ISL6308 Datasheet, PDF (13/27 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with High Current Integrated MOSFET Drivers

ISL6308
lumped quantity, as shown in Figure 7. The channel current,
I_L, flowing through the inductor, passes through the DCR.
Equation 6 shows the s-domain equivalent voltage, V_L,
across the inductor.
V_L(s) = I_L ⋅ (s ⋅ L + DCR)
(EQ. 6)
The inductor DCR is important because the voltage dropped
across it is proportional to the channel current. By using a
simple R-C network and a current sense amplifier, as shown
in Figure 7, the voltage drop across all of the inductors DCRs
can be extracted. The output of the current sense amplifier,
V_DROOP, can be shown to be proportional to the channel
currents I_L1, I_L2, and I_L3, shown in Equation 7.
VD _{R O O P} ( s )
=
(^---s^----⋅^----R^----_C^-----_O^{-----}_M^-D----s-----_P^{-C---⋅------L-R}⋅^------C^--+---_C^--1--_O^{-----}_M^------_P^-----+-----1^----) ^â‹
(EQ. 7)
-^R----^C-----^O-----^M------^P--
RS
⋅
(IL1
+
IL2
+
IL3)
⋅
DCR
If the R-C network components are selected such that the
R-C time constant matches the inductor L/DCR time
constant, then V_DROOP is equal to the sum of the voltage
drops across the individual DCRs, multiplied by a gain. As
Equation 8 shows, V_DROOP is therefore proportional to the
total output current, I_OUT.
V_DROOP
=
R_{-----C----O-----M-----P-}
R_S
⋅
I_OUT
⋅
DCR
(EQ. 8)
PHASE1
PHASE2
ISUM
V_L(s)
L1 _DCR
INDUCTOR
IL₁
R_S
L2 _DCR
INDUCTOR
IL₂
R_S
I_OUT
V_OUT
C_OUT
ICOMP
-
DROOP
V_DROOP
+ IREF
ISL6308
C_COMP ^RCOMP
C_SUM
(Optional)
By simply adjusting the value of R_S, the load line can be set
to any level, giving the converter the right amount of droop at
all load currents. It may also be necessary to compensate for
any changes in DCR due to temperature. These changes
cause the load line to be skewed, and cause the R-C time
constant to not match the L/DCR time constant. If this
becomes a problem a simple negative temperature
coefficient resistor network can be used in the place of
R_COMP to compensate for the rise in DCR due to
temperature.
Output Voltage Offset Programming
The ISL6308 allows the designer to accurately adjust the
offset voltage by connecting a resistor, R_OFS, from the OFS
pin to VCC or GND. When R_OFS is connected between OFS
and VCC, the voltage across it is regulated to 1.5V. This
causes a proportional current (I_OFS) to flow into the OFS pin
and out of the FB pin. If R_OFS is connected to ground, the
voltage across it is regulated to 0.5V, and I_OFS flows into the
FB pin and out of the OFS pin. The offset current flowing
through the resistor between VDIFF and FB will generate
the desired offset voltage which is equal to the product
(I_OFS x R₁). These functions are shown in Figures 8 and 9.
VDIFF
+
V_{OFS R1}
-
VREF
E/A
FB
I_OFS
R_OFS
OFS
ISL6308
GND
+
0.5V
-
-
1.5V
+
GND
VCC
FIGURE 8. POSITIVE OFFSET OUTPUT VOLTAGE
PROGRAMMING
FIGURE 7. DCR SENSING CONFIGURATION
13
FN9208.2
October 19, 2005