ISL6326 Datasheet, PDF (18/30 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision rDS ON or DCR Differential Current Sensing

ISL6326
Load-Line Regulation
Some microprocessor manufacturers require a
precisely-controlled output resistance. This dependence of
output voltage on load current is often termed “droop” or
“load line” regulation. By adding a well controlled output
impedance, the output voltage can effectively be level shifted
in a direction which works to achieve the load-line regulation
required by these manufacturers.
In other cases, the designer may determine that a more
cost-effective solution can be achieved by adding droop.
Droop can help to reduce the output voltage spike that
results from fast load-current demand changes.
The magnitude of the spike is dictated by the ESR and ESL
of the output capacitors selected. By positioning the no-load
voltage level near the upper specification limit, a larger
negative spike can be sustained without crossing the lower
limit. By adding a well controlled output impedance, the
output voltage under load can effectively be level shifted
down so that a larger positive spike can be sustained without
crossing the upper specification limit.
As shown in Figure 5, a current proportional to the average
current of all active channels (I_AVG) flows from FB through a
load-line regulation resistor R_FB. The resulting voltage drop
across R_FB is proportional to the output current, effectively
creating an output voltage droop with a steady-state value
defined as Equation 8:
V_DROOP = I_AVG R_FB
(EQ. 8)
The regulated output voltage is reduced by the droop voltage
V_DROOP. The output voltage as a function of load current is
derived by combining Equation 8 with the appropriate
sample current expression defined by the current sense
method employed in Equation 9.
V_OUT
=
V_REF
–
V_O
F
S
–
⎛
⎜
⎝
-I-^O-----^U----^T--
N
------R----^X-------
R_ISEN
⎞
R _{F B⎠⎟}
(EQ. 9)
Where V_REF is the reference voltage, V_OFS is the
programmed offset voltage, I_OUT is the total output current
of the converter, R_ISEN is the sense resistor connected to
the ISEN+ pin, and R_FB is the feedback resistor, N is the
active channel number, and R_X is the DCR, or R_SENSE
depending on the sensing method.
Therefore the equivalent loadline impedance, i.e. Droop
impedance, is equal to Equation 10:
R_LL
=
--R----^F----^B--
N
-----R-----^X-------
R_ISEN
(EQ. 10)
Output Voltage Offset Programming
The ISL6326 allows the designer to accurately adjust the
offset voltage. When a resistor, R_OFS, is connected between
OFS to VCC, the voltage across it is regulated to 1.6V. This
causes a proportional current (I_OFS) to flow into OFS. If
R_OFS is connected to ground, the voltage across it is
regulated to 0.4V, and I_OFS flows out of OFS. A resistor
between DAC and REF, R_REF, is selected so that the
product (I_OFS x R_OFS) is equal to the desired offset voltage.
These functions are shown in Figure 6.
Once the desired output offset voltage has been determined,
use Equations 11 and 12 to set R_OFS:
For Positive Offset (connect R_OFS to VCC):
R_OFS
=
1_----._--6_----×_-----R_{-----R----E----F-}
V_OFFSET
(EQ. 11)
For Negative Offset (connect R_OFS to GND):
R_OFS
=
0_----._--4_----×_-----R_{-----R----E----F-}
V_OFFSET
(EQ. 12)
FB
-
E/A
+
DYNAMIC
VID D/A
DAC
R_REF
REF
C_REF
+
-
-
-
1.6V
+
+
+
0.4V
-
VCC
OR
GND
R_OFS
OFS
ISL6326B
VCC
GND
FIGURE 6. OUTPUT VOLTAGE OFFSET PROGRAMMING
18
FN9262.1
May 5, 2008