Datasheet

( )
straightwire
4 L
L 0.2 L ln 0.75 nH
D
´
æ ö
» ´ ´ -
ç ÷
è ø
( )
NOM LOAD
SPIKE absolute
L
V V I
C
= + ´
2
IN
MIN
V
R
12
=
TPS2420
SLUS903E –JANUARY 2009–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
If EN is tied to GND at startup and V
IN
does not ramp quickly the TPS2420 may momentarily turn off then on
during startup. This can happen if a capacitive load momentarily pulls down the input voltage below the UV
threshold. If necessary, this can be avoided by delaying EN assertion until V
IN
is fully up.
Maximum Load
The power limiting function of the TPS2420 provides very effective protection for the internal FET. Expectedly,
there is a supply voltage dependent maximum load which the device will be able to power up. Loads above this
level may cause the device to shut off current before startup is complete. Neglecting any load capacitance, the
maximum load ( minimum load resistance ) is calculated using the equation;
(7)
Adding load capacitance may reduce the maximum load which can be present at start up.
Transient Protection
The need for transient protection in conjunction with hot-swap controllers should always be considered. When
the TPS2420 interrupts current flow, input inductance generates a positive voltage spike on the input and output
inductance generates a negative voltage spike on the output. Such transients can easily exceed twice the supply
voltage if steps are not taken to address the issue. Typical methods for addressing transients include;
• Minimizing lead length/inductance into and out of the device
• Voltage Suppressors (TVS) on the input to absorb inductive spikes
• Shottky diode across the output to absorb negative spikes
• A combination of ceramic and electrolytic capacitors on the input and output to absorb energy
• Use PCB GND plane
The following equation estimates the magnitude of these voltage spikes:
where
• V
NOM
is the nominal supply voltage
• I
LOAD
is the load current
• C is the capacitance present at the input or output of the TPS2420
• L equals the effective inductance seen looking into the source or the load (8)
Calculating the inductance due to a straight length of wire is shown in Equation 9.
where
• L is the length of the wire
• D is diameter of the wire (9)
Some applications may require the addition of a TVS to prevent transients from exceeding the absolute ratings if
sufficient capacitance cannot be included.
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