TPS2456A Datasheet, PDF (22/32 Pages) Texas Instruments – Dual 12 V Protection and Blocking Controller

TPS2456A
SLVSBI8 – JULY 2012
www.ti.com
An analysis must be performed to determine the need for transient protection. Equation 23 allows the designer to
estimate the voltage spike due to current interruptions.
V_SPIKE = V_INIT + I_LOAD
L
C
(23)
Where:
V_INIT = initial voltage at terminal being analyzed before the current is interrupted
L = combined inductance of feed and RTN lines in series with interrupted current
C = capacitance at point of computation
I_LOAD = current immediately before circuit is opened
An approximation for the inductance of a straight wire is:
L ≈ [0.2 × length_in_meters × (In(4 × length/diameter) – 0.75)] nH
The capacitance in Equation 23, at INPUT, consists of parasitic capacitance and any intentional bypass
capacitance. This implies that the transients can be controlled by the addition of sufficient capacitance.
Equation 24 can be used to calculate the capacitance required to limit the voltage spike to a desired level above
the nominal voltage.
C=
L
´
I_LO
2
AD
( ) V_SPIKE - V_NOM ²
(24)
TRANSIENT PROTECTION SOLUTIONS
Typical protection solutions involve capacitors, TVSs ( Transient Voltage Suppressors ) and/or a Schottky diode.
A TVS and small bypass capacitor at INPUT (see Figure 22) are the most likely solutions to solve input voltage
overshoot. The TVS must be selected so that its clamping is below the Absolute Maximum Rating (17 V) at the
anticipated fault current. For example, the SMCJ13A data sheet specifies a maximum clamping voltage of 21.5 V
(which exceeds the Absolute Maximum voltage) at 69.7 A. The actual clamping voltage at the fault current (I_FT)
may be within the Absolute Maximum; however, the clamp voltage at lower currents must be estimated to verify.
By modeling the TVS as a perfect voltage clamp in series with a resistor, the clamping voltage may be estimated
at different currents per Equation 25.
R_SERIES
=
V_{CL_MAX} -
I_{PEAK_PULSE}
V_{BR _M IN}
- I_TEST
= ^{21.5 V - 14.4 V}
69.7 A - 1 mA
= 0.102 W
(25)
The maximum permitted clamping current for this device is found in Equation 26. This is a worst case (low)
number.
I_{CL_MAX}
=
V_{ABS_MAX} - V_{BR_MAX}
R_SERIES
=
17 V - 15.9 V
0.102
= 10.8 A
(26)
A Schottky diode and capacitor across the OUTPUT (Figure 22) are the most likely solutions to clamp the
transient energy and limit the negative voltage excursion. Although the Schottky diode absorbs most of the
energy, the extremely fast di/dt at shutoff allows some of the leading edge energy to couple through the parasitic
capacitances of the protection and blocking MOSFET (C_DS, C_GS, C_GD) to the GAT1x and GAT2x pins. Protection
for these pins is provided by 100 Ω resistors which have little effect on normal operation but provide good
isolation during transient events.
Equation 24 gives insight into selection of transient protection capacitors for both INPUT and OUTPUT; however,
there are concerns with adding a lot of capacitance in some situations. See the BYPASS CAPACITORS section
regarding considerations and limitations
PCB layout of the protection is critical to its performance. The layout should minimize the impedance between
the TPS2456A and the protection in order to provide the best clamping.
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