isl6312a Intersil Corporation, isl6312a Datasheet - Page 28

• Manufacturer Part Number: isl6312a
• Description: Four-phase Buck Pwm Controller With Integrated Mosfet Drivers For Intel Vr10, Vr11, And Amd Applications
• Manufacturer: Intersil Corporation

Inductor DCR Current Sensing Component
Selection
The ISL6312A senses each individual channel’s inductor
current by detecting the voltage across the output inductor
DCR of that channel (As described in the “Continuous Current
Sampling” on page 12). As Figure 18 illustrates, an R-C
network is required to accurately sense the inductor DCR
voltage and convert this information into a current, which is
proportional to the total output current. The time constant of
this R-C network must match the time constant of the inductor
L/DCR.
The R-C network across the inductor also sets the
overcurrent trip threshold for the regulator. Before the R-C
components can be selected, the desired overcurrent
protection level should be chosen. The minimum overcurrent
trip threshold the controller can support is dictated by the
DCR of the inductors and the number of active channels. To
calculate the minimum overcurrent trip level, I
Equation 33, where N is the number of active channels, and
DCR is the individual inductor’s DCR.
If the desired overcurrent trip level is equal to or less
then the minimum overcurrent trip level, follow the steps
below to choose the component values for the R-C
current sensing network:
I
OCP min
1. Choose an arbitrary value for C
2. Plug the inductor L and DCR component values, and the
MOSFET
DRIVER
value is 0.1μF.
value for C
calculate the value for R
,
ISL6312A INTERNAL CIRCUIT
SAMPLE
FIGURE 18. DCR SENSING CONFIGURATION
=
I
n
0.0375 N
-------------------------- -
DCR
1
UGATE(n)
LGATE(n)
I
chosen in step 1, into Equation 34 to
SEN
⋅
+
-
1
V
R
V
28
IN
ISEN
C
(s)
-
1
R
INDUCTOR
. The recommended
1
L
V
ISEN-(n)
ISEN+(n)
L
I
(s)
L
V
C
DCR
R
*R
-
(s)
C
2*
1
2
OCP,min
-
is OPTIONAL
C
V
OUT
(EQ. 33)
, use
OUT
ISL6312A
.
If the desired overcurrent trip level is greater then the
minimum overcurrent trip level, I
divider R-C circuit should be used to set the desired trip
level. Follow the steps below to choose the component
values for the resistor divider R-C current sensing
network:
Due to errors in the inductance or DCR it may be necessary
to adjust the value of R
correctly. The effects of time constant mismatch can be seen
in the form of droop overshoot or undershoot during the
initial load transient spike, as shown in Figure 19. Follow the
steps below to ensure the R-C and inductor L/DCR time
constants are matched accurately.
3. Resistor R
1. Choose an arbitrary value for C
2. Plug the inductor L and DCR component values, the
1. Capture a transient event with the oscilloscope set to
2. Record ΔV1 and ΔV2 as shown in Figure 19.
3. Select new values, R
4. Replace R
value is 0.1μF.
value for C
channels N, and the desired overcurrent protection level
I
R
about L/DCR/2 (sec/div). For example, with L = 1μH and
DCR = 1mΩ, set the oscilloscope to 500μs/div.
constant resistors based on the original values, R
and R
that the error is corrected. Repeat the procedure if
necessary.
R
R
R
R
R
OCP
1
1
1
2
1 NEW
2 NEW
,
,
and R
=
=
=
into Equations 35 and 36 to calculate the values for
-------------------------
DCR C
-------------------------------------- -
C
--------------------------------------------------------------------------------- -
C
2,OLD
1
1
L I
⋅
⋅
=
=
2
L
0.0375 N
(
1
2
.
⋅
I
1
⋅
R
R
OCP
, using Equations 37 and 38.
and R
OCP
should be left unpopulated.
chosen in step 1, the number of active
1 OLD
2 OLD
1
,
,
⋅
L I
⋅
DCR 0.0375 N
⋅
2
1
⋅
⋅
OCP
with the new values and check to see
Δ
----------
Δ
Δ
----------
Δ
and R
1,NEW
V
V
V
V
–
1
2
1
2
2
OCP,min
and R
to match the time constants
I
I
⋅
OCP
OCP
1
. The recommended
)
2,NEW
≤
>
, then a resistor
I
I
OCP min
OCP min
, for the time
,
,
August 1, 2007
(EQ. 34)
(EQ. 35)
(EQ. 36)
(EQ. 37)
(EQ. 38)
1,OLD
FN9290.3