ISL6308 Datasheet, PDF (20/27 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with High Current Integrated MOSFET Drivers

ISL6308
dissipating their heat so that the affected channels run hotter
than desired, choose new, smaller values of R_ISEN for the
affected phases (see the section entitled Channel Current
Balance). Choose R_ISEN,2 in proportion to the desired
decrease in temperature rise in order to cause proportionally
less current to flow in the hotter phase.
R _{I S E N ,2}
=
R_ISEN ⋅
∆_-----T----2-
∆T₁
(EQ. 24)
In Equation 24, make sure that ∆T₂ is the desired
temperature rise above the ambient temperature, and ∆T₁ is
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_ISEN two or more times to achieve
optimal thermal balance between all channels.
Load Line Regulation Component Selection (DCR
Current Sensing)
For accurate load line regulation, the ISL6308 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_COMP and C_COMP must be
chosen so that the time constant of this R_COMP-C_COMP
network matches the time constant of the inductor L/DCR.
Then the resistor R_S must be chosen to set the current
sense network gain, obtaining the desired full load droop
voltage. Follow the steps below to choose the component
values for this R-C network.
1. Choose an arbitrary value for C_COMP. The recommended
value is 0.01µF.
2. Plug the inductor L and DCR component values, and the
values for C_COMP chosen in steps 1, into Equation 25 to
calculate the value for R_COMP.
R_COMP
=
-------------------^L--------------------
DCR ⋅ C_COMP
(EQ. 25)
3. Use the new value for R_COMP obtained from Equation 25,
as well as the desired full load current, I_FL, full load droop
voltage, V_DROOP, and inductor DCR in Equation 26 to
calculate the value for R_S.
R_S
=
---------^I--^F---^L----------
V_DROOP
⋅
R_C
OM
P
⋅
DC
R
(EQ. 26)
Due to errors in the inductance or DCR it may be necessary
to adjust the value of R_COMP 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 18. 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 ∆V1 and ∆V2 as shown in Figure 18.
3. Select a new value, R_COMP,2, for the time constant
resistor based on the original value, R_COMP,1, using the
following equation.
R_{COMP, 2}
=
R_C
O
M P,
1
⋅
∆_----V_----1--
∆V₂
(EQ. 27)
4. Replace R_COMP with the new value and check to see that
the error is corrected. Repeat the procedure if necessary.
After choosing a new value for R_COMP, it will most likely be
necessary to adjust the value of R_S to obtain the desired full
load droop voltage. Use Equation 26 to obtain the new value
for R_S.
∆V₂
∆V₁
V_OUT
I_TRAN
∆I
FIGURE 18. TIME CONSTANT MISMATCH BEHAVIOR
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, R₂ and C₁.
20
FN9208.2
October 19, 2005