Datasheet

48
OUT SHUNT
AO ( I R ) IO= ´ ´ +
POS
NEG AO
I0
+
+
V
OUT
SW
L
8.33 kΩ 400 kΩ
8.33 kΩ 400 kΩ
Curre nt SenseAmp
IO Buffe r
Amp
R
SHUNT
POS
NEG AO
I0
+
+
SW
C
L
+
R
8.33 kΩ
R
NEG
R
POS
400 kΩ
8.33 kΩ 400kΩ
Curre nt SenseAmp
IO Buffe r
Amp
V
OUT
C
OUT
C
OUT
OUT COPPER
AO ( A I R ) IO= ´ ´ +
UCD7230
SLUS741D –NOVEMBER 2006–REVISED JANUARY 2010 www.ti.com
Current Sensing and Overload Protection
Since the UCD7230 is physically collocated with the high-current elements of the power converter, it is logical
that current be monitored by the chip. An internal instrumentation amplifier conditions current sense signals so
that they can be used by the control chip generating the PWM signal.
POS and NEG are inputs to an instrumentation amplifier circuit. This amplifier has a nominal gain of 48 and
presents its output at AO. This can be used to monitor either an external current sense shunt or a parallel RC
around the buck inductor shown in Figure 5. The shunt yields the highest accuracy and will be insensitive to
inductor core saturation effects. It comes with the price of added power dissipation. Using the shunt, AO is given
by:
(1)
The internal configuration of the instrumentation amplifier is such that AO is 0.6 V when POS – NEG = 0.
Because of this output offset, the amplifier can accurately pass information for both positive and negative load
current. The offset is controlled by IO. If IO is left to float, the offset is 0.6 V. 0.6 V is present at IO through an
internal 10-kΩ resistor and should be bypassed to AGND. If a higher value of offset is desired, a voltage in
excess of 0.66 V can be externally applied to IO. Once IO is forced above 0.66 V, the internal 10 kΩ is
disconnected, and the AO output offset is now equal to the voltage applied to IO.
Figure 5. Current Sense Using External Shunt and Lossless Average Output Current Sensing Using DC
Resistance of the Output Inductor.
Figure 5 also shows lossless current sensing utilizing an RC across the buck inductor to generate an analog of
the IR drop on the copper of the inductor. As long as the R
POS
x C time constant is the same as the L/R of the
inductor and its parasitic equivalent series resistance, then the voltage on C is the same as the IR drop on the
parasitic inductor resistance. A resistor, R
NEG
= R
POS
is used for amplifier bias current cancellation. The transfer
function of the amplifier is given by:
(2)
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Product Folder Link(s): UCD7230