ISL6334A_14 Datasheet, PDF (25/31 Pages) Intersil Corporation – VR11.1, 4-Phase PWM Controller with Light Load Efficiency Enhancement and Load Current Monitoring Features

ISL6334, ISL6334A
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, one does not need the pull-down resistor R_TC2.
If otherwise, obtain R_TC2 using Equation 23:
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 connected to the 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.
External Temperature Compensation
By pulling the TCOMP pin to GND, the integrated
temperature compensation function is disabled. In addition,
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
IS L 6 3 3 4 ,
IS L 6 3 3 4 A
IN TER N A L
C IR C U IT
FB
o_C
IS E N
V D IFF
FIGURE 16. EXTERNAL TEMPERATURE COMPENSATION
The sensed current will flow out of the FB pin and develop a
droop voltage across the resistor equivalent (R_FB) between
the 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 the FB
and VDIFF pins 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 ISL6334, ISL6334A.
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 in the
following. In addition to this guide, Intersil provides complete
reference designs, which 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 upon 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 25A. 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---^,---^P----^--^P---²---⁽--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 forward voltage at I_M, V_D(ON); the switching
frequency, F_sw; and the length of dead times, t_d1 and t_d2, at
the beginning and the end of the lower-MOSFET conduction
interval respectively.
P_{LOW, 2}
=
V_D(ON) F_sw
⎛
⎝
I--_M---
N
+
I--_{P--2-----P--⎠⎞}
t_d1
+
⎛
⎜
-I-^M---
⎝N
–
-I-^P----^---^P--⎟⎞
2⎠
t_d2
(EQ.
25)
25
FN6482.2
February 1, 2013