ISL6326 Datasheet, PDF (25/30 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision rDS ON or DCR Differential Current Sensing

ISL6326
conducted through the upper MOSFET across VIN. The
power dissipated as a result is P_UP,3 and is approximately
P_UP,3 = V_IN Q_rr f_S
(EQ. 28)
Finally, the resistive part of the upper MOSFET’s is given in
Equation 29 as P_UP,4.
The total power dissipated by the upper MOSFET at full load
can now be approximated as the summation of the results
from Equations 26, 27, and 28. Since the power equations
depend on MOSFET parameters, choosing the correct
MOSFETs can be an iterative process involving repetitive
solutions to the loss equations for different MOSFETs and
different switching frequencies.
P_UP,4 ≈ r_DS(ON)
⎛
⎜
⎝
-I-^M---⎟⎞
N⎠
2
d
+
-I-^P----^P--²-
12
d
(EQ. 29)
Current Sensing Resistor
The resistors connected to the ISEN+ pins determine the
gains in the load-line regulation loop and the channel-current
balance loop as well as setting the overcurrent trip point.
Select values for these resistors by using Equation 30:
R_ISEN
=
8----5----R-×^{---1-X--0----–--6--}
I--_{O-----C----P--}
N
(EQ. 30)
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 1.3x 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 30
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 30 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 RISEN for the
affected phases (see the section entitled “Channel-Current
Balance” on page 13). 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 in
Equation 31:
R_ISEN,2 = R_ISEN ^ΔΔ-----TT----²¹-
(EQ. 31)
Make sure that ΔT₂ is the desired temperature rise above the
ambient temperature, and ΔT₁ is the measured temperature
rise above the ambient temperature. While a single
adjustment according to Equation 31 is usually sufficient, it
may occasionally be necessary to adjust R_ISEN two or more
times to achieve optimal thermal balance between all
channels.
Load-Line Regulation Resistor
The load-line regulation resistor is labelled R_FB in Figure 5.
Its value depends on the desired loadline requirement of the
application.
The desired loadline can be calculated by using
Equation 32:
R_LL
=
V-----_{D----R----O-----O----P--}
I_FL
(EQ. 32)
where I_FL is the full load current of the specific application,
and VR_DROOP is the desired voltage droop under the full
load condition.
Based on the desired loadline R_LL, the loadline regulation
resistor can be calculated by using Equation 33:
R_FB
=
N-----R-----_{I--S----E----N----R----L----L-}
R_X
(EQ. 33)
where N is the active channel number, R_ISEN is the sense
resistor connected to the ISEN+ pin, and R_X is the
resistance of the current sense element, either the DCR of
the inductor or R_SENSE depending on the sensing method.
If one or more of the current sense resistors are adjusted for
thermal balance, as in Equation 31, the load-line regulation
resistor should be selected based on the average value of
the current sensing resistors, as given in Equation 34:
∑ ^R_FB
=
R-----_L---L--
R_X
R_ISEN(n)
n
(EQ. 34)
wthheenre_thRIS_{ISEENN+(np)iins.} the current sensing resistor connected to
Compensation
The two opposing goals of compensating the voltage
regulator are stability and speed. Depending on whether the
regulator employs the optional load-line regulation as
described in “Load-Line Regulation” on page 18, there are
two distinct methods for achieving these goals.
25
FN9262.1
May 5, 2008