ISL6568_06 Datasheet, PDF (23/29 Pages) Intersil Corporation – Two-Phase Buck PWM Controller with Integrated MOSFET Drivers for VRM9, VRM10, and AMD Hammer Applications

ISL6568
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 26 to
calculate the value for R_COMP.
R_COMP
=
-------------------^L--------------------
DCR ⋅ C_COMP
(EQ. 26)
3. Use the new value for R_COMP obtained from Equation
26, as well as the desired full load current, I_FL, full load
droop voltage, V_DROOP, and inductor DCR in Equation
27 to calculate the value for R_S.
R_S
=
---------^I--^F---^L----------
V_DROOP
⋅
R_C
OM
P
⋅
DC
R
(EQ. 27)
PHASE1
PHASE2
ISUM
V_L(s)
L
DCR
INDUCTOR
IL₁
R_S
L
DCR
INDUCTOR
IL₂
R_S
I_OUT
V_OUT
C_OUT
ICOMP
-
V_DROOP
+ ^IREF
C_COMP R_COMP
(optional)
ISL6568
FIGURE 18. DCR SENSING CONFIGURATION
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 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 ∆V1 and ∆V2 as shown in Figure 19.
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. 28)
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 27 to obtain the new value
for R_S.
∆V₂
∆V₁
V_OUT
I_TRAN
∆I
FIGURE 19. TIME CONSTANT MISMATCH BEHAVIOR
Compensation
The two opposing goals of compensating the voltage
regulator are stability and speed.
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_C and C_C.
23
FN9187.4
March 9, 2006