ISL6322G Datasheet, PDF (25/39 Pages) Intersil Corporation – Two-Phase Buck PWM Controller with Integrated MOSFET Drivers, I2C Interface and Phase Dropping

ISL6322G
MOSFET should have a gate threshold well below the
maximum voltage rating of the load/microprocessor.
In the event that during normal operation the PVCC or VCC
voltage falls back below the POR threshold, the pre-POR
overvoltage protection circuitry reactivates to protect from
any more pre-POR overvoltage events.
Open Sense Line Prevention
In the case that either of the remote sense lines, VSEN or
GND, become open, the ISL6322G is designed to prevent
the controller from regulating. This is accomplished by
means of a small 5µA pull-up current on VSEN, and a pull-
down current on RGND. If the sense lines are opened at any
time, the voltage difference between VSEN and RGND will
increase until an overvoltage event occurs, at which point
overvoltage protection activates and the controller stops
regulating. The ISL6322G will be latched off and cannot be
restarted until the controller is reset.
Overcurrent Protection
The ISL6322G takes advantage of the proportionality
between the load current and the total channel sense
current, I_SEN1+I_SEN2, to detect an overcurrent condition.
Two different methods of detecting overcurrent events are
available on the ISL6322G. The first method continually
compares the average sense current with an OCP reference
current as shown in Figure 14. The value of the OCP
reference current is dependant upon the number of channels
operating. If the controller is operating in 2-phase mode the
OCP reference current is 240µA. If the controller is operating
in single phase mode the OCP reference current is 120µA.
This ensures that the OCP trip current scales properly with
the number of channels firing. Once the total sense current
exceeds the OCP reference current, a comparator triggers
the converter to begin overcurrent protection procedures.
This first method for detecting overcurrent events limits the
minimum overcurrent trip threshold because of the fact the
ISL6322G uses set internal R_ISEN current sense resistors.
For this first method the minimum overcurrent trip threshold
is dictated by the DCR of the inductors and the number of
active channels. To calculate the minimum overcurrent trip
level, I_OCP,min, use Equation 18, where N is the number of
active channels, DCR is the individual inductor’s DCR, and
R_ISEN is the 300Ω internal current sense resistor.
I_{OCP, min}
=
-¹---²---⁰-----^â‹---¹---⁰----^–---⁶----^â‹---^R-----^I--^S----^E---^N------^â‹---^N--
DCR
(EQ. 18)
If the desired overcurrent trip level is greater than the
minimum overcurrent trip level, I_OCP,min, then the resistor
divider R-C circuit around the inductor shown in Figure 15
should be used to set the desired trip level.
I_OCP
=
⎛
⎜
⎝
1_----2_---0_-----⋅_---1_---0_----^–_-D--⁶_---C-⋅_---RR_{-----I--S----E---N------}⋅_---N_{-- ⎠⎟⎞}
⋅
⎛
⎜
⎝
-^R----¹--_R--+---₂--^R-----²-_{⎠⎟⎞}
(EQ. 19)
I_OCP > I_{OCP, min}
If an overcurrent trip level lower then I_OCP,min is desired,
then a second method for setting the OCP trip level is
available.
The second method for detecting overcurrent events
continuously compares the voltage on the IOUT pin, V_IOUT,
to the overcurrent protection voltage, V_OCP, as shown in
Figure 14. The total channel sense current flows out the
IOUT pin and through R_IOUT, creating the IOUT pin voltage
which is proportional to the output current. When the IOUT
pin voltage exceeds the V_OCP voltage of 2.0V, the
overcurrent protection circuitry activates. Since the IOUT pin
voltage is proportional to the output current, the overcurrent
trip level, I_OCP, can be set by selecting the proper value for
R_IOUT, as shown in Equation 20.
I_OCP
=
-^V----^O----^C----^P-----^â‹---^R-----^I--^S----^E----^N--
DCR ⋅ R_IOUT
I_OCP < I_{OCP, min (EQ. 20)}
Once the output current exceeds the overcurrent trip level,
V_IOUT will exceed V_OCP and a comparator will trigger the
converter to begin overcurrent protection procedures.
At the beginning of overcurrent shutdown, the controller sets
all of the UGATE and LGATE signals low and forces PGOOD
low. This turns off all of the upper and lower MOSFETs. The
system remains in this state for a fixed period of 12ms. 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 until either the 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.
OUTPUT CURRENT, 50A/DIV
0A
OUTPUT VOLTAGE,
500mV/DIV
0V
3ms/DIV
FIGURE 15. OVERCURRENT BEHAVIOR IN HICCUP MODE
Individual Channel Overcurrent Limiting
The ISL6322G has the ability to limit the current in each
individual channel without shutting down the entire regulator.
25
FN6715.0
May 22, 2008