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Current Sensing and Overload Protection

48

OUT

SHUNT

AO

(

I

R

) IO

=

´

´

+

POS

NEG

AO

I0

+

+

V

OUT

SW

L

8.33k

400k

8.33k

400k

Current SenseAmp

IO Buffer

Amp

R

SHUNT

POS

NEG

AO

I0

+

+

SW

C

L

+

R

8.33k

R

NEG

R

POS

400k

8.33k

400k

Current SenseAmp

IO Buffer

Amp

V

OUT

C

OUT

C

OUT

OUT

COPPER

AO

( A I

R

) IO

=

´

´

+

UCD7230

SLUS741C – NOVEMBER 2006 – REVISED MARCH 2007

APPLICATION INFORMATION (continued)

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:

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-k

Ω 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 k

Ω 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

inductor and its parasitic equivalent series resistance, then the voltage on C is the same as the IR drop on the

function of the amplifier is given by:

12