ISL6308 Datasheet, PDF (12/27 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with High Current Integrated MOSFET Drivers

ISL6308
to VSEN pin to set the output voltage level as shown in
Figure 6. This method is good for generating voltages up to
2.3V (with the REF voltage set to 1.5V).
For this case, the output voltage can be obtained as follows:
OUT
=
V_R
E
F
⋅
(_---R_{-----S---1-----+-----}R_{----P----1----})
R_P1
+−
V_O
F
S
–
V_DR
O
O
P
(EQ. 4)
It is recommended to choose resistor values of less than
500Ω for R_S1 and R_P1 resistors in order to get better output
voltage DC accuracy.
TABLE 1. ISL6308 DAC VOLTAGE SELECTION TABLE
REF1
REF0
VDAC
0
0
0.600V
0
1
0.900V
1
0
1.200V
1
1
1.500V
Voltage Regulation
In order to regulate the output voltage to a specified level, the
ISL6308 uses the integrating compensation network shown in
Figure 6. This compensation network insures that the steady
state error in the output voltage is limited only to the error in
the reference voltage (output of the DAC or the external
voltage reference) and offset errors in the OFS current
source, remote sense and error amplifiers. Intersil specifies
the guaranteed tolerance of the ISL6308 to include the
combined tolerances of each of these elements, except when
an external reference or voltage divider is used, then the
tolerances of these components has to be taken into account.
The ISL6308 incorporates an internal differential remote-
sense amplifier in the feedback path. The amplifier removes
the voltage error encountered when measuring the output
voltage relative to the controller ground reference point,
resulting in a more accurate means of sensing output
voltage. Connect the microprocessor sense pins to the non-
inverting input, VSEN, and inverting input, RGND, of the
remote-sense amplifier. The droop voltage, VDROOP, also
feeds into the remote-sense amplifier. The remote-sense
output, VDIFF, is therefore equal to the sum of the output
voltage, VOUT, and the droop voltage. VDIFF is connected
to the inverting input of the error amplifier through an
external resistor.
The output of the error amplifier, V_COMP, is compared to the
sawtooth waveform to generate the PWM signals. The PWM
signals control the timing of the Internal MOSFET drivers
and regulate the converter output so that the voltage at FB is
equal to the voltage at REF. This will regulate the output
voltage to be equal to Equation 5. The internal and external
circuitry that controls voltage regulation is illustrated in
Figure 6.
V_OUT = V_REF ± V_OFST – V_DROOP
(EQ. 5)
EXTERNAL CIRCUIT
R₂ C_{1 COMP}
DAC
ISL6308 INTERNAL CIRCUIT
VID DAC
REF
C_REF
FB
+
R₁
V_OFS
-
VDIFF
R_S1
+
V_OUT
VSEN
R_P1
-
RGND
-
V_DROOP
+
C_SUM
DROOP
IREF
ICOMP
ISUM
+
-
V_COMP
I_OFS ^{ERROR AMPLIFIER}
+
+
-
- ^DIFFERENTIAL
REMOTE-SENSE
AMPLIFIER
+
ISENSE
AMP-
FIGURE 6. OUTPUT VOLTAGE AND LOAD-LINE
REGULATION WITH OFFSET ADJUSTMENT
Load-Line (Droop) Regulation
In some high current applications, a requirement on a
precisely controlled output impedance is imposed. This
dependence of output voltage on load current is often
termed “droop” or “load line” regulation.
The Droop is an optional feature in the ISL6308. It can be
enabled by connecting ICOMP pin to DROOP pin as shown
in Figure 6. To disable it, connect the DROOP pin to IREF
pin.
As shown in Figure 6, a voltage, V_DROOP, proportional to the
total current in all active channels, I_OUT, feeds into the
differential remote-sense amplifier. The resulting voltage at
the output of the remote-sense amplifier is the sum of the
output voltage and the droop voltage. As Equation 4 shows,
feeding this voltage into the compensation network causes
the regulator to adjust the output voltage so that it’s equal to
the reference voltage minus the droop voltage.
The droop voltage, V_DROOP, is created by sensing the
current through the output inductors. This is accomplished
by using a continuous DCR current sensing method.
Inductor windings have a characteristic distributed
resistance or DCR (Direct Current Resistance). For
simplicity, the inductor DCR is considered as a separate
12
FN9208.2
October 19, 2005