ISL6313B Intersil Corporation, ISL6313B Datasheet - Page 18

• Manufacturer Part Number: ISL6313B
• Description: Two-Phase Buck PWM Controller
• Manufacturer: Intersil Corporation

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 7, a current proportional to the average
current of all active channels, I
load-line regulation resistor R
across R
creating an output voltage droop with a steady-state value
defined as Equation 9:
The regulated output voltage is reduced by the droop voltage
V
derived by combining Equations 6, 8, and 9.
In Equation 10, V
programmed offset voltage, I
of the converter, R
connected to the ISEN+ pin, R
is the active channel number, and DCR is the Inductor DCR
value.
Therefore the equivalent loadline impedance, i.e. droop
impedance, is equal to:
Output-Voltage Offset Programming
The ISL6313B allows the designer to accurately adjust the
offset voltage by connecting a resistor, R
pin to VCC or GND. When R
and VCC, the voltage across it is regulated to 1.6V. This
causes a proportional current (I
and out of the FB pin, providing a negative offset. If R
connected to ground, the voltage across it is regulated to
0.3V, and I
providing a positive offset. The offset current flowing through
the resistor between VSEN and FB will generate the desired
offset voltage which is equal to the product (I
These functions are shown in Figures 8 and 9.
Once the desired output offset voltage has been determined,
use formulas in Equations 12 and 13 to set R
V
V
R
DROOP
DROOP
OUT
LL
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=
=
------------
R
. The output voltage as a function of load current is
N
FB
V
FB
=
REF
OFS
I
is proportional to the output current, effectively
⋅
AVG
----------------- -
R
DCR
–
ISEN
flows into the FB pin and out of the OFS pin,
V
⋅
REF
OFS
ISEN
R
FB
is the reference voltage, V
–
⎛
⎜
⎝
is the internal sense resistor
I
------------ -
OUT
N
OFS
OUT
18
FB
⋅
AVG
FB
OFS
----------------- - R
R
. The resulting voltage drop
DCR
ISEN
is connected between OFS
is the total output current
is the feedback resistor, N
, flows from FB through a
) to flow into the OFS pin
⋅
OFS
FB
⎞
⎟
⎠
OFS
, from the OFS
OFS
OFS
:
x R
(EQ. 10)
(EQ. 11)
FB
is the
OFS
(EQ. 9)
).
ISL6313B
is
For Negative Offset (connect R
For Positive Offset (connect R
R
R
V
R
V
R
OFS
OFS
OFS
OFS
OFS
OFS
+
FIGURE 8. POSITIVE OFFSET OUTPUT VOLTAGE
FIGURE 9. NEGATIVE OFFSET OUTPUT VOLTAGE
-
+
-
VSEN
VSEN
VCC
=
=
I
GND
OFS
R
R
I
--------------------------
V
--------------------------
V
OFS
1.6 R
0.3 R
FB
FB
OFS
OFS
OFFSET
OFFSET
FB
FB
⋅
⋅
PROGRAMMING
PROGRAMMING
FB
FB
ISL6313B
ISL6313B
VREF
VREF
OFS
OFS
to GND):
to VCC):
E/A
GND
E/A
GND
+
+
-
-
0.3V
0.3V
November 6, 2008
VCC
VCC
(EQ. 12)
(EQ. 13)
FN6809.0
+
-
+
-
1.6V
1.6V