ISL6334 Datasheet, PDF (22/30 Pages) Intersil Corporation – VR11.1, 4-Phase PWM Controller with Light Load Efficiency Enhancement and Load Current Monitoring

ISL6334, ISL6334A
measured. The average current is continually compared with
a constant 105µA reference current, as shown in Figure 10.
Once the average current exceeds the reference current, a
comparator triggers the converter to shutdown.
The current out of IMON pin is equal to the sensed average
current I_AVG. With a resistor from IMON to GND, the voltage
at IMON will be proportional to the sensed average current
and the resistor value. The ISL6334, ISL6334A continuously
monitors the voltage at IMON pin. If the voltage at IMON pin
is higher than 1.11V, a comparator triggers the overcurrent
shutdown. By increasing the resistor between IMON and
GND, the overcurrent protection threshold can be adjusted
to be less than 105µA. For example, the overcurrent
threshold for the sensed average current I_AVG can be set to
95µA by using a 11.8kΩ resistor from IMON to GND.
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 11) 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
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 12. 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 13 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 RTM1.
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 39.1% of VCC
voltage, and is pulled to GND when the TM voltage
increases to above 45.1% of VCC voltage. The VR_FAN
signal is set to high when the TM voltage goes below 33.3%
of VCC voltage, and is pulled to GND when the TM voltage
goes back to above 39.1% of VCC voltage. Figure 14 shows
the operation of those signals.
OUTPUT VOLTAGE
0V
2ms/DIV
FIGURE 11. OVERCURRENT BEHAVIOR IN HICCUP MODE.
F_SW = 500kHz
For the individual channel overcurrent protection, ISL6334,
ISL6334A continuously compares the sensed current signal
of each channel with the 129µA reference current. If one
channel current exceeds the reference current, ISL6334,
ISL6334A 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
129µA reference current. The peak current limit of individual
channel will not trigger the converter to shutdown.
VCC
VR_FAN
R_TM1
TM
0.391V_CC
VR_HOT
o_c
R
NTC
0.333V_CC
FIGURE 12. BLOCK DIAGRAM OF THERMAL MONITORING
FUNCTION
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FN6482.0
February 26, 2008