V

ENABLE

V

COMP

V

OUT

SC Threshold

Configured

(1 ms)

Soft Start Time (4 ms)

Compensation

Network Zeroed

(1 ms)

UDG-06062

11

TPS40192, TPS40193

www.ti.com

SLUS719F – MARCH 2007 – REVISED NOVEMBER 2016

Product Folder Links: TPS40192 TPS40193

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Copyright © 2007–2016, Texas Instruments Incorporated

Feature Description (continued)

7.3.5 Startup Sequence and Timing

After input power is applied, the 5-V onboard regulator comes up. Once this regulator comes up, the device goes

through a period where it samples the impedance at the COMP pin and determines the short circuit protection

threshold voltage, by placing 400 mV on the COMP pin for approximately 1 ms. During this time, the current is

measured and compared against internal thresholds to select the short circuit protection threshold. After this, the

COMP pin is brought low for 1 ms. This ensures that the feedback loop is preconditioned at startup and no

sudden output rise occurs at the output of the converter when the converter is allowed to start switching. After

these initial two milliseconds, the internal soft-start circuitry is engaged and the converter is allowed to start. See

Figure 13.

Figure 13. Startup Sequence

7.3.6 Selecting the Short Circuit Current

A short circuit in the devices is detected by sensing the voltage drop across the low-side MOSFET when it is on,

and across the high-side MOSFET when it is on. If the voltage drop across either MOSFET exceeds the short

circuit threshold in any given switching cycle, a counter increments one count. If the voltage across the high-side

MOSFET was higher that the short circuit threshold, that MOSFET is turned off early. If the voltage drop across

either MOSFET 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 increases 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 MOSFET conduction time decreases.

These devices provide three selectable short circuit protection thresholds for the low-side MOSFET: 100 mV,

200 mV and 280 mV. The particular threshold is selected by connecting a resistor from COMP to GND. Table 1

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.