ISL6334B Datasheet, PDF (25/30 Pages) Intersil Corporation – VR11.1, 4-Phase PWM Controller with Phase Dropping, Droop Disabled and Load Current Monitoring Features

ISL6334B, ISL6334C
Power Stages
The first step in designing a multiphase converter is to
determine the number of phases. This determination
depends heavily upon the cost analysis, which in turn
depends on system constraints that differ from one design to
the next. Principally, the designer will be concerned with
whether components can be mounted on both sides of the
circuit board; whether through-hole components are
permitted; and the total board space available for power
supply circuitry. Generally speaking, the most economical
solutions are those in which each phase handles between
15A and 25A. All surface-mount designs will tend toward the
lower end of this current range. If through-hole MOSFETs
and inductors can be used, higher per-phase currents are
possible. In cases where board space is the limiting
constraint, current can be pushed as high as 40A per phase,
but these designs require heat sinks and forced air to cool
the MOSFETs, inductors and heat-dissipating surfaces.
MOSFETs
The choice of MOSFETs depends on the current each
MOSFET will be required to conduct; the switching
frequency; the capability of the MOSFETs to dissipate heat;
and the availability and nature of heat sinking and air flow.
LOWER MOSFET POWER CALCULATION
The calculation for heat dissipated in the lower MOSFET is
simple, since virtually all of the heat loss in the lower
MOSFET is due to current conducted through the channel
resistance (r_DS(ON)). In Equation 23, I_M is the maximum
continuous output current; I_P-P is the peak-to-peak inductor
current (see Equation 1); d is the duty cycle (V_OUT/V_IN); and
L is the per-channel inductance.
P_{LOW, 1}
=
r_DS(ON)
⎛
⎜
⎝
I--_{M---⎟⎞}
N⎠
2
(
1
–
d
)
+
-I-^L---^,---^P----^--^P--²--⁽---1-----^–----d----⁾
12
(EQ. 23)
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_sw; 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_{LOW, 2}
=
V_D(ON) F_sw
⎛
⎝
I--_M---
N
+
I--_{P--2-----P--⎠⎞}
t_d1
+
⎛
⎜
-I-^M---
⎝N
–
I--_{P-------P--⎟⎞}
2⎠
t_d2
(EQ.
24)
Thus the total maximum power dissipated in each lower
MOSFET is approximated by the summation of P_LOW,1 and
P_LOW,2.
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 25,
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. 25)
At turn on, the upper MOSFET begins to conduct and this
transition occurs over a time t₂. In Equation 26, 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. 26)
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 approximated in Equation 27:
P_UP,3 = V_IN Q_rr f_S
(EQ. 27)
Finally, the resistive part of the upper MOSFET’s is given in
Equation 28 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 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, as shown in Equation 28.
P_UP,4 ≈ r_DS(ON)
⎛
⎜
⎝
I--_{M---⎟⎞}
N⎠
2
d
+
-I-^P----^---^P--²d
12
(EQ. 28)
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 29:
R_ISEN
=
-1---0---5---R--_×---^X_{1---0----–}---₆-
I--_{O-----C----P--}
N
(EQ. 29)
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
25
FN6689.2
August 31, 2010