ISL6565A Datasheet, PDF (20/28 Pages) Intersil Corporation – Multi-Phase PWM Controller with Precision rDS(ON) or DCR Current Sensing for VR10.X Application

ISL6565A, ISL6565B
the required time for this commutation is t₁ and the
approximated associated power loss is P_UP,1.
P _{U P,1}
≈
V_IN


I--_M---
N
+
-I-^P--2--^P--_



t--_1--


2_
f_S
(EQ. 22)
At turn on, the upper MOSFET begins to conduct and this
transition occurs over a time t₂. In Equation 23, the
approximate power loss is P_UP,2.
P_{UP, 2}
≈
V_IN


I--_M---
N
–
I--_{P-2---P--}



t--_2--


2_
f_S
(EQ. 23)
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 recover all of Q_rr, it is conducted
through the upper MOSFET across VIN. The power
dissipated as a result is P_UP,3.
P_UP,3 = V_IN Q_rr f_S
(EQ. 24)
Finally, the resistive part of the upper MOSFETs is given in
Equation 25 as P_UP,4.
P_UP,4 ≈ r_DS(ON)



-I-^M---
N
2
d
+
-I-^P----^P--²-
12
(EQ. 25)
The total power dissipated by the upper MOSFET at full load
can now be approximated as the summation of the results
from Equations 22, 23, 24 and 25. 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 Component Selection
The ISL6565A supports MOSFET r_DS(ON) current sensing,
while the ISL6565B uses inductor DCR current sensing. The
procedures for choosing the components for each method of
current sensing are very different and are described in the
next two sections.
MOSFET r_DS(ON) SENSING (ISL6565A ONLY)
The ISL6565A senses the channel load current by sampling
the voltage across the lower MOSFET r_DS(ON), as shown in
Figure 15. The ISEN pins are denoted ISEN1, ISEN2, and
ISEN3. The resistors connected between these pins and the
respective phase nodes determine the gains in the load-line
regulation loop and the channel-current balance loop as well
as setting the overcurrent trip point.
V_IN
CHANNEL N
UPPER MOSFET
I_L
ISL6565A
ISEN(n)
R_ISEN
-
IL rDS_(ON)
+
CHANNEL N
LOWER MOSFET
FIGURE 15. ISL6565A INTERNAL AND EXTERNAL CURRENT-
SENSING CIRCUITRY
Select values for these resistors based on the room
temperature r_DS(ON) of the lower MOSFETs; the full-load
operating current, I_FL; and the number of phases, N using
Equation 26.
R_ISEN
=
7-r--^D-0---S--×^{--(-1-O--0---N-–--6-)-}
-I-^F----^L-
N
(EQ. 26)
In certain circumstances, it may be necessary to adjust the
value of one or more ISEN resistor. 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 the section entitled Voltage Regulation). 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_ISEN,2 = R_ISEN ^∆∆-----TT----²¹-
(EQ. 27)
In Equation 27, 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 27 is usually
sufficient, it may occasionally be necessary to adjust R_ISEN
two or more times to achieve optimal thermal balance
between all channels.
INDUCTOR DCR SENSING (ISL6565B ONLY)
The ISL6565B senses the channel load current by sampling
the voltage across the output inductor DCR, as described in
the Current Sensing section. As Figure 16 illustrates, an R-C
network across the inductor is required to sense the channel
current accurately.
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