Datasheet

rails Iout
N T
R 0.45
C
=
t
OC_thres CS_nom Imon
V V V (1 e )
- t
= + D -
( )
( )
det
Imon Imon OC_thres CS_nom
T
1
R
C
ln V ln V V V
= ´
D - D - +
( ) ( )
( )
t
smoothed 1 2 1
I t I I I 1 e
- t
= + - -
2 1
lag
2 limit
I I
t ln
I I
æ ö
-
= t
ç ÷
-
è ø
UCD9244
SLVSAL6A –NOVEMBER 2010– REVISED FEBRUARY 2011 www.ti.com
(7)
where N
rails
is the number of rails configured and T
Iout
is the sample period for the current sense inputs.
Therefore, when the comparator is not used, the recommended component values for the RC network are C = 10
nF and R = 35.7 kΩ.
When the fast over-current comparator is used, the filter corner frequency based on the ADC sample rate may
be too slow and a corner frequency that is a compromise between the requirements of fast over-current detection
and attenuating aliased content in the sampled current must be sought. In this case, the filter corner frequency
can be calculated based on the time to cross the over-current threshold.
(8)
where V
OC_thres
is the programmed OC comparator threshold, V
CS_nom
is the nominal CS voltage, ΔV
Imon
is the
change in CS voltage due to an over-current fault and τ is the filter time constant. Using the equation for the
comparator voltage above, the RC network values can be calculated as
(9)
where T
det
is the time to cross the over-current comparator threshold. For T
det
= 10 µs, ΔV
Imon
= 1.5V, V
OC_thres
=
2.0V and V
CS_nom
= 1.5V, the corner frequency is 6.4 kHz and the recommended RC network component values
are C = 10 nF and R = 2.49 kΩ.
Over-Current Detection
Several mechanisms are provided to sense output current fault conditions. This allows for the design of power
systems with multiple layers of protection.
1. Integrated gate drivers such as the UCD72xx family can be used to generate the FLT signal. The driver
monitors the voltage drop across the high side FET and if it exceeds a resistor/voltage programmed
threshold, the driver activates its fault output. A logic high signal on the FLT input causes a hardware
interrupt to the internal CPU, which then disables the DPWM output. This process takes about 14
microseconds.
2. Inputs CS1A, CS2A, CS3A, and CS4A 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 threshold that
can be set by the FAST_OC_FAULT_LIMIT PMBus command. Though the command is specified in
amperes, the hardware threshold is programmed with a value between 31mV and 2V in 64 steps. The
relationship between amperes to sensed volts is configured by the IOUT_CAL_GAIN command. When the
current sense voltage exceeds the threshold, the corresponding DPWM output is driven low on the voltage
rail with the fault.
3. Each Current Sense input to the UCD9244 is also monitored by the 12-bit ADC. Each measured value is
scaled using the IOUT_CAL_GAIN and IOUT_CAL_OFFSET commands and then passed through a digital
smoothing filter. The smoothed current measurements are compared to fault and warning limits set by the
IOUT_OC_FAULT_LIMIT and IOUT_OC_WARN_LIMIT commands. The action taken when an OC 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 over-current
condition depends on a combination of the time constant (τ) from Table 8, the recent measurement history, and
how much the measured value exceeds the over-current 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
(10)
At the point when I
smoothed
exceeds the limit, the smoothing filter lags time, t
lag
is
(11)
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