ISL6327A Datasheet, PDF (24/29 Pages) Intersil Corporation – Enhanced 6-Phase PWM Controller with 8-Bit VID Code and Differential Inductor DCR or Resistor Current Sensing

ISL6327A
At turn-on, the upper MOSFET begins to conduct and this
transition occurs over a time t₂. In Equation 25, the
approximate power loss is P_UP,2.
P_{UP, 2}
≈
V_IN
⎛
⎜
I--_M---
⎝N
–
I--_{P-------P--⎟⎞}
2_⎠
⎛
⎜
⎝
-t-²--
2
⎞
⎟
⎠
f_S
(EQ. 25)
A third component involves the lower MOSFET’s reverse
recovery charge, Q_rr. Since the inductor current has fully
commutated to the upper MOSFET before the lower
MOSFET’s body diode can draw all of Q_rr, it is 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. 26)
Finally, the resistive part of the upper MOSFET’s is given in
Equation 27 as P_UP,4.
P_UP,4 ≈ r_DS(ON)
⎛
⎜
⎝
I--_{M---⎟⎞}
N⎠
2
d
+
-I-^P----^---^P--²d
12
(EQ. 27)
The total power dissipated by the upper MOSFET at full load
can now be approximated as the summation of the results
from Equations 24, 25, 26 and 27. 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.
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 the Equation 28.
R_ISEN
=
8----5----R-×^{---1-X--0----–--6--}
I--_{O-----C----P--}
N
(EQ. 28)
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 28
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 28 should be the maximum DC
resistance of the current sense element at all the 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 titled “Channel-Current
Balance” on page 12). 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.
R _{I S E N ,2}
=
R_ISEN
Δ_-----T----2-
ΔT₁
(EQ. 29)
In Equation 29, 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 29 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 using Equation 30:
R_LL
=
V-----_{D----R----O-----O----P--}
I_FL
(EQ. 30)
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 using Equation 31:
R_FB
=
N-----R-----_{I--S----E----N----R----L----L-}
R_X
(EQ. 31)
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 29, the load-line regulation
resistor should be selected based on the average value of
the current sensing resistors, as given in Equation 32:
∑ ^R_FB
=
R-----_L---L--
R_X
R_ISEN(n)
n
(EQ. 32)
wthheenre_thRIS_{ISEENN+(np)iins.} the current sensing resistor connected to
24
FN6833.0
February 17, 2009