Datasheet

To
A/D
A0
5
BLANK DLY
t ( ns ) R ( k )» W
1 2
CS
CS( in )
DLY
R
V .
R
+
æ ö
= ´
ç ÷
è ø
10
LIM
CS( out )
I
V =
UCD7230
SLUS741D –NOVEMBER 2006–REVISED JANUARY 2010 www.ti.com
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:
(4)
where R
DLY
is the resistor from DLY to AGND. R
DLY
should be limited to a range of 25 kΩ to 100 kΩ.
Once R
DLY
has been chosen, the threshold for the input comparator, i.e., the drop allowed across the high-side
MOSFET, is given by:
(5)
Where V
CS(in)
is the threshold of allowed voltage across the high-side MOSFET and RCS+ is a resistor
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:
(6)
where V
CS(out)
is the threshold of allowed voltage between the POS and NEG pins and I
LIM
is the voltage on the
ILIM pin. Note that the ILIM is internally connected to 0.5 V through a 42 kΩ resistor. Any voltage between 0.25
V and 1.0 V can be applied to ILIM. For voltages above 1.0 V, the maximum V
CS(OUT)
threshold is clamped to 0.1
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.
14 Copyright © 2006–2010, Texas Instruments Incorporated
Product Folder Link(s): UCD7230