Datasheet

Overcurrent Detection
( )
( )
1 2 1
1 e
t
-
æ ö
ç ÷
= + - -
ç ÷
è ø
smoothed
t
I t
I I I
(3)
2 1
2
lnt
æ ö
-
=
ç ÷
-
è ø
lag
limit
I I
t
I I
(4)
Current Foldback Mode
UCD9240
SLUS766C – JULY 2008 – REVISED NOVEMBER 2008 ................................................................................................................................................... www.ti.com
Several mechanisms are provided to sense output current fault conditions. This allows for the design of power
systems with multiple layers of protection.
1. A logic high signal on the FAULT input causes a hardware interrupt to the internal CPU. The CPU then
determines which DPWM outputs are configured to be associated with the voltage rail that contained the fault
and disables those DPWM and SRE outputs. This process takes about 14 microseconds. An integrated gate
driver such as the UCD7230 can be used to generate the FAULT signal. The UCD7230 monitors the voltage
drop across the high side FET and if it exceeds a resistor/voltage programmed threshold, the UCD7230
activates its fault output. The FAULT input can be disabled by reconfiguring the FAULT pin to be a
sequencing pin.
2. Inputs CS-1A, CS-2A, CS-3A and CS-4A each drive an internal analog comparator. These comparators can
be used to detect the voltage output of a current sense circuit. Each comparator has a separate PMBus
configurable threshold. This threshold is set by issuing the FAST_OC_FAULT_LIMIT command. Though the
command is specified in amperes, the hardware threshold is programmed with a value between 31mV and
2V in 64 steps. The conversion from amperes to volts is accomplished by issuing the IOUT_CAL_GAIN
command. When the current sense voltage exceeds the configured threshold the corresponding DPWM and
SRE outputs are driven low on the voltage rail with the fault.
3. Each Current Sense input to the UCD9240 is also monitored by the 12-bit ADC. Each measured value is
scaled using the IOUT_CAL_GAIN and IOUT_CAL_OFFSET commands. The currents for each power stage
configured as part of a voltage rail are summed and compared to the OC limit set by the
IOUT_OC_FAULT_LIMIT command. The action taken when a fault is detected is defined by the
IOUT_OC_FAULT_RESPONSE command.
Because the current measurement is averaged with a smoothing filter, the response time to an Overcurrent
condition depends on a combination of the time constant ( τ ) from Table 4 , the recent measurement history,
and how much the measured value exceeds the overcurrent limit. When the current steps from a current (I
1
)
that is less than the limit to a higher current (I
2
) that is greater than the limit, the output of the smoothing filter
is:
At the point when I
smoothed
exceeds the limit, the smoothing filter lags time, t
lag
is:
The worst case response time to an overcurrent condition is the sum of the sampling interval (Table 4 ) and the
smoothing filter lag, t
lag
from the equation above.
When the measured output current exceeds the value specified by the IOUT_OC_FAULT_LIMIT command, the
UCD9240 attempts to continue to operate by reducing the output voltage in order to maintain the output current
at the value set by IOUT_OC_FAULT_LIMIT. This continues indefinitely as long as the output voltage remains
above the minimum value specified by IOUT_OC_LV_FAULT_LIMIT. If the output voltage is pulled down to less
than that value, the device shuts down, if programmed to do so by the IOUT_OC_LV_FAULT_RESPONSE
command.
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Product Folder Link(s): UCD9240