ISL6314CRZ-T Datasheet, PDF (23/32 Pages) Intersil Corporation – Single-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR11 and AMD Applications

ISL6314
Open Sense Line Protection
In the case that either of the remote sense lines (VSEN or
RGND) become open, the ISL6314 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 V_SEN and R_GND
will increase until an overvoltage event occurs, at which
point overvoltage protection activates and the controller
stops regulating. The ISL6314 will be latched off and cannot be
restarted until the controller is reset.
Overcurrent Protection
The ISL6314 detects overcurrent events by comparing the
droop voltage, V_DROOP, to the OCSET voltage, V_OCSET, as
shown in Figure 14. The droop voltage, set by the external
current sensing circuitry, is proportional to the output current
as shown in Equation 19. A constant 100µA flows through
R_OCSET, creating the OCSET voltage. When the droop
voltage exceeds the OCSET voltage, the overcurrent
protection circuitry activates. Since the droop voltage is
proportional to the output current, the overcurrent trip level,
I_MAX, can be set by selecting the proper value for R_OCSET,
as shown in Equation 20.
V_DROOP
=
R_{-----C----O-----M-----P-}
R_S
⋅
I_OUT
⋅
D
CR
(EQ. 19)
R_OCSET
=
-^I-^M-----^A----^X-----^â‹---^R-----^C----^O----^M-----^P-----^â‹---^D-----^C-----^R--
100μA ⋅ R_S
(EQ. 20)
Once the output current exceeds the overcurrent trip level,
V_DROOP will exceed V_OCSET, and a comparator will trigger
the converter to begin overcurrent protection procedures.
At the beginning of an overcurrent shutdown, the controller
turns off both upper and lower MOSFETs and lowers
PGOOD. The controller will then immediately attempt to
soft-start (which includes the 1.1ms delay of either t_d1 or
t_dA). If the overcurrent fault remains, the trip-retry cycles will
continue until either the controller is disabled or the fault is
cleared. But if five overcurrent events occur without
successfully completing soft-start, the controller will latch off
after the fifth try and must be reset by toggling EN before a
soft-start can be reinitiated. Note that the energy delivered
during trip-retry cycling is much less than during full-load
operation, so there is no thermal hazard. Figure 15 shows an
example.
OUTPUT CURRENT, 25A/DIV
0A
OUTPUT VOLTAGE,
500mV/DIV
0V
1ms/DIV
FIGURE 15. OVERCURRENT BEHAVIOR IN HICCUP MODE
General Design Guide
This design guide is intended to provide a high-level
explanation of the steps necessary to create a power converter.
It is assumed that the reader is familiar with many of the basic
skills and techniques referenced in the following. 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 Stage
The first step in designing a power converter using the
ISL6314 is to determine if one phase is sufficient (if not,
Intersil offers other parts, such as the ISL6313, which has
two phases with similar features). 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, the
total board space available for power-supply circuitry, and
the maximum amount of load current. Generally speaking,
the most economical solutions are those in which the output
handles between 25A and 30A. All surface-mount designs
will tend toward the lower end of this current range. If
through-hole MOSFETs and inductors can be used, higher
currents are possible. In cases where board space is the
limiting constraint, current can be pushed as high as 40A,
but these designs require heat sinks and forced air to cool
the MOSFETs, inductors and heat-dissipating surfaces.
MOSFETS
The choice of MOSFETs depends on the current each
MOSFET will be required to conduct, the switching frequency,
the capability of the MOSFETs to dissipate heat, and the
availability and nature of heat sinking and air flow.
23
FN6455.2
October 8, 2009