ISL6569A Datasheet, PDF (16/22 Pages) Intersil Corporation – Multi-Phase PWM Controller

ISL6569A
current (see Equation 1); d is the duty cycle (V_OUT/V_IN); and
L is the per-channel inductance.
P_L
=
r_DS(ON)



I--_{M---}
2
2
(
1
–
d
)
+
-I-^L---^,---^2P----^P----⁽--1-----^–-----d----⁾
12
(EQ. 13)
An additional term can be added to the lower-MOSFET loss
equation to account for additional loss accrued during the
dead time when inductor current is flowing through the
lower-MOSFET body diode. This term is dependent on the
diode forward voltage at I_M, V_D(ON); the switching frequency,
f_S; and the length of dead times, t_d1 and t_d2, at the
beginning and the end of the lower-MOSFET conduction
interval respectively.
P_D
=
V_D(ON) f_S


I--_M---
2
+
-I-^P-2---^P--_
t_d1
+


I--_M---
2
–
I--_{P----P--}
2
t_d2
(EQ. 14)
Thus the total maximum power dissipated in each lower
MOSFET is approximated by the summation of P_L and P_D.
UPPER MOSFET POWER CALCULATION
In addition to r_DS(ON) losses, a large portion of the upper-
MOSFET losses are due to currents conducted across the
input voltage (V_IN) during switching. Since a substantially
higher portion of the upper-MOSFET losses are dependent
on switching frequency, the power calculation is more
complex. Upper MOSFET losses can be divided into
separate components involving the upper-MOSFET
switching times; the lower-MOSFET body-diode reverse-
recovery charge, Q_rr; and the upper MOSFET r_DS(ON)
conduction loss.
When the upper MOSFET turns off, the lower MOSFET does
not conduct any portion of the inductor current until the
voltage at the phase node falls below ground. Once the
lower MOSFET begins conducting, the current in the upper
MOSFET falls to zero as the current in the lower MOSFET
ramps up to assume the full inductor current. In Equation 15,
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---
2
+
I--_{P-2---P--}



t--_1--


2_
f_S
(EQ. 15)
The upper MOSFET begins to conduct and this transition
occurs over a time t₂. In Equation 16, the approximate power
loss is P_UP,2.
P_U
P,
2
≈
V_IN



I--_M---
2
–
I--_{P----P--}
2



t--_2--
2



f_S
(EQ. 16)
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. 17)
Finally, the resistive part of the upper MOSFET’s dissipation
is given in Equation 18 as P_UP,4.
P _{U P,4}
=
r_DS(ON)


I--_{M---}
 2
2
d
+
-I-^P----^P--²-
12
(EQ. 18)
In this case, of course, r_DS(ON) is the on resistance of the
upper MOSFET.
The total power dissipated by the upper MOSFET at full load
can now be approximated as the summation of the results
from Equations 15, 16, 17 and 18. Since the power
equations depend on MOSFET parameters, choosing the
correct MOSFETs can be an iterative process that involves
repetitively solving the loss equations for different MOSFETs
and different switching frequencies until converging upon the
best solution.
Current Sensing
The ISEN pins are denoted ISEN1 and ISEN2. The resistors
connected between these pins and their respective phase
nodes determine the gain in the load-line regulation loop and
the channel-current balance loop. Select the values for these
resistors based on the room temperature r_DS(ON) of the
lower MOSFETs; the full-load operating current, I_FL;
according to Equation 19 (see also Figure 4).
R_ISEN
=
5-r--^D-0---S--×^{--(-1-O--0---N–---6-)-}
-I-^F----^L-
2
(EQ. 19)
In certain circumstances, it may be necessary to adjust the
value of one or both of the ISEN resistors. This can arise
when the components of one channel are inhibited from
dissipating their heat so that the affected channel runs hotter
than desired (see the section entitled Channel-Current
Balance). In this case, chose a new, smaller value of R_ISEN
for the affected phase. 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
∆_-----T----2-
∆T₁
(EQ. 20)
In Equation 20, 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 20 is usually
sufficient, it may occasionally be necessary to adjust R_ISEN
two or more times to achieve perfect thermal balance between
both channels.
16
FN9092.2
December 29, 2004