ISL6366
Two actions are taken by ISL6366 to protect the microprocessor
load when an overvoltage condition occurs.
average current is measured. The average current is continually
compared with a constant 100µA reference current, as shown in
Figure 17. Once the average current exceeds the reference
current, a comparator triggers the converter to shutdown. In
addition, the current out of the IMON pin is equal to the sensed
At the inception of an overvoltage event, all PWM outputs are
commanded low instantly. This causes the Intersil drivers to turn on
the lower MOSFETs and pull the output voltage below a level to avoid
damaging the load. When the output voltage falls below the DAC
plus 107mV, PWM signals enter a high-impedance state. The Intersil
drivers respond to the high-impedance input by turning off both
upper and lower MOSFETs. If the overvoltage condition reoccurs,
ISL6366 will again command the lower MOSFETs to turn on.
ISL6366 will continue to protect the load in this fashion as long as
the overvoltage condition occurs.
average current I
. With a resistor from IMON to GND, the
AVG
voltage at IMON will be proportional to the sensed average current
and the resistor value. The ISL6366 continuously monitors the
voltage at the IMON pin. If the voltage at the IMON pin is higher
than 1.12V, a precision comparator triggers the overcurrent
shutdown. Since the internal current comparator has wider
tolerance than the voltage comparator, the IMON voltage
comparator is the preferred one for OCP trip. Hence, the resistor
between IMON and GND can be scaled such that the overcurrent
protection threshold is tripping lower than 100µA. For example,
Once an overvoltage condition is detected, the respective VR#
ceases the normal PWM operation and pulls its VR_Ready low
until ISL6366 is reset. Cycling the voltage VCC below the
POR-falling threshold will reset the controller. Cycling the EN_VTT,
or EN_PWR will also reset the controller.
the overcurrent threshold for the sensed average current I
can
AVG
be set to 95µA by using a 11.8kΩ resistor from IMON to GND.
Thus, the internal 100µA comparator might only be triggered at its
lower corner. However, IMON OCP trip should NOT be too far away
from 140µA, which is used for cycle-by-cycle protection and
inductor saturation.
VR_RDY
OUTPUT CURRENT
100µA
+
OC
-
I
AVG
0A
SOFT-START, FAULT
AND CONTROL LOGIC
OUTPUT VOLTAGE
1.12V
IMON
VSEN
+
+
OV
OC
-
-
0V
2ms/DIV
ISL6366
VID + 0.179V
FIGURE 18. OVERCURRENT BEHAVIOR IN HICCUP MODE. F
500kHz
=
SW
FIGURE 17. VR_RDY AND PROTECTION CIRCUITRY
At the beginning of overcurrent shutdown, the controller places all
PWM signals in a high-impedance state, commanding the Intersil
MOSFET driver ICs to turn off both upper and lower MOSFETs. The
system remains in this state a period of 8ms. 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 18) 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.
Overcurrent Protection
ISL6366 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.
For the individual channel overcurrent protection, ISL6366
continuously compares the sensed peak current (~50ns filter)
signal of each channel with the 140µA reference current. If one
channel current exceeds the reference current, ISL6366 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 140µA reference current. The
peak current limit of individual channel will only use for cycle-by-
cycle current limiting and will not trigger the converter to
shutdown.
Thermal Monitoring (VR_HOT#)
VR_HOT# indicates the temperature status of the voltage
regulator. VR_HOT# is an open-drain output, and an external pull-
up resistor is required. This signal is valid only after the controller
is enabled.
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.
In instantaneous protection mode, ISL6366 utilizes the sensed
average current I
to detect an overcurrent condition. See
AVG
“Current Sensing” on page 17 for more details on how the
FN6964.0
January 3, 2011
29
INTERSIL [ Intersil ]
