ISL6323 Datasheet, PDF (27/34 Pages) Intersil Corporation – Hybrid SVI/PVI

ISL6323
4. Using Equation 40 (also derived from Equation 20),
calculate the value of K for the North bridge regulator.
K
=
---¹---⁰---⁰----^μ----^A----
I_CoreNB
⋅
----------¹-----------
DCR_NB
⋅
⎛
⎝
----³-----
400
⋅
R_S
E
⎞
T⎠
(EQ. 40)
5. Choose a capacitor value for the North Bridge RC filter. A
0.1µF capacitor is a recommended starting point.
6. Calculate the values for R₁ and R₂ for North Bridge.
Equations 41 and 42 will allow for their computation.
K
=
-----------^R-----²---^N----^B-------------
R_1NB + R_2NB
(EQ. 41)
------^L---^N-----^B-------
DCR_NB
=
-^R-----¹---^N----^B-----^â‹---^R-----²---^N----^B---
R_1NB + R_2NB
⋅
C_N
B
(EQ. 42)
CASE 3
I_NBMAX ⋅ DCR_NB
=
I_{--C-----o---r--e---M-----A----X--}
N
⋅
D
CR_C
o
r
e
(EQ. 43)
In Case 3, the DC voltage across the North Bridge inductor
at full load is equal to the DC voltage across a single phase
of the Core regulator while at full load. Here, the full scale
DC inductor voltages for both North Bridge and Core will be
impressed across the ISEN pins without any gain. So, the R₂
resistors for the Core and North Bridge inductor RC filters
are left unpopulated and K = 1 for both regulators.
For this Case, it is recommended that the overcurrent trip
point for the North Bridge regulator be equal to the
overcurrent trip point for the Core regulator divided by the
number of core phases.
1. Choose a capacitor value for the North Bridge RC filter. A
0.1µF capacitor is a recommended starting point.
2. Calculate the value for the North Bridge resistor R₁:
R_1NB = _D-----_C-----_R----^L-_N---^N-_B---^B--_â‹---_C----_N-----_B-
(EQ. 44)
3. Choose a capacitor value for the Core RC filter. A 0.1µF
capacitor is a recommended starting point.
4. Calculate the value for the Core resistor R₁:
R_1Core = -_D----_C-----_R-----_C--^L--_o-^C--_r--_eo----^r-_â‹-^e--_C----_C----_o----_r--_e-
(EQ. 45)
3. Calculate the value for the R_SET resistor using Equation 46:
R_SET
=
4_----0_---0_--
3
⋅
I_{--O-----C----P----N----B--}
100 μ A
⋅
D
C
R_N
B
⋅
K
(EQ. 46)
Where: K = 1
NOTE: The values of R_SET must be greater than 20kΩ and
less than 80kΩ. For all of the 3 cases above, if the calculated
value of R_SET is less than 20kΩ, then either the OCP trip
point needs to be increased or the inductor must be changed
to an inductor with higher DCR. If the R_SET resistor is
greater than 80kΩ, then a value of R_SET that is less than
80kΩ must be chosen and a resistor divider across both
North Bridge and Core inductors must be set up with proper
gain. This gain will represent the variable “K” in all equations.
It is also very important that the R_SET resistor be tied
between the RSET pin and the VCC pin of the ISL6323.
Inductor DCR Current Sensing Component Fine
Tuning
MOSFET
V_IN
UGATE(n)
IL_n
L
DCR
V_OUT
DRIVER
LGATE(n)
INDUCTOR
V_L(s)
C_OUT
V_C(s)
R₁
C
ISL6323 INTERNAL CIRCUIT
R₂
I_n
SAMPLE
+
-
I_SEN
V_C(s)
R_ISEN
ISENn-
ISENn+
VCC
RSET
R_SET
C_SET
FIGURE 20. DCR SENSING CONFIGURATION
Due to errors in the inductance and/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 21.
Follow the steps below to ensure the RC and inductor
L/DCR time constants are matched accurately.
1. If the regulator is not utilizing droop, modify the circuit by
placing the frequency set resistor between FS and
Ground for the duration of this procedure.
2. 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.
3. Record ΔV1 and ΔV2 as shown in Figure 21.Select new
values, R_1(NEW) and R_2(NEW) for the time constant
27
FN9278.2
April 7, 2008