ISL6333_14 Datasheet, PDF (21/40 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with Integrated MOSFET Drivers and Light Load Efficiency Enhancements for Intel VR11.1 Applications

ISL6333, ISL6333A, ISL6333B, ISL6333C
Continuous Current Sensing
In order to realize proper current-balance, the currents in
each channel are sensed continuously every switching
cycle. During this time the current-sense amplifier uses the
ISEN inputs to reproduce a signal proportional to the
inductor current, I_L. This sensed current, I_SEN, is simply a
scaled version of the inductor current.
The controllers support inductor DCR current sensing to
continuously sense each channel’s current for channel-current
balance. The internal circuitry, shown in Figure 6 represents
one channel of the controller. This circuitry is repeated for each
channel in the converter, but may not be active depending on
how many channels are operating.
MOSFET
DRIVER
UGATE
LGATE
V_IN
IL
L
DCR
INDUCTOR
V_L(s)
V_OUT
C_OUT
V_C(s)
R₁
C₁
I_n
ISL6333 INTERNAL
CIRCUIT
SENSE
+
-
V_C(s)
ISEN-
R_ISEN
ISEN+
I_SEN
RSET
R_SET
VCC
FIGURE 6. INDUCTOR DCR CURRENT SENSING
CONFIGURATION
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 6. The channel current
I_L, flowing through the inductor, passes through the DCR.
Equation 4 shows the S-domain equivalent voltage, V_L,
across the inductor.
V_L(s) = I_L ⋅ (s ⋅ L + DCR)
(EQ. 4)
A simple R-C network across the inductor (R₁ and C)
extracts the DCR voltage, as shown in Figure 6. The voltage
across the sense capacitor, V_C, can be shown to be
proportional to the channel current I_L, shown in Equation 5.
⎛
⎝
--^s-----^â‹---^L---
DCR
+
1_⎠⎞
V_C(s)
=
----------------------------------------
(s ⋅ R₁ ⋅ C₁ + 1)
⋅
D
C
R
⋅
I_L
(EQ. 5)
If the R₁-C₁ network components are selected such that their
time constant matches the inductor L/DCR time constant, then
V_C is equal to the voltage drop across the DCR.
The capacitor voltage V_C, is then replicated across the
effective internal sense resistance, R_ISEN. This develops a
current through R_ISEN which is proportional to the inductor
current. This current, I_SEN, is continuously sensed and is
then used by the controllers for load-line regulation,
channel-current balancing, and overcurrent detection and
limiting. Equation 6 shows that the proportion between the
channel-current, I_L, and the sensed current, I_SEN, is driven
by the value of the effective sense resistance, R_ISEN, and
the DCR of the inductor.
I_SEN
=
I_L
⋅
---^D----^C-----^R-----
R_ISEN
(EQ. 6)
The effective internal R_ISEN resistance is important to the
current sensing process because it sets the gain of the load
line regulation loop as well as the gain of the channel-current
balance loop and the overcurrent trip level. The effective
internal R_ISEN resistance is user programmable and is set
through use of the R_SET pin. Placing a single resistor, R_SET,
from the R_SET pin to the VCC pin programs the effective
internal R_ISEN resistance according to Equation 7.
R_ISEN
=
----³-----
400
⋅
R_S
E
T
(EQ. 7)
The current sense circuitry operates in a very similar manner
for negative current feedback, where inductor current is
flowing from the output of the regulator to the PHASE node,
opposite of flow pictured in Figure 6. However, the range of
proper operation with negative current sensing is limited to
~60% of full positive current OCP threshold. Care should be
taken to avoid operation with negative current feedback
exceeding this threshold, as this may lead to momentary
loss of current balance between phases and disruption of
normal circuit operation.
Output Voltage Setting
The controllers use a digital to analog converter (DAC) to
generate a reference voltage based on the logic signals at
the VID pins. The DAC decodes the logic signals into one of
the discrete voltages shown in Table 2. Each VID pin is
pulled up to an internal 1.2V voltage by a weak current
source (40µA), which decreases to 0A as the voltage at the
VID pin varies from 0 to the internal 1.2V pull-up voltage.
External pull-up resistors or active-high output stages can
augment the pull-up current sources, up to a voltage of 5V.
TABLE 2. VR11 VOLTAGE IDENTIFICATION CODES
VID7 VID6 VID5 VID4 VID3 VID2 VID1 VID0 VDAC
0
0
0
0
0
0
0
0
OFF
0
0
0
0
0
0
0
1
OFF
0
0
0
0
0
0
1
0 1.60000
0
0
0
0
0
0
1
1 1.59375
0
0
0
0
0
1
0
0 1.58750
0
0
0
0
0
1
0
1 1.58125
0
0
0
0
0
1
1
0 1.57500
21
FN6520.3
October 8, 2010