Datasheet

TPS24720
www.ti.com SLVSAL1C –MARCH 2011–REVISED SEPTEMBER 2013
START-UP OF HOT-SWAP CIRCUIT BY VCC OR EN
The connection and disconnection between a load and the input power bus are controlled by turning on and
turning off the MOSFET, M
1
.
The TPS24720 has two ways to turn on MOSFET M
1
:
• Increasing V
VCC
above UVLO upper threshold while EN is already higher than its upper threshold sources
current to the GATE pin. After an inrush period, the TPS24720 fully turns on MOSFET M
1
.
• Increasing EN above its upper threshold while V
VCC
is already higher than the UVLO upper threshold sources
current to the GATE pin. After an inrush period, the TPS24720 fully turns on MOSFET M
1
.
The EN pin can be used to start up the TPS24720 at a selected input voltage V
VCC
.
To isolate the load from the input power bus, the GATE pin sinks current and pulls the gate of MOSFET M
1
low.
The MOSFET can be disabled by any of the following conditions: UVLO, EN, ENSD, load current above the
current-limit threshold, hard short at load, OV, or OTSD. Three separate mechanisms pull down the GATE pin:
1. GATE is pulled down by an 11-mA current source when any of the following occurs.
– The fault timer expires during an overload current fault (V
IMON
> 675 mV).
– V
EN
is below its falling threshold.
– V
VCC
drops below the UVLO threshold.
– V
OV
is above its rising threshold.
2. GATE is pulled down by a 1-A current source for 13.5 μs when a hard output short circuit occurs and V
(VCC –
SENSE)
is greater than 60 mV, i.e., the fast-trip shutdown threshold. After fast-trip shutdown is complete, an
11-mA sustaining current ensures that the external MOSFET remains off.
3. GATE is discharged by a 20-kΩ resistor to GND if the chip die temperature exceeds the OTSD rising
threshold or ENSD is pulled low.
MINIMIZATION OF POWER DISSIPATION AT STANDY BY ENSD
The ENSD pin enables the use of TPS24720 in applications requiring a low-power standby mode. When this pin
is pulled below its threshold voltage, all the internal circuitry is switched off and the GATE pin is discharged to
GND through a 20-kΩ resistor. Thus, the MOSFET is disabled and power consumption is kept to a minimum.
The correct procedure to go into standby mode is first to shut down the TPS24720 by using the EN pin and then
to pull the ENSD pin low.
FAULT DETECTION OF MOSFET SHORT WITH FFLTb
One of the salient features of the TPS24720 is the detection of short-circuited MOSFETs by the FFLTb pin. The
FFLTb is pulled low to indicate a FET short if all the following conditions occur.
• EN is below its threshold voltage.
• V
VCC
is above the UVLO threshold.
• V
IMON
> 103 mV.
The fact that GATE is turned off but current is still flowing through R
SENSE
indicates a drain-to-source short.
DESIGN EXAMPLE: POWER-LIMITED START-UP
This design example assumes a 12-V system voltage with an operating tolerance of ±2 V. The rated load current
is 10 A, corresponding to a dc load of 1.2 Ω. If the current exceeds 12 A, then the controller should shut down
and then attempt to restart. Ambient temperatures may range from 20°C to 50°C. The load has a minimum input
capacitance of 470 μF. Figure 37 shows a simplified system block diagram of the proposed application.
This design procedure seeks to control the junction temperature of MOSFET M
1
under both static and transient
conditions by proper selection of package, cooling, r
DS(on)
, current limit, fault timeout, and power limit. The design
procedure further assumes that a unit running at full load and maximum ambient temperature experiences a brief
input power interruption sufficient to discharge C
OUT
, but short enough to keep M
1
from cooling. A full C
OUT
recharge then takes place. Adjust this procedure to fit the application and design criteria.
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