
SLVSAL2C
– JANUARY 2011 – REVISED MAY 2011
Figure 26. Inrush Mode at Hot-Swap Circuit Insertion
INRUSH OPERATION
When the TPS24710/11/12/13 activates the pass MOSFET, M1, a current flows into the downstream bulk storage
capacitors. When this current exceeds the limit set by the power limit engine, the gate of the MOSFET is
regulated by a feedback loop to make the MOSFET current rise in a controlled manner. This not only limits the
inrush current charging capacitance but it also limits the power dissipation of the MOSFET to safe levels. A more
complete explanation of the power limiting scheme is given in the section entitled Action of the Constant Power
Engine. At the instant when the current in RSENSE reaches the programmed limit, the TIMER pin begins to charge
the timing capacitor CT with a current of approximately 10 μA. The TIMER pin continues to charge CT until
V(GATE – VCC) reaches the timer activation voltage (6 V for VVCC = 12 V). The TIMER then begins to discharge CT
with a current of approximately 10
μA. This indicates that the inrush mode is finished. If the TIMER exceeds its
upper threshold of 1.35 V before V(GATE – VCC) reaches the timer activation voltage, the GATE pin is pulled to
GND and the hot-swap circuit enters either latch mode (TPS24710/12) or auto-retry mode (TPS24711/13).
The power limit feature is disabled once the inrush operation is finished and the hotswap circuit becomes a
circuit breaker. The TPS24710/11/12/13 will turn off the MOSFET, M1, after a fault timer period once the load
exceeds the current limit threshold.
ACTION OF THE CONSTANT-POWER ENGINE
Figure 27 illustrates the operation of the constant-power engine during start-up. The circuit used to generate the
waveforms of
Figure 27 was programmed to a power limit of 29.3 W by means of the resistor connected between
PROG and GND. At the moment current begins to flow through the MOSFET, a voltage of 12 V appears across
it (input voltage VVCC = 12 V), and the constant-power engine therefore allows a current of 2.44 A (equal to 29.3
W divided by 12 V) to flow. This current increases in inverse ratio as the drain-to-source voltage diminishes, so
as to maintain a constant dissipation of 29.3 W. The constant-power engine adjusts the current by altering the
reference signal fed to the current limit amplifier. The lower part of
Figure 28 shows the measured power
dissipated within the MOSFET, labeled FET PWR, remaining substantially constant during this period of
operation, which ends when the current through the MOSFET reaches the current limit ILIM. This behavior can be
considered a form of foldback limiting, but unlike the standard linear form of foldback limiting, it allows the power
device to operate near its maximum capability, thus reducing the start-up time and minimizing the size of the
required MOSFET.
Copyright
2011, Texas Instruments Incorporated
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