ISL6327 Datasheet, PDF (23/30 Pages) Intersil Corporation – Enhanced 6-Phase PWM Controller with 8-Bit VID Code and Differential Inductor DCR or Resistor Current Sensing

ISL6327
5. Use the following equation to calculate the TCOMP factor N:
N
=
2----0---9----x---(_{---T----C----S----C-----}–_{-----T----N----T---C-----})
3xT_NTC + 400
+
4
(EQ. 22)
6. Choose an integral number close to the above result for
the TCOMP factor. If this factor is higher than 15, use
N = 15. If it is less than 1, use N = 1.
7. Choose the pull-up resistor R_TC1 (typical 10kΩ);
8. If N = 15, do not need the pull-down resistor R_TC2,
otherwise obtain R_TC2 by the following equation:
R_TC2
=
-N----x----R----^T----^C----¹-
15 – N
(EQ. 23)
9. Run the actual board under full load again with the proper
resistors to TCOMP pin.
10. Record the output voltage as V1 immediately after the
output voltage is stable with the full load; Record the
output voltage as V2 after the VR reaches the thermal
steady state.
11. If the output voltage increases over 2mV as the
temperature increases, i.e. V2-V1>2mV, reduce N and
redesign R_TC2; if the output voltage decreases over 2mV
as the temperature increases, i.e. V1-V2>2mV, increase
N and redesign R_TC2.
The design spreadsheet is available for those calculations.
External Temperature Compensation
By pulling the TCOMP pin to GND, the integrated
temperature compensation function is disabled. And one
external temperature compensation network, shown in
Figure 16, can be used to cancel the temperature impact on
the droop (i.e. load line).
COMP
ISL6327
Internal
circuit
IDROOP
o_C
FB
VDIFF
FIGURE 16. EXTERNAL TEMPERATURE COMPENSATION
The sensed current will flow out of IDROOP pin and develop
the droop voltage across the resistor (R_FB) between FB and
VDIFF pins. If R_FB resistance reduces as the temperature
increases, the temperature impact on the droop can be
compensated. An NTC resistor can be placed close to the
power stage and used to form R_FB. Due to the non-linear
temperature characteristics of the NTC, a resistor network is
needed to make the equivalent resistance between FB and
VDIFF pin reverse proportional to the temperature.
The external temperature compensation network can only
compensate the temperature impact on the droop, while it
has no impact to the sensed current inside ISL6327.
Therefore this network cannot compensate for the
temperature impact on the overcurrent protection function.
General Design Guide
This design guide is intended to provide a high-level
explanation of the steps necessary to create a multiphase
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 multiphase 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 15A and 20A.
All surface-mount designs will tend toward the lower end of
this current range. If through-hole MOSFETs and inductors
can be used, higher per-phase currents are possible. In
cases where board space is the limiting constraint, current
can be pushed as high as 40A per phase, 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.
LOWER MOSFET POWER CALCULATION
The calculation for heat dissipated in the lower MOSFET is
simple, since virtually all of the heat loss in the lower
MOSFET is due to current conducted through the channel
resistance (R_DS(ON)). In Equation 24, I_M is the maximum
continuous output current; I_PP is the peak-to-peak inductor
current (see Equation 1); d is the duty cycle (V_OUT/V_IN); and
L is the per-channel inductance.
P_{LOW, 1}
=
r_DS(ON)
⎛
⎜
⎝
-I-^M---⎟⎞
N⎠
2
(
1
–
d)
+
-I-^L---^,---^2P----^P----⁽--1-----^–-----d----⁾
12
(EQ. 24)
An additional term can be added to the lower-MOSFET loss
equation to account for additional loss accrued during the dead
time when inductor current is flowing through the lower-
MOSFET body diode. This term is dependent on the diode
23
FN9276.2
December 20, 2006