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UCD7230

SLUS741C – NOVEMBER 2006 – REVISED MARCH 2007

APPLICATION INFORMATION (continued)

The amp output can go up to 3.3 V, so reasonable designs limits full scale to 3.0 V. Should attenuation be

necessary, use a resistive divider between AO and the control chip A/D input as shown in Figure 7.

Figure 7. Attenuating and Filtering the Voltage Representation of the Average Output Current

While the current sense amplifier is useful for accurate current monitoring or controlling overload conditions,

extreme overload conditions must be handled in timeframes that are generally much shorter than the A/D of a

control chip can achieve. Therefore, there are two comparators on the UCD7230 to sense extreme overload and

protect the driven power MOSFETs.

Extreme current overload is handled in two ways by the UCD7230. One is a comparator that monitors the

voltage between POS and NEG, or effectively the output current of the converter.. The other is a comparator

that monitors the voltage drop across the high-side MOSFET, or effectively the input current. Should either

condition exceed a preset value, OUT1 is immediately turned off for the remainder of the cycle.

To program the current limit, a value of resistance from DLY to AGND must first be chosen to establish a

blanking time during which the comparators will be blinded to switching noise. The blanking time starts with the

rising edge on IN for the input comparator and from both the rising and falling edge of IN for the output

comparator. Blanking time is given by:

MOSFET, is given by:

connected from CS+ to the drain of the high-side MOSFET.

The blanking time for the output comparator is identical to the input comparator. The output comparator

threshold is given by:

ILIM pin. Note that the ILIM is internally connected to 0.5 V through a 42 k

Ω resistor. Any voltage between 0.25

V. Possible methods for setting ILIM are shown in Figure 8.

When using the output comparator to monitor the voltage on the parallel sensing capacitor across the inductor,

the same caveats apply as described for the current sense amplifier.

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