ISL6308A Datasheet, PDF (13/28 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with High Current Integrated MOSFET Drivers

ISL6308A
EXTERNAL CIRCUIT
R₂ C_{1 COMP}
DAC
ISL6308A 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 ISL6308A. 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
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-----^â‹---^L---
DCR
+
1_⎠⎞
-------------------------------------------------------------------------
(s ⋅ RCOMP ⋅ CCOMP + 1)
⋅
R_{-----C-----O-----M------P--}
RS
⋅
(
IL
1
+
IL2
(EQ. 7)
+ 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_O
U
T
⋅
D
C
R
(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
ISL6308A
C_COMP ^RCOMP
C_SUM
(OPTIONAL)
FIGURE 7. DCR SENSING CONFIGURATION
13
FN6669.0
September 9, 2008