ISL6326 Datasheet, PDF (21/30 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision rDS ON or DCR Differential Current Sensing

ISL6326
Overcurrent Protection
ISL6326 has two levels of overcurrent protection. Each
phase is protected from a sustained overcurrent condition by
limiting its peak current, while the combined phase currents
are protected on an instantaneous basis.
In instantaneous protection mode, the ISL6326 utilizes the
sensed average current I_AVG to detect an overcurrent
condition. See the “Channel-Current Balance” on page 13
for more detail on how the average current is measured. The
average current is continually compared with a constant
85µA reference current, as shown in Figure 9. Once the
average current exceeds the reference current, a
comparator triggers the converter to shutdown.
At the beginning of overcurrent shutdown, the controller
places all PWM signals in a high-impedance state within
20ns, commanding the Intersil MOSFET driver ICs to turn off
both upper and lower MOSFETs. The system remains in this
state a period of 4096 switching cycles. If the controller is still
enabled at the end of this wait period, it will attempt a
soft-start. If the fault remains, the trip-retry cycles will
continue indefinitely (as shown in Figure 10) until either
controller is disabled or the fault is cleared. Note that the
energy delivered during trip-retry cycling is much less than
during full-load operation, so there is no thermal hazard
during this kind of operation.
OUTPUT CURRENT
0A
OUTPUT VOLTAGE
Thermal Monitoring (VR_HOT/VR_FAN)
There are two thermal signals to indicate the temperature
status of the voltage regulator: VR_HOT and VR_FAN. Both
VR_FAN and VR_HOT pins are open-drain outputs, and
external pull-up resistors are required. Those signals are
valid only after the controller is enabled.
The VR_FAN signal indicates that the temperature of the
voltage regulator is high and more cooling airflow is needed.
The VR_HOT signal can be used to inform the system that
the temperature of the voltage regulator is too high and the
CPU should reduce its power consumption. The VR_HOT
signal may be tied to the CPU’s PROC_HOT signal.
The diagram of thermal monitoring function block is shown in
Figure 11. One NTC resistor should be placed close to the
power stage of the voltage regulator to sense the operational
temperature, and one pull-up resistor is needed to form the
voltage divider for the TM pin. As the temperature of the
power stage increases, the resistance of the NTC will
reduce, resulting in the reduced voltage at the TM pin.
Figure 12 shows the TM voltage over the temperature for a
typical design with a recommended 6.8kΩ NTC (P/N:
NTHS0805N02N6801 from Vishay) and 1kΩ resistor R_TM1.
We recommend using those resistors for the accurate
temperature compensation.
There are two comparators with hysteresis to compare the
TM pin voltage to the fixed thresholds for VR_FAN and
VR_HOT signals respectively. The VR_FAN signal is set to
high when the TM voltage is lower than 33% of VCC voltage,
and is pulled to GND when the TM voltage increases to
above 39% of VCC voltage. The VR_FAN signal is set to
high when the TM voltage goes below 28% of VCC voltage,
and is pulled to GND when the TM voltage goes back to
above 33% of VCC voltage. Figure 13 shows the operation
of those signals.
0V
2ms/DIV
FIGURE 10. OVERCURRENT BEHAVIOR IN HICCUP MODE.
F_SW = 500kHz
For the individual channel overcurrent protection, the
ISL6326 continuously compares the sensed current signal of
each channel with the 120µA reference current. If one
channel current exceeds the reference current, ISL6326 will
pull PWM signal of this channel to low for the rest of the
switching cycle. This PWM signal can be turned on next
cycle if the sensed channel current is less than the 120µA
reference current. The peak current limit of individual
channel will not trigger the converter to shutdown.
VCC
VR_FAN
R _TM1
TM
°C R _NTC
0.33VCC
0.28VCC
VR_HOT
FIGURE 11. BLOCK DIAGRAM OF THERMAL MONITORING
FUNCTION
21
FN9262.1
May 5, 2008