ISL6265 Datasheet, PDF (16/24 Pages) Intersil Corporation – Multi-Output Controller with Integrated MOSFET Drivers for AMD SVI Capable

ISL6265
Estimating the value of R_{FSET_NB} is written as shown in
Equation 5:
R_FSET
=
----------¹----------
K ⋅ F_SW
(EQ. 5)
Where F_SW is the PWM switching frequency, R_{FSET_NB} is
the programming resistor and K = 1.5 x 10^-10.
It is recommended that whenever the control loop
compensation network is modified, the switching frequency
should be checked and adjusted by changing R_{FSET_NB} if
necessary.
Current Sense
Core and Northbridge regulators feature two different types
of current sense circuits.
CORE CONTINUOUS CURRENT SENSE
The ISL6265 provides for load current to be measured using
either resistors in series with the individual output inductors
or using the intrinsic series resistance of the inductors as
shown in the applications circuits in Figures 2 and 3. The
load current in a particular output is sampled continuously
every switching cycle. During this time the current-sense
amplifier uses the current sense inputs to reproduce a signal
proportional to the inductor current. This sensed current is a
scaled version of the inductor current.
V_IN
UGATE
MOSFET
DRIVER
LGATE
ISL6265 INTERNAL CIRCUIT
IL
L
DCR
INDUCTOR
V_L(s)
V_C(s)
R₁
C₁
R₂
V_OUT
C_OUT
CURRENT
SENSE
R_{NTC OPTIONAL}
ISP
NTC
R_{3 NETWORK}
ISN
FIGURE 9. DCR SENSING COMPONENTS
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 9. The inductor 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)
A simple R-C network across the inductor (R₁, R₂ and C)
extracts the DCR voltage, as shown in Equation 7. The
voltage across the sense capacitor, V_C, can be shown to be
proportional to the output current I_L, shown in Equation 7.
⎛
⎝
--^s-----^â‹---^L---
DCR
+
1_⎠⎞
V_C(s)
=
----------------------------------------------------------
⎛
⎜
⎝
s
⋅
(_---R_-----1----⋅_---R_-----2---)
R₁ + R₂
⋅
C₁
+
⎞
1⎟
⎠
⋅
K
⋅
DCR
⋅
I_L
(EQ. 7)
Where
K
=
-------^R-----²--------
R₂ + R₁
(EQ. 8)
Sensing the time varying inductor current accurately
requires that the parallel R-C network time constant match
the inductor L/DCR time constant. If the R-C network
components are selected such that the R-C time constant
matches the inductor L/DCR time constant (see Equation 9),
then V_C is equal to the voltage drop across the DCR
multiplied by the ratio of the resistor divider, K.
------^L-------
DCR
=
-^R-----¹----^â‹---^R-----²--
R₁ + R₂
⋅
C₁
(EQ. 9)
The inductor current sense information is used for current
balance in dual plane applications, overcurrent detection in
core outputs and output voltage droop depending on
controller configuration.
CORE DCR TEMPERATURE COMPENSATION
It may also be necessary to compensate for changes in
inductor DCR due to temperature. DCR shifts due to
temperature cause time constant mismatch, skewing
inductor current accuracy. Potential problems include output
voltage droop and OC trip point, both shifting significantly
from expected levels. The addition of a negative temperature
coefficient (NTC) resistor to the R-C network compensates
for the rise in DCR due to temperature. Typical NTC values
are in the 10kΩ range. A second resistor, R₃, in series with
the NTC allows for more accurate time-constant and
resistor-ratio matching as the pair of resistors are placed in
parallel with R₂ (Figure 9). The NTC resistor must be placed
next to the inductor for good heat transfer, while R₁, R₂, R₃,
and C₁ are placed close to the controller for interference
immunity.
CORE DCR COMPONENT SELECTION FOR DROOP
By adjusting the ratio between inductor DCR drop and the
voltage measured across the sense capacitor, the load line
can be set to any level, giving the converter the correct
amount of droop at all load currents.
Equation 10 shows the relation between droop voltage,
maximum output current (I_MAX), OC trip level and current
sense capacitor voltage at the OC current level, V_C(OC).
V_DROOP
=
I_{--M-----A----X--}
I_OC
⋅
5
⋅
V_C,
OC
(EQ. 10)
16
FN6599.1
May 13, 2009