ISL6312A Datasheet, PDF (28/35 Pages) Intersil Corporation – Four-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR10, VR11, and AMD Applications

ISL6312A
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.
MOSFET
DRIVER
V_IN
UGATE(n)
LGATE(n)
ISL6312A INTERNAL CIRCUIT
IL
L
DCR
INDUCTOR
V_L(s)
V_C(s)
R₁
C₁
R_2*
V_OUT
C_OUT
I_n
SAMPLE
+
-
I_SEN
V_C(s)
R_ISEN
ISEN-(n)
ISEN+(n)
*R₂ is OPTIONAL
FIGURE 18. DCR SENSING CONFIGURATION
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_OCP,min, use
Equation 33, where N is the number of active channels, and
DCR is the individual inductor’s DCR.
I_{OCP, min}
=
0_----._--0_---3_---7_----5_----⋅_---N_---
DCR
(EQ. 33)
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:
1. Choose an arbitrary value for C₁. The recommended
value is 0.1µF.
2. Plug the inductor L and DCR component values, and the
value for C₁ chosen in step 1, into Equation 34 to
calculate the value for R₁
.
R₁ = _D-----_C-----_R--^L----_â‹---_C-----₁-
I_OCP ≤ I_{OCP, min} ^{(EQ. 34)}
3. Resistor R₂ should be left unpopulated.
If the desired overcurrent trip level is greater then the
minimum overcurrent trip level, I_OCP,min, then a resistor
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:
1. Choose an arbitrary value for C₁. The recommended
value is 0.1µF.
2. Plug the inductor L and DCR component values, the
value for C₁ chosen in step 1, the number of active
channels N, and the desired overcurrent protection level
I_OCP into Equations 35 and 36 to calculate the values for
R₁ and R₂.
R₁
=
---------^L-----^â‹---^I--^O----^C-----^P----------
C₁ ⋅ 0.0375 ⋅ N
I_OCP > I_{OCP, min} ^{(EQ. 35)}
R₂
=
-------------------------------^L-----^â‹---^I--^O----^C----^P--------------------------------
C₁ ⋅ (I_OCP ⋅ DCR – 0.0375 ⋅ N)
(EQ. 36)
Due to errors in the inductance or DCR it may be necessary
to adjust the value of R₁ and R₂ to match the time constants
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.
1. Capture a transient event with the oscilloscope set to
about L/DCR/2 (sec/div). For example, with L = 1µH and
DCR = 1mΩ, set the oscilloscope to 500μs/div.
2. Record ΔV1 and ΔV2 as shown in Figure 19.
3. Select new values, R_1,NEW and R_2,NEW, for the time
constant resistors based on the original values, R_1,OLD
and R_2,OLD, using Equations 37 and 38.
R_{1, NEW} = R_{1, OLD} ⋅ ^ΔΔ----_VV----¹²--
(EQ. 37)
R_{2, NEW} = R_{2, OLD} ⋅ ^ΔΔ----_VV----¹²--
(EQ. 38)
4. Replace R₁ and R₂ with the new values and check to see
that the error is corrected. Repeat the procedure if
necessary.
28
FN9290.4
April 29, 2010