ISL6313_14 Datasheet, PDF (18/33 Pages) Intersil Corporation – Two-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR11 and AMD Applications

ISL6313
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_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 9:
V_DROOP = I_AVG ⋅ R_FB
(EQ. 9)
For Negative Offset (connect R_OFS to VCC):
R_OFS
=
--¹---^.--⁶-----^â‹---^R----^F----^B---
V_OFFSET
For Positive Offset (connect R_OFS to GND):
R_OFS
=
--⁰---^.--³-----^â‹---^R----^F----^B---
V_OFFSET
VSEN
+
V_{OFS RFB}
-
VREF
E/A
FB
I_OFS
(EQ. 12)
(EQ. 13)
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 Equations 6, 8, and 9.
V_OUT
=
V_REF
–
V_O
F
S
–
⎛
⎜
⎝
-I-^O-----^U----^T--
N
⋅
---D----C-----R-----
R_ISEN
⋅
⎞
R _{F B⎠⎟}
(EQ. 10)
In Equation 10, 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 internal sense resistor
connected to the ISEN+ pin, R_FB is the feedback resistor, N
is the active channel number, and DCR is the Inductor DCR
value.
Therefore the equivalent loadline impedance, i.e. droop
impedance, is equal to:
R_LL
=
--R----^F----^B-- ⋅
N
---D----C-----R-----
R_ISEN
(EQ. 11)
Output-Voltage Offset Programming
The ISL6313 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 1.6V. This
causes a proportional current (I_OFS) to flow into the OFS pin
and out of the FB pin, providing a negative offset. If R_OFS is
connected to ground, the voltage across it is regulated to
0.3V, and I_OFS flows into the FB pin and out of the OFS pin,
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_OFS x R_FB).
These functions are shown in Figures 8 and 9.
Once the desired output offset voltage has been determined,
use Equations 12 and 13 to set R_OFS:
18
R_OFS
OFS
ISL6313
GND
+
0.3V
-
-
1.6V
+
GND
VCC
FIGURE 8. POSITIVE OFFSET OUTPUT VOLTAGE
PROGRAMMING
VSEN
-
V_{OFS RFB}
+
VREF
E/A
FB
I_OFS
VCC
R_OFS
OFS
ISL6313
+
0.3V
-
-
1.6V
+
GND
VCC
FIGURE 9. NEGATIVE OFFSET OUTPUT VOLTAGE
PROGRAMMING
FN6448.2
September 2, 2008