ISL8103 INTERSIL [Intersil Corporation], ISL8103 Datasheet - Page 20

• Manufacturer Part Number: ISL8103
• Description: Three-Phase Buck PWM Controller with High Current Integrated MOSFET Drivers
• Manufacturer: INTERSIL [Intersil Corporation]

dissipating their heat so that the affected channels run hotter
than desired, choose new, smaller values of R
affected phases (see the section entitled Channel Current
Balance). Choose R
decrease in temperature rise in order to cause proportionally
less current to flow in the hotter phase.
In Equation 24, make sure that ∆T
temperature rise above the ambient temperature, and ∆T
the measured temperature rise above the ambient
temperature. While a single adjustment according to
Equation 24 is usually sufficient, it may occasionally be
necessary to adjust R
optimal thermal balance between all channels.
Load Line Regulation Component Selection (DCR
Current Sensing)
For accurate load line regulation, the ISL8103 senses the
total output current by detecting the voltage across the
output inductor DCR of each channel (As described in the
Load Line Regulation section). As Figure 7 illustrates, an
R-C network is required to accurately sense the inductor
DCR voltage and convert this information into a droop
voltage, which is proportional to the total output current.
Choosing the components for this current sense network is a
two step process. First, R
chosen so that the time constant of this R
network matches the time constant of the inductor L/DCR.
Then the resistor R
sense network gain, obtaining the desired full load droop
voltage. Follow the steps below to choose the component
values for this R-C 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 18. Follow the
R
1. Choose an arbitrary value for C
2. Plug the inductor L and DCR component values, and the
3. Use the new value for R
ISEN 2 ,
value is 0.01µF.
values for C
calculate the value for R
as well as the desired full load current, I
voltage, V
calculate the value for R
R
R
COMP
S
=
=
------------------------ - R
V
R
DROOP
=
ISEN
I
FL
DROOP
---------------------------------------
DCR C
COMP
⋅
⋅
∆T
----------
∆T
S
⋅
ISEN,2
L
, and inductor DCR in Equation 26 to
must be chosen to set the current
COMP
ISEN
2
1
COMP
chosen in steps 1, into Equation 25 to
COMP
COMP
two or more times to achieve
COMP
in proportion to the desired
⋅
COMP
S
DCR
20
.
to match the time constants
and C
obtained from Equation 25,
2
.
COMP
is the desired
COMP
. The recommended
COMP
FL
, full load droop
must be
ISEN
-C
COMP
for the
(EQ. 24)
(EQ. 25)
(EQ. 26)
1
is
ISL8103
steps below to ensure the R-C and inductor L/DCR time
constants are matched accurately.
After choosing a new value for R
necessary to adjust the value of R
load droop voltage. Use Equation 26 to obtain the new value
for R
Compensation
The two opposing goals of compensating the voltage
regulator are stability and speed. Depending on whether the
regulator employs the optional load-line regulation as
described in “Load-Line Regulation”, there are two distinct
methods for achieving these goals.
Compensating the Load-Line Regulated Converter
The load-line regulated converter behaves in a similar
manner to a peak current mode controller because the two
poles at the output filter L-C resonant frequency split with the
introduction of current information into the control loop. The
final location of these poles is determined by the system
function, the gain of the current signal, and the value of the
compensation components, R
1. Capture a transient event with the oscilloscope set to
2. Record ∆V1 and ∆V2 as shown in Figure 18.
3. Select a new value, R
4. Replace R
about L/DCR/2 (sec/div). For example, with L = 1µH and
DCR = 1mΩ, set the oscilloscope to 500µs/div.
resistor based on the original value, R
following equation.
the error is corrected. Repeat the procedure if necessary.
R
S
FIGURE 18. TIME CONSTANT MISMATCH BEHAVIOR
COMP 2
.
∆V
1
,
=
COMP
R
COMP 1
with the new value and check to see that
,
COMP,2
⋅
∆
----------
∆
V
V
1
2
2
and C
COMP
, for the time constant
S
to obtain the desired full
1
, it will most likely be
.
COMP,1
∆V
∆I
2
February 15, 2006
, using the
V
I
TRAN
(EQ. 27)
OUT
FN9246.0