ISL6556A Intersil Corporation, ISL6556A Datasheet - Page 16

• Manufacturer Part Number: ISL6556A
• Description: Optimized Multi-Phase PWM Controller with 6-Bit DAC for VR10.X Application
• Manufacturer: Intersil Corporation

ISL6556A to protect the microprocessor load when an over-
voltage condition occurs.
At the inception of an over-voltage event, all PWM outputs
are commanded low until the voltage at VSEN falls below
0.6V with valid VCC or 1.5V otherwise. This causes the
Intersil drivers to turn on the lower MOSFETs and pull the
output voltage below a level that might cause damage to the
load. The PWM outputs remain low until VDIFF falls to the
programmed DAC level when they enter a high-impedance
state. The Intersil drivers respond to the high-impedance
input by turning off both upper and lower MOSFETs. If the
over-voltage condition reoccurs, the ISL6556A will again
command the lower MOSFETs to turn on. The ISL6556A will
continue to protect the load in this fashion as long as the
over-voltage condition recurs.
Simultaneous to the protective action of the PWM outputs,
the OVP pin pulls to VCC delivering up to 100mA to the gate
of a crowbar MOSFET or SCR placed either on the input rail
or the output rail. Turning on the MOSFET or SCR collapses
the power rail and causes a fuse placed further up stream to
blow. The fuse must be sized such that the MOSFET or SCR
will not overheat before the fuse blows. The OVP pin is
tolerant to 12V (see Absolute Maximum Ratings), so an
external resistor pull up can be used to augment the driving
capability. If using a pull up resistor in conjunction with the
internal over-voltage protection function, care must be taken
to avoid nuisance trips that could occur when VCC is below
2V. In that case, the controller is incapable of holding OVP
low.
Once an over-voltage condition is detected, normal PWM
operation ceases until the ISL6556A is reset. Cycling the
voltage on EN or ENLL or VCC below the POR-falling
threshold will reset the controller. Cycling the VID codes will
not reset the controller.
Over-Current Protection
ISL6556A has two levels of over-current protection. Each
phase is protected from a sustained over-current condition
on a delayed basis, while the combined phase currents are
protected on an instantaneous basis.
In instantaneous protection mode, the ISL6556A takes
advantage of the proportionality between the load current
and the average current, I
condition. See the Channel-Current Balance section for
more detail on how the average current is measured. The
average current is continually compared with a constant
100 A reference current as shown in Figure 10. Once the
average current exceeds the reference current, a
comparator triggers the converter to shutdown.
In individual over-current protection mode, the ISL6556A
continuously compares the current of each channel with the
same 100 A reference current. If any channel current
exceeds the reference current continuously for eight
AVG
16
to detect an over-current
ISL6556A
consecutive cycles, the comparator triggers the converter to
shutdown.
At the beginning of over-current 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 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, trip-retry cycles 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, there is no thermal hazard
during this kind of operation.
General Design Guide
This design guide is intended to provide a high-level
explanation of the steps necessary to create a multi-phase
power converter. It is assumed that the reader is familiar with
many of the basic skills and techniques referenced below. In
addition to this guide, Intersil provides complete reference
designs that include schematics, bills of materials, and
example board layouts for all common microprocessor
applications.
Power Stages
The first step in designing a multi-phase converter is to
determine the number of phases. This determination
depends heavily on the cost analysis which in turn depends
on system constraints that differ from one design to the next.
Principally, the designer will be concerned with whether
components can be mounted on both sides of the circuit
board; whether through-hole components are permitted; and
the total board space available for power-supply circuitry.
Generally speaking, the most economical solutions are
those in which each phase handles between 15 and 20A. All
surface-mount designs will tend toward the lower end of this
FIGURE 11. OVERCURRENT BEHAVIOR IN HICCUP MODE.
0A
0V
OUTPUT CURRENT, 50A/DIV
F
OUTPUT VOLTAGE,
500mV/DIV
SW
= 500kHz
2ms/DIV