Datasheet
APPLICATION INFORMATION
INPUT AND OUTPUT CAPACITANCE
PROGRAMMING THE CURRENT-LIMIT THRESHOLD
TPS2550
TPS2551
www.ti.com
........................................................................................................................................ SLVS736B – FEBRUARY 2008 – REVISED NOVEMBER 2008
Input and output capacitance improve the performance of the device; the actual capacitance should be optimized
for the particular application. For all applications, a 0.01 µ F to 0.1 µ F ceramic bypass capacitor between IN and
GND is recommended as close to the device as possible for local noise de-coupling. This precaution reduces
ringing on the input due to power-supply transients. Additional input capacitance may be needed on the input to
reduce voltage overshoot from exceeding the absolute maximum voltage of the device during heavy transients.
This is especially important during bench testing when long, inductive cables are used to connect the evaluation
board to the bench power-supply.
Placing a high-value electrolytic capacitor on the output pin is recommended when the large transient currents
are expected on the output. Additionally, bypassing the output with a 0.01 µ F to 0.1 µ F ceramic capacitor
improves the immunity of the device to short-circuit transients.
The overcurrent threshold is user programmable via an external resistor. Many applications require that the
minimum current-limit is above a certain current level or that the maximum current-limit is below a certain current
level, so it is important to consider the tolerance of the overcurrent threshold when selecting a value for R
ILIM
.
The following equations and Figure 21 can be used to calculate the resulting overcurrent threshold for a given
external resistor value
ILIM
). Figure 21 includes current-limit tolerance due to variations caused by temperature
and process. The traces routing the R
ILIM
resistor to the TPS2550/51should be as short as possible to reduce
parasitic effects on the current-limit accuracy.
There are two important current-limit thresholds for the device and are related by Figure 4 . The first threshold is
the short-circuit current threshold I
OS
. I
OS
is the current delivered to the load if the part is enabled into a
short-circuit or a short-circuit is applied during normal operation. The second threshold is the overcurrent
threshold I
OC
. I
OC
is the peak DC current that can be delivered to the load before the device begins to limit
current. I
OC
is important if ramped loads or slow transients are common to the application. It is important to
consider both I
OS
and I
OC
when choosing R
ILIM
. R
ILIM
can be selected to provide a current-limit threshold that
occurs 1) above a minimum load current or 2) below a maximum load current.
To design above a minimum current-limit threshold, find the intersection of R
ILIM
and the maximum desired load
current on the I
OS(min)
curve and choose a value of R
ILIM
below this value. Programming the current-limit above a
minimum threshold is important to ensure start-up into full-load or heavy capacitive loads. The resulting
maximum DC load current is the intersection of the selected value of R
ILIM
and the I
OC(max)
curve.
To design below a maximum DC current level, find the intersection of R
ILIM
and the maximum desired load
current on the I
OC(max)
curve and choose a value of R
ILIM
above this value. Programming the current-limit below a
maximum threshold is important to avoid current-limiting upstream power supplies causing the input voltage bus
to droop. The resulting minimum short-circuit current is the intersection of the selected value of R
ILIM
and the
I
OS(min)
curve.
Overcurrent Threshold Equations (I
OC
):
• I
OC(max)
(mA) = (24500 V) /
ILIM
k Ω )
0.975
• I
OC(typ)
(mA) = (23800 V) /
ILIM
k Ω )
0.985
• I
OC(min)
(mA) = (23100 V) /
ILIM
k Ω )
0.996
Short-Circuit Current Equations (I
OS
):
• I
OS(max)
(mA) = (25500 V) /
ILIM
k Ω )
1.013
• I
OS(typ)
(mA) = (28700 V) /
ILIM
k Ω )
1.114
• I
OS(min)
(mA) = (39700 V) /
ILIM
k Ω )
1.342
where 14.3 k Ω ≤ R
ILIM
≤ 80.6 k Ω . I
OS(typ)
and I
OS(max)
are not plotted to improve graph clarity.
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