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8 33

POS

+ ç

500

1500

2000

POS - W

1000

Current Sense AMP Gain

RPOS

Normal Gain

Minimum Gain Corner

(minimum sheet and hot

temperature)

Maximum Gain Corner

(minimum sheet and Cold

temperature)

UCD7230

SLUS741C – NOVEMBER 2006 – REVISED MARCH 2007

APPLICATION INFORMATION (continued)

With the addition of R

POS and RNEG, the natural gain, A, of the current sense is predictably decreased as:

For RPOS << 8.33 kΩ, the gain is 48. While the 400 kΩ and 8.33 kΩ are well matched, it is important to keep

RPOS as small as possible since they have absolute variation from chip-to-chip and over temperature. The graph

in Figure 6 shows the band of expected gain for A as a function of RPOS. The gain variation at RPOS = 1 kΩ

results in around ±4% error. However, the tolerance of the value of R in the inductor has a more significant

effect on measurement accuracy as does the temperature coefficient of R. Copper has a temperature coefficient

of approximately 3800 ppm/°C. For a 100°C rise in winding temperature, the dc resistance of the inductor

increases by 38%. The worst case scenario would be a cracked core or under-designed inductor in which cases

the core could tend towards saturation. In that scenario, inductor current could change slope drastically and is

not correctly modeled by the capacitor voltage.

Note that inferring inductor current by use of a parallel RC has an additional caveat. As long as TRC = RPOS C is

the same as TLR = L/R, then the voltage across C is the same as the IR drop across the equivalent R of the

inductor. If the time constants don't match, the average voltage across C is still the same as the average voltage

across R, but the indication of ripple current amplitude will be off. Furthermore, load transients results in reported

current that appears to have overshoot or undershoot if TRC is respectively faster or slower than TLR.

While the amp faithfully passes the sensed dc current signal, it should be noted that the amplifier is bandwidth

limited for normal switching frequencies. Therefore, AO represents a moving average of the sensed current.

Figure 6. Current Sense Amp Gain as a Function of RPOS

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