DCR

L

R1

C

Vout

CSN

CSP

SW

R2

2.49 kW

2.49 kW

OUT

LOAD

R2

V(I

) = 0.5 + 47.8

DCR

I

R1 + R2

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ö

´

´

´ ç

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UCD7232

SLUSAH3

– MAY 2011

www.ti.com

Figure 4. Attenuating the DCR Sense Signal

The amount of attenuation is equal to R2/(R1 + R2). The equivalent resistance value to use in the L/DCR = RC

formula is the parallel combination of R1 and R2. Thus, when using the circuit of Figure 4,

L/DCR = C

× R1 × R2/(R1 + R2)

(3)

IMON

The IMON signal is a voltage proportional to the output current delivered by the power stage. The voltage

magnitude obeys the following equation when using the circuit of Figure 3. This equation takes into account the

gain reduction caused by the series 2.49k resistors.

V(IOUT) = 0.5 + 47.8 × DCR × ILOAD

(4)

If the calculated value of V(IMON) exceeds the range of the analog-to-digital converter (ADC) or, if used, the

maximum fault comparator threshold limit of a controller monitoring this voltage, then the circuit of Figure 4

should be used. When using the circuit of Figure 4, the voltage on IMON obeys this modified equation:

(5)

In either case, the output voltage is 500mV at no load. Current that is sourced to the load causes the IMON

voltage to rise above 500mV. Current that is forced into the power stage (sinking current) is considered

“negative” current and will cause the IMON voltage to fall below 500mV. The usable dynamic range of the IMON

signal is approximately 100mV to 3.1V. Keep in mind that this signal swing could exceed not just the maximum

range of an analog to digital converter (ADC) that may be used to read or monitor the IMON signal, but also the

maximum programmable limit for the fault OC threshold. For example, the UCD92xx family of digital controllers

has maximum limit of 2.5V for the ADC converter and 2.0V for the fault OC threshold, even though the input pin

can tolerate voltages up to 3.3V.

The IMON voltage is internally fed to the non-inverting input of the output over-current fault comparator. Good

practice dictates that the over-current threshold should be set at approximately 150% of the rated power stage

output current plus one half of the peak-to-peak inductor ripple current. This mandates that the IMON signal should

remain within its linear dynamic range at this threshold load current level. This requirement may force the use of

the attenuation circuit of Figure 4. Note that the IMON voltage (that goes to the output over-current fault

comparator) is held during the blanking interval set by the resistor on the RDLY pin. This means that the IMON pin

will not reflect output current changes during the blanking interval, and that a fault will not be flagged until the

blanking interval terminates.

ILIM

The ILIM pin feeds the inverting input of the output over-current fault comparator. The voltage applied to this pin

sets the over-current fault threshold. When the voltage on the IMON pin exceeds the voltage on this pin, a fault is

flagged. The voltage on this pin can be set by a voltage divider, a DAC, or by a filtered PWM output. The usable

voltage range of the ILIM pin is approximately 0.6V to 3.1V. This represents the linear range of the IMON signal for

sourced output current. When using a voltage divider to set the threshold, a (0.01

µF) capacitor to BP3 can be

added to improve noise immunity.

12

Copyright

© 2011, Texas Instruments Incorporated