ISL6565A Datasheet, PDF (22/28 Pages) Intersil Corporation – Multi-Phase PWM Controller with Precision rDS(ON) or DCR Current Sensing for VR10.X Application

ISL6565A, ISL6565B
important to note that when using Equations 33 and 34 the
resistor divider ratio of the corresponding phase RC network
is being changed. In the phase being adjusted, this new ratio,
K_new (described in Equation 32), can not exceed 1.0.
K_new
=
K
∆_-----T----1-
∆T₂
(EQ. 35)
If this occurs, the current in the hot phase cannot be reduced
any more. Instead of decreasing the current in the hot phase,
the current must be increased in the colder phases. To
accomplish this, use Equations 33 and 34 to get the desired
temperature rise in the cold phases.
While a single adjustment, according to Equations 33 and 34,
is usually sufficient, it may occasionally be necessary to adjust
R₁ and R₂ in the corresponding channels two or more times
to achieve optimal thermal balance between all phases.
Load-Line Regulation Resistor
The load-line regulation resistor is labeled R_FB in Figure 7.
Its value depends on the desired full-load droop voltage
(V_DROOP in Figure 7). Once the ISEN resistor has been
chosen, the load-line regulation resistor can be calculated
using Equation 36.
R_FB = ^V-7---0-D----^R_×---1-^O---0--^O-_–---_6P--
(EQ. 36)
If one or more of the ISEN resistors is adjusted for thermal
balance, as in Equation 26, the load-line regulation resistor
should be selected according to Equation 37 where I_FL is the
full-load operating current and R_ISEN(n) is the ISEN resistor
connected to the n^th ISEN pin.
∑ ^R_FB
=
-----V----^D----^R----^O-----^O----^P------
I_FL r_DS(ON)
R_ISEN(n)
n
(EQ. 37)
Temperature Compensation Resistor
By combining Equations 17 and 18 found in the Temperature
Compensation section, the value of the TCOMP resistor can
be determined using Equation 38.
R_TCOMP
=
---------^α------------
K_T K_TC
(EQ. 38)
In Equation 38, K_T is the temperature coupling coefficient
between the ISL6565A and the closest lower MOSFET, or
the ISL6565B and the output inductor. It represents how
closely the controller temperature tracks the lower MOSFET
or inductor temperature. The value of K_T is typically between
75% and 100%. K_TC is the temperature dependant
transconductance of the internal compensation circuit. Its
value is designed as 2µA/V/°C. The temperature coefficient
of MOSFET r_DS(ON) or inductor DCR is given by α. This is
the ratio of the change in resistance to the change in
temperature. Resistance is normalized to the value at 25°C
and the value of α is typically between 0.35%/°C and
0.50%/°C.
According to Equation 38, a voltage regulator with 80%
thermal coupling coefficient between the controller and lower
MOSFET and 0.4%/°C temperature coefficient of MOSFET
r_DS(ON) requires a 2.5kΩ TCOMP resistor.
If the exact value for K_T and α are not known, Equation 38
can give an incorrect value for R_TCOMP. If this is the case,
follow the steps below to obtain an accurate value for
R_TCOMP. This procedure works by making two output
voltage measurements. The first is made by using too much
temperature compensation, and the second with too little.
Each of the measurements produces an error and a linear
interpolation is used to find a TCOMP resistor value to
produce zero error. Make all measurements using a digital
multimeter accurate to 100µV or better.
1. Install a 5kΩ resistor (R₁) for R_TCOMP.
2. Start the regulator at room temperature and apply full
load current. Record the output voltage, V₁, immediately
after loading the regulator.
3. Allow the board to heat until the output voltage stabilizes
(usually several minutes). Record the output voltage, V₂.
4. Install a 1kΩ resistor (R₂) for R_TCOMP.
5. Start the regulator at room temperature and apply full
load current. Record the output voltage, V₃, immediately
after loading the regulator.
6. Allow the board to heat until the output voltage stabilizes
(usually several minutes). Record the output voltage, V₄.
7. Calculate the correct value for R_TCOMP using
Equation 39.
R_TCOMP = R₁ – (R₁ – R₂) ₍---_V----₂-----–----_V⁽---^V-₁---²-₎---+–-----^V₍---_V-¹---₃⁾----–-----_V----₄----₎
(EQ. 39)
Compensation
The two opposing goals of compensating the voltage
regulator are stability and speed.
The load-line regulated converter behaves in a similar
manner to a peak-current mode controller because the two
poles at the output-filter L-C resonant frequency split with
the introduction of current information into the control loop.
The final location of these poles is determined by the system
function, the gain of the current signal, and the value of the
compensation components, R_C and C_C.
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