Datasheet

C002
I_IN:InputCurrent,2 A/div
GAT:VoltageonGATEPin,5V/div
V_DS:Drain-to-SourceVoltageofM1,5V/div
TIMER:Voltageon TIMERPin,500mV/div
FET_PWR:PoweronM1(productofV_DSandI_IN),19W/div
Time:10ms/div
TPS24720
www.ti.com SLVSAL1C –MARCH 2011–REVISED SEPTEMBER 2013
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 I
LIM
. 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.
Figure 30. Computation of M
1
Power Stress During Startup
CIRCUIT BREAKER AND FAST TRIP
The TPS24720 monitors load current by sensing the voltage across R
SENSE
. The TPS24720 incorporates two
distinct thresholds: a current-limit threshold and a fast-trip threshold.
The functions of circuit breaker and fast-trip turn off are shown in Figure 31 through Figure 34.
Figure 31 shows the behavior of the TPS24720 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 TPS24720 either latches
off or commences a restart cycle, depending upon the state of the LATCH pin. In either event, fault pin FLTb
pulls low to signal a fault condition. 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 turn off current. The fast-trip circuit holds the MOSFET off
for only a few microseconds, after which the TPS24720 turns back on slowly, allowing the current-limit feedback
loop to take over the gate control of M
1
. Then the hot-swap circuit goes into latch mode or auto-retry mode,
depending on pre-determined conditions. Figure 33 and Figure 34 illustrate the behavior of the system when the
current exceeds the fast-trip threshold.
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