Datasheet

( )
( )
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
UCD9222
www.ti.com SLVSAL7A –NOVEMBER 2010–REVISED FEBRUARY 2011
(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 and CS2A 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 UCD9222 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 7, 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)
The worst case response time to an over-current condition is the sum of the sampling interval (Table 7) and the
smoothing filter lag, t
lag
from Equation 11.
Current Foldback Mode
When the measured output current exceeds the value specified by the IOUT_OC_FAULT_LIMIT command, the
UCD9222 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 responds as programmed by the IOUT_OC_LV_FAULT_RESPONSE command.
Input Voltage Monitoring
The VinMon pin on the UCD9222 monitors the input voltage. The VinMon pin is monitored using the internal
12-bit ADC which has a dynamic range of 0 to 2.5V. The fault thresholds for the input voltage are set using the
VIN_OV_FAULT_LIMIT and VIN_UV_FAULT_LIMIT commands. The scaling for Vin is set using the
VIN_SCALE_MONITOR command.
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