Datasheet

det
Imon Imon OC_thres CS_nom
T
1
R =
C ln( V ) ln(ΔV V + V )D - -
( )
t
OC_thres CS_nom Imon
V = V + V 1 e
-
D -
t
rails Iout
N T
R = 0.45
C
UCD9248
www.ti.com SLVSA33A –JANUARY 2010–REVISED AUGUST 2012
Table 4. Output Current Filter Time Constants
OUTPUT CURRENT SAMPLING FILTER TIME CONSTANTS
NUMBER OF OUTPUT RAILS
INTERVALS (µs) (ms)
1 200 0.7
2 400 1.4
3 600 2.1
4 800 2.8
For example, with a single rail, the filter has the transfer function characteristics that shows the signal magnitude
at the output of the averaging filter due to a sine wave input for a range of frequencies. This plot includes an RC
analog low pass network, with a corner frequency of 3 kHz, on the current sense inputs.
This averaged current measurement is used for output current fault detection; see “Over-Current Detection”
section.
In response to a PMBus request for a current reading, the device returns an average current value. When the
UCD9248 is configured to drive a multi-phase power converter, the device adds the average current
measurement for each of the power stages tied to a power rail.
Current Sense Input Filtering
Each power stage current is monitored by the device at the CS pins. There are 4 "A" channel pins and 4 "B"
channel pins. The B channel monitors the current with a 12-bit ADC and samples each current sense voltage in
turn. The A channels monitor the current with the same12-bit ADC and also monitor the current with a digitally
programmable analog comparator. The comparator can be disabled by writing a zero to the
FAST_OC_FAULT_LIMIT.
Because the current sense signal is both digitally sampled and compared to the programmable over-current
threshold, it should be conditioned with an RC network acting as an anti-alias filter. If the comparator is disabled,
the CS input should be filtered at 35% of the sampling rate. An RC network with this characteristic can be
calculated as
(8)
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.
(9)
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 is the filter time constant. Using the equation for the
comparator voltage above, the RC network values can be calculated as
(10)
Where T
det
is the time to cross the over-current comparator threshold. For T
det
= 10 µsec, 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.
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