ISL6336D Datasheet, PDF (26/30 Pages) Intersil Corporation – VR11.1, 6-Phase PWM Controller with Phase Dropping,Droop Disabled and Load Current Monitoring Features

ISL6336D
where R_ISEN is the sense resistor connected to the ISEN+ pin, N
is the active channel number, R_X is the resistance of the current
sense element, either the DCR of the inductor or R_SENSE
depending on the sensing method, and I_OCP is the desired
overcurrent trip point. Typically, I_OCP can be chosen to be 1.2x
the maximum load current of the specific application.
With integrated temperature compensation, the sensed current
signal is independent on the operational temperature of the
power stage, i.e., the temperature effect on the current sense
element R_X is cancelled by the integrated temperature
compensation function. R_X in Equation 29 should be the
resistance of the current sense element at the room
temperature.
When the integrated temperature compensation function is
disabled by pulling the TCOMP pin to GND, the sensed current will
be dependent on the operational temperature of the power
stage, since the DC resistance of the current sense element may
be changed according to the operational temperature. R_X in
Equation 29 should be the maximum DC resistance of the
current sense element at the all operational temperature.
In certain circumstances, it may be necessary to adjust the value
of one or more ISEN resistors. When the components of one or
more channels are inhibited from effectively dissipating their
heat so that the affected channels run hotter than desired,
choose new, smaller values of R_ISEN for the affected phases (see
“Voltage Regulation” on page 16). Choose R_ISEN,2 in proportion to
the desired decrease in temperature rise in order to cause
proportionally less current to flow in the hotter phase, as shown
in Equation 30:
R _{I S E N ,2}
=
R_ISEN
_-----T----2-
T₁
(EQ. 30)
In Equation 30, make sure that T₂ is the desired temperature
rise above the ambient temperature, and T₁ is the measured
temperature rise above the ambient temperature. While a single
adjustment according to Equation 30 is usually sufficient, it may
occasionally be necessary to adjust R_ISEN two or more times to
achieve optimal thermal balance between all channels.
Compensation
The ISL6336D converter can be accurately modeled as a
voltage-mode regulator with two poles at the L-C resonant
frequency and a zero at the ESR frequency. A type III controller,
as shown in Figure 22, provides the necessary compensation.
C₂
R_C ^CC
COMP
C₁
R₁
R_FB
FB
ISL6336D
VDIFF
FIGURE 22. COMPENSATION CIRCUIT
The first step is to choose the desired bandwidth, f₀, of the
compensated system. Choose a frequency high enough to assure
adequate transient performance but not higher than 1/3 of the
switching frequency. The type III compensator has an extra
high-frequency pole, f_HF. This pole can be used for added noise
rejection or to assure adequate attenuation at the error-amplifier
high-order pole and zero frequencies. A good general rule is to
choose f_HF = 10f₀, but it can be higher if desired. Choosing f_HF to
be lower than 10f₀ can cause problems with too much phase
shift below the system bandwidth.
R₁
=
R_FB
 ------------C------^---E----S-----R-------------
L  C – C  ESR
C₁
=
-----L-----^---C-----^–-----C------^---E----S-----R--
R_FB
C₂
=
----------------------------------------------V-----^I--^N------------------------------------------------
2  ²  f₀  f_HF   L  C  R_FB  V_P-P
R_C
=
-V----^P----^P-----^---^---2----^---^---²-----^---f--⁰----^---f--^H----^F-----^---L-----^---C-----^---R-----^F----^B--
V_IN ^{ 2    f}_HF ^{ L  C–1}
(EQ. 31)
C_C
=
---------------V-----^I-^N------^---^--2-----^---^-----^---f--^H----^F-----^-------L-----^---C----^–----1----^---------------
2  ²  f₀  f_HF   L  C  R_FB  V_P-P
In the solutions to the compensation equations, there is a single
degree of freedom. For the solutions presented in Equation 31,
R_FB can be arbitrarily chosen as 1kΩ to 2kΩ. The remaining
compensation components are then selected.
In Equation 31, L is the per-channel filter inductance divided by
the number of active channels; C is the sum total of all output
capacitors; ESR is the equivalent-series resistance of the bulk
output-filter capacitance; and V_P-P is the peak-to-peak sawtooth
signal amplitude, typically 1.5V.
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FN8320.0
October 6, 2014