Datasheet

( )
( )
OC2 OC1 CU OC1
V V T2 T1 1 TC V- = - ´ + ´
( )
( )
( )
( )
ISNS max
MAX
ISNS MAX CU
V
I
R T 25 TC 1
=
´ - ´ +
TPS40400
SLUS930B –APRIL 2011– REVISED OCTOBER 2011 www.ti.com
• R
ISNS
is the resistance of the current sensing element, either the inductor DC resistance or the resistance of
the current sense resistor used (11)
Combining these two resolution limits shows that for current sense elements with a resistance below 3.75 mΩ,
the overcurrent resolution is given by Equation 11. For current sense element resistances above 3.75 mΩ, the
overcurrent warning and fault resolution is 500 mA.
The TPS40400 has built in temperature correction for the temperature coefficient of resistance for copper wound
inductors used as current sense elements. As the temperature of a copper wound inductor increases, its
resistance increases, resulting in a higher DC component of voltage across it for a given current. This leads to a
decrease in the current that would actually trip the overcurrent thresholds. The voltages that the TPS40400 uses
to represent the overcurrent thresholds is automatically adjusted higher as the die temperature of the TPS40400
increases. The temperature coefficient for the increase of the thresholds is chosen close to the temperature
coefficient of copper at 4000 ppm/°C. The change in overcurrent threshold voltage from one temperature to
another is given in Equation 12.
where
• V
OC1
and V
OC2
are the overcurrent threshold voltages
• T1 and T2 are the corresponding temperatures in °C
• T
CCU
is the temperature coefficient, 0.004 (12)
The change in overcurrent threshold voltages given in Equation 12 maintains the actual overcurrent trip points
near constant only if the die temperature of the TPS40400 and the copper temperature of the inductor are closely
coupled. If the inductor copper temperature rises higher than the TPS40400 die temperature, the overcurrent
thresholds appears to decrease and vice versa.
Temperature compensation applied to the overcurrent thresholds must be considered. The threshold voltage
must not be or become greater (with the internal temperature compensation) than 110 mV referred to the voltage
at the ISNS pins. For instance, if a 10 mΩ resistance inductor was used as the current sense element, a current
of 10 A would cause a 100mV DC level at the current sense pins. At first this looks just fine and within the
bounds of the 110 mV limit of the controller. However, the temperature compensation of the threshold inside the
device raises the effective threshold as the TPS40100 die temperature increases. For 100°C increase in die
temperature, for example, the effective threshold crossed at the ISNS pins to trip an overcurrent is approximately
140 mV at the ISNS pins. The TPS40400 cannot respond this high and the result is a failure of the overcurrent
mechanism to respond at higher die temperatures. For a given maximum temperature defined by the
characteristics of the particular application, the maximum overcurrent setting that should be made for the
TPS40400 is calculated in Equation 13.
where
• I
MAX
is the maximum overcurrent threshold setting permissible (using the IOUT_OC_FAULT_LIMIT command)
in A
• V
ISNS(max)
is the maximum allowable voltage differential at the ISNS pins, 120 mV R
ISNS
is the resistance of the
current sensing element – either inductor or current sense resistor
• T
MAX
is the maximum junction temperature expected for the TPS40400 in °C
• TC
CU
is the temperature coefficient of resistance for copper, 0.004 (13)
Equation 13 is illustrated in Figure 20. This figure shows the variation of the internal overcurrent threshold as the
die temperature increases. In this example, the designated maximum die temperature is 125°C. For the
overcurrent threshold to be valid at this temperature (110 mV or below), the maximum overcurrent threshold that
should be set using the IOUT_OC_FAULT or IOUT_OC_WARN commands should correspond to no more than
75.7 mV. The current level that achieves this is what is calculated in Equation 13. If the maximum expected die
temperature is less than 125°C, then the maximum 25°C overcurrent threshold increases accordingly.
20 Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated
Product Folder Link(s) :TPS40400