Datasheet

TPS24700
TPS24701
SLVSAL3B – MARCH 2011– REVISED MAY 2011 www.ti.com
INRUSH OPERATION
When the TPS2470/1 activates MOSFET M1, a current flows into the downstream bulk storage capacitors. When
this current exceeds the limit threshold set by Equation 1, the gate of the MOSFET is regulated by a feedback
loop to make the MOSFET current stay at a current level no more than the current limit threshold. This limits the
inrush current charging capacitance. The TIMER pin begins to charge the timing capacitor C
T
with a current of
approximately 10 μA. The TIMER pin continues to charge C
T
until V
(GATE–VCC)
reaches the timer activation
voltage (6 V for V
VCC
= 12 V). The TIMER then begins to discharge C
T
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 (TPS24700) or auto-retry mode (TPS24701).
The current limit feature is disabled once the inrush operation is finished and the hot-swap circuit becomes a
circuit breaker. The TPS24700/1 turns off the MOSFET, M1, after a fault timer period once the load exceeds the
current limit threshold.
CIRCUIT BREAKER AND FAST TRIP
The TPS24700/1 monitors load current by sensing the voltage across R
SENSE
. The TPS24700/1 incorporates two
distinct thresholds: a current-limit threshold and a fast-trip threshold.
Figure 24 shows the behavior of the TPS24700/1 when a fault in the output load causes the current passing
through R
SENSE
to increase to a value above the current limit but less than the fast-trip threshold. When the
current exceeds the current-limit threshold, a current of approximately 10 μA begins to charge timing capacitor
C
T
. If the voltage on C
T
reaches 1.35 V, then the external MOSFET is turned off. The TPS24700 latches off and
the TPS24701 commences a restart cycle. Overload between the current limit and the fast-trip threshold is
permitted for this period. This shutdown scheme is sometimes called an electronic circuit breaker.
The fast-trip threshold protects the system against a severe overload or a dead short circuit. When the voltage
across the sense resistor R
SENSE
exceeds the 60-mV fast-trip threshold, the GATE pin immediately pulls the
external MOSFET gate to ground with approximately 1 A of current. This extremely rapid shutdown may
generate disruptive transients in the system, in which case a low-value resistor inserted between the GATE pin
and the MOSFET gate can be used to moderate the turnoff current. The fast-trip circuit holds the MOSFET off for
only a few microseconds, after which the TPS24700/1 turns back on slowly, allowing the current-limit feedback
loop to take over the gate control of M1. Then the hot-swap circuit goes into either latch mode (TPS24700) or
auto-retry mode (TPS24701). Figure 26 and Figure 27 illustrate the behavior of the system when the current
exceeds the fast-trip threshold.
The functions of circuit breaker and fast-trip turnoff are shown in Figure 24 through Figure 27.
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