Datasheet

-470.02
R
(RSET)
V - 0.00314
(OFF)
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TPS2419
SLVS998B –FEBRUARY 2010–REVISED SEPTEMBER 2011 www.ti.com
SYSTEM DESIGN AND NOISE ISSUES
In noisy system environments, the low impedance of a MOSFET coupled with a default positive turn off threshold
voltage might result in unwanted ON/OFF GATE cycling. Ideally the best way to approach the problem is with a
clean layout and noise free system design. Since design constraints limit the ability to improve this, the following
suggestions can be employed with the TPS2419.
• Set the turn off threshold negative using the RSET pin. This is required to operate at light load, but does
permit reverse current.
• If current monitoring is used in the system, take advantage of the shunt resistor and connect the A and C pins
across the shunt and FET. This increases the sense resistance, reducing noise sensitivity by increasing the
signal levels while reducing the permitted reverse current.
• Disable the device using EN under light load conditions.
RECOMMENDED OPERATING RANGE
The maximum recommended bus voltage is lower than the absolute maximum voltage ratings on A and C, solely
to provide some margin for transients on the bus. The TPS2419 will operate properly up to the absolute
maximum voltage ratings on A and C.
Most power systems experience transient voltages above or below the normal operating level. Short transients,
or voltage spikes, may be clamped by the ORing MOSFET to an output capacitor and/or voltage rail depending
on the system design. Protection may be required on the input or output if the system design does not inherently
limit transient voltages between the TPS2419 absolute maximum ratings (positive or negative).
Protection for positive transients that would exceed the absolute maximum limits may be accomplished with a
TVS diode (transient voltage suppressor) clamp to ground, or a diode clamp to a safe voltage rail. If a TVS is
required, it must protect to the absolute maximum ratings at the worst case clamping current. Protection for
negative transients that would drive pins (e.g. C) below the absolute maximum limits may be accomplished with a
diode clamp to ground. Limit transient current in or out of the TPS2419 to less than 50 mA. Transients can also
be controlled by bus capacitance or composite snubber/clamps such as a zener-blocked large capacitor with a
discharge resistor in parallel.
MOSFET SELECTION AND R
(RSET)
MOSFET selection criteria include voltage rating, voltage drop, power dissipation, size, and cost. The voltage
rating consists of both the ability to withstand the rail voltage with expected transients, and the gate breakdown
voltage. The MOSFET gate rating should exceed be the maximum of the controlled rail voltage or 11.5 V.
While r
DS(on)
is often chosen with the power dissipation, voltage drop, size and cost in mind, there are several
other factors to be concerned with in ORing applications. When using a TPS2419 with RSET programmed to a
negative voltage, the permitted static reverse current is equal to the turn-off threshold divided by the MOSFET's
r
DS(on)
. While this current may actually be desirable in some systems, the amount may be controlled by selection
of r
DS(on)
and RSET. The practical range of r
DS(on)
for a single MOSFET runs from the low milliohms to 40 mΩ for
a single MOSFET.
MOSFETs may be paralleled for lower voltage drop (power loss) at high current. Current sharing depends on the
resistance match including both the r
DS(on)
, connection resistance, and thermal coupling.
The TPS2419 may only be operated without an RSET programming resistor if the loading provides a V
(A-C)
greater than 3 mV. A negative turnoff threshold reduces sensitivity to false tripping due to noise on the bus, but
permits larger static reverse current. Installing a resistor from RSET to ground creates a negative shift in the
turn-off threshold per Equation 2.
(2)
12 Copyright © 2010–2011, Texas Instruments Incorporated
Product Folder Link(s): TPS2419