Datasheet
ESR
L
R5 C4
R
æ ö
´ ³
ç ÷
è ø
L
R5
V
IN
To load
ISNS+
ISNS–
C4
UDG-09076
L
V
IN
To load
ISNS+
ISNS–
UDG-09077
R
ISNS
TPS40400
SLUS930B –APRIL 2011– REVISED OCTOBER 2011
www.ti.com
where (from Figure 18)
• R5 and R
ESR
are in Ω
• C4 is in F (suggest 100 nF, 10
-7
F)
• L is in H (2)
The maximum voltage that the TPS40400 is designed to accept across the ISNS pins is 110 mV. Because most
all inductors have a copper conductor and because copper has a fairly large temperature coefficient of
resistance, the resistance of the inductor and the current through the inductor should make a DC voltage less
than 110 mV when the inductor is at the maximum temperature for the converter. This also applies for the
external resistor in Figure 19. The full load output current multiplied by the sense resistor value, must be less that
110 mV at the maximum converter operating temperature.
There is also a constraint on the negative (reverse current) voltage that can be applied to the ISNS pins of the
TPS40400. The voltage differential from ISNS+ to ISNS- should not be less than –45 mV. If this condition is not
met, inaccurate results from the READ_IOUT command is the result. This is intended to be a ripple voltage
limitation. The net current through the inductor must flow towards the load from the input voltage. Current sinking,
while possible for the controller to accommodate, is not supported for overcurrent detection or for the
READ_IOUT command.
In all cases, C4 should be placed as close to the ISNS pins as possible to help avoid problems with noise.
Figure 18. Current Sensing Using Inductor Figure 19. Current Sensing Using Sense Resistor
Resistance
Once the current sensing method is chosen, the TPS40400 needs to be told what the resistance of the current
sense element is. This allows the proper calculation of thresholds for the overcurrent fault and warning, as well
as more accurate reporting of the actual output current. The IOUT_CAL_GAIN command is used to set the value
of the sense element resistence of the device. IOUT_OC_WARN_LIMIT and IOUT_OC_FAULT_LIMIT set the
levels for the overcurrent warning and fault levels respectively. (See the PMBus Functionality and Additional
Setup section for more details.)
Linear regulators. The TPS40400 has two on board linear regulators primarily intended to provide suitable
power for the internal circuitry of the device. These pins, BP3 and BP6 must be properly bypassed to function
properly. BP3 needs a minimum of 100nF connected to GND and BP6 should have approximately 1µF
connected to GND.
It is permissible to use the external regulator to power other circuits if desired, but care must be taken to ensure
that the loads placed on the regulators do not adversely affect operation of the controller. The main consideration
is to avoid loads with heavy transient currents that can affect the regulator outputs. Transient voltages on these
outputs could result in noisy or erratic operation of the TPS40400.
.
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