETs. After a timeout of
approximately 50 ms, the controller attempts to restart. If a short circuit is still present at the output, the current
quickly ramps up to the short circuit threshold and another fault condition is declared and the process of waiting
for the 50 ms an attempting to restart repeats. The low side threshold will increase as the low side on time
decreases due to blanking time and comparator response time. See Figure 11 for changes in the threshold as
the low side FET conduction time decreases.
The TPS40192/3 provides three selectable short circuit protection thresholds for the low side FET: 100 mV,
200 mV and 280 mV. The particular threshold is selected by connecting a resistor from COMP to GND. Table 2
shows the short circuit thresholds for corresponding resistors from COMP to GND. When designing the
compensation for the feedback loop, remember that a low impedance compensation network combined with a
long network time constant can cause the short circuit threshold setting to not be as expected. The time constant
and impedance of the network connected from COMP to FB should be as in Equation 1 to ensure no interaction
with the short circuit threshold setting.
where
• t is 1 ms, the sampling time of the short circuit threshold setting circuit
• R1 and C1 are the values of the components in Figure 14 (1)
Figure 14. Short Circuit Threshold Feedback Network
Table 2. Short Circuit Threshold Voltage Selection
COMPARATOR RESISTANCE CURRENT LIMIT THRESHOLD
R
COMP
(kΩ) VOLTAGE (mV)
V
ILIM
(V)
12 ±10% 280
Open 200
4 ±10% 100
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