Datasheet

rails Iout
N T
R 0.45
C
=
t
OC_thres CS_nom Imon
V V V (1 e )
- t
= + D -
UCD9222
SLVSAL7A – NOVEMBER 2010– REVISED FEBRUARY 2011 www.ti.com
SyncIn pin is used as a sync source, the delay is applied using a different register (EV1) than when using the
other sources (which use the PhaseTrig registers). Using the EV1 register introduces delay in the control loop
calculation that will introduce phase loss that must be taken into consideration when calculating the loop
compensation. Therefore, under most conditions it will be desirable to set the delay to zero for the PWM signal
synchronized by the SyncIn pin.
Output Current Measurement
Pins CS1A and CS2A are used to measure either output current or inductor current in each of the controlled
power stages. PMBus commands IOUT_CAL_GAIN and IOUT_CAL_OFFSET are used to calibrate each
measurement. See the UCD92xx PMBus Command Reference (SLUU337) for specifics on configuring this
voltage to current conversion.
When the measured current is outside the range of either the over-current or under-current fault threshold, a
current limit fault is declared and the UCD9222 performs the PMBus configured fault recovery. ADC current
measurements are digitally averaged before they are compared against the over-current and under-current
warning and fault thresholds. The output current is measured at a rate of one output rail per t
Iout
microseconds.
The current measurements are then passed through a digital smoothing filter to reduce noise on the signal and
prevent false errors. The output of the smoothing filter asymptotically approaches the input value with a time
constant that is approximately 3.5 times the sampling interval.
Table 7. Output Current Filter Time Constants
NUMBER OF OUTPUT CURRENT FILTER
OUTPUT RAILS SAMPLING INTERVALS (µs) TIME CONSTANT τ (ms)
1 200 0.7
2 400 1.4
This smoothed current measurement is used for output current fault detection; see the Over-current Detection
section. The smoothed current measurement is also reported in response to a PMBus request for a current
reading.
Current Sense Input Filtering
Each power stage current is monitored by the device at the CS pins. The device monitors the current with a
12-bit ADC and also monitors 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
(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
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