isl6312a Intersil Corporation, isl6312a Datasheet - Page 25

• Manufacturer Part Number: isl6312a
• Description: Four-phase Buck Pwm Controller With Integrated Mosfet Drivers For Intel Vr10, Vr11, And Amd Applications
• Manufacturer: Intersil Corporation

power supply cuts off. For complete protection, the low side
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.
Undervoltage Detection
The undervoltage threshold is set at 60% of the VID code.
When the output voltage (VSEN-RGND) is below the
undervoltage threshold, PGOOD gets pulled low. No other
action is taken by the controller. PGOOD will return high if
the output voltage rises above 70% of the VID code.
Open Sense Line Prevention
In the case that either of the remote sense lines, VSEN or
GND, become open, the ISL6312A 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 ISL6312A will be latched off and cannot be
restarted until the controller is reset.
Overcurrent Protection
The ISL6312A takes advantage of the proportionality
between the load current and the average current, I
detect an overcurrent condition. Two different methods of
detecting overcurrent events are available on the ISL6312A.
The first method continually compares the average sense
current with a constant 125μA OCP reference current as
shown in Figure 14. Once the average 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
ISL6312A uses set internal R
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
active channels, DCR is the individual inductor’s DCR, and
R
If the desired overcurrent trip level is greater then the
minimum overcurrent trip level, I
divider R-C circuit around the inductor shown in Figure 5
should be used to set the desired trip level.
I
OCP min
ISEN
,
OCP,min
is the 300Ω internal current sense resistor.
=
125 10
--------------------------------------------------------- -
, use Equation 21, where N is the number of
⋅
–
DCR
6
⋅
R
ISEN
25
ISEN
⋅
N
OCP,min
current sense resistors.
, then the resistor
AVG
(EQ. 21)
, to
ISL6312A
If an overcurrent trip level lower then I
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
to the overcurrent protection voltage, V
Figure 14. The average channel sense current flows out the
IOUT pin and through R
which is proportional to the output current. When the IOUT
pin voltage exceeds the V
overcurrent protection circuitry activates. Since the IOUT pin
voltage is proportional to the output current, the overcurrent
trip level, I
R
Once the output current exceeds the overcurrent trip level,
V
converter to begin overcurrent protection procedures.
At the beginning of an overcurrent shutdown, the controller
turns off both upper and lower MOSFETs. The system
remains in this state for 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.
I
I
I
OCP
OCP
OCP
IOUT
IOUT
FIGURE 15. OVERCURRENT BEHAVIOR IN HICCUP MODE
0A
0V
>
=
=
, as shown in Equation 23.
will exceed V
I
OCP min
⎛
⎜
⎝
V
--------------------------------------------- -
125 10
--------------------------------------------------------- -
OCP
DCR R
OUTPUT CURRENT, 50A/DIV
OCP
,
⋅
OUTPUT VOLTAGE,
500mV/DIV
⋅
, can be set by selecting the proper value for
⋅
R
–
DCR
ISEN
6
IOUT
⋅
R
OCP
ISEN
⋅
n
IOUT
and a comparator will trigger the
OCP
⋅
N
3ms/DIV
, creating the IOUT pin voltage
⎞
⎟
⎠
⋅
voltage of 2.0V, the
⎛
⎜
⎝
R
-------------------- -
1
R
I
+
OCP
2
R
OCP,min
2
OCP
⎞
⎟
⎠
<
I
OCP min
, as shown in
,
is desired,
August 1, 2007
(EQ. 22)
(EQ. 23)
FN9290.3
IOUT
,