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ISL6336B Datasheet(PDF) 27 Page - Renesas Technology Corp

Part No. ISL6336B
Description  6-Phase PWM Controller with Light Load Efficiency Enhancement and Current Monitoring
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Manufacturer  RENESAS [Renesas Technology Corp]
Direct Link  http://www.renesas.com
Logo RENESAS - Renesas Technology Corp

ISL6336B Datasheet(HTML) 27 Page - Renesas Technology Corp

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ISL6336B
FN6696 Rev 3.00
Page 27 of 32
May 5, 2016
The feedback resistor, RFB, has already been chosen as
outlined in “Load-Line Regulation Resistor” on page 26. Select
a target bandwidth for the compensated system, f0. The target
bandwidth must be large enough to ensure adequate transient
performance, but generally smaller than 1/3 of the per-channel
switching frequency. The values of the compensation
components depend on the relationships of f0 to the L-C pole
frequency and the ESR zero frequency. For each of the three
cases which follow, there are a separate set of equations for
the compensation components.
In Equation 37, L is the per-channel filter inductance divided by
the number of active channels; C is the sum total of all output
capacitors; ESR is the equivalent-series resistance of the bulk
output-filter capacitance; and VP-P is the peak-to-peak
sawtooth signal amplitude as described in the “Electrical
Specifications” table beginning on page 7.
The optional capacitor C2, is sometimes needed to bypass
noise away from the PWM comparator (see Figure 19). Keep a
position available for C2, and be prepared to install a high
frequency capacitor between 22pF and 150pF in case
excessive jitter is noted.
Once selected, the compensation values in Equation 37
ensure a stable converter with reasonable transient
performance. In most cases, transient performance can be
improved by making adjustments to RC. Slowly increase the
value of RC while observing the transient performance on an
oscilloscope until no further improvement is noted. Normally,
CC will not need adjustment. Keep the value of CC from
Equation 37 unless some performance issue is noted.
C1 and R1 can also be added to improve transient
performance per the type III compensation discussion in the
following.
COMPENSATION WITHOUT LOAD-LINE REGULATION
The non load-line regulated converter is accurately modeled as
a voltage-mode regulator with two poles at the L-C resonant
frequency and a zero at the ESR frequency. A type III
controller, as shown in Figure 20, provides the necessary
compensation.
The first step is to choose the desired bandwidth, f0, of the
compensated system. Choose a frequency high enough to
ensure adequate transient performance but generally not
higher than 1/3 of the switching frequency. The type-III
compensator has an extra high-frequency pole, fHF. This pole
can be used for added noise rejection or to ensure adequate
attenuation at the error-amplifier high-order pole and zero
frequencies. A good general rule is to choose fHF = 10f0, but it
can be higher if desired. Choosing fHF to be lower than 10f0
can cause problems with too much phase shift below the
system bandwidth.
In the solutions to the compensation equations, there is a
single degree of freedom. For the solutions presented in
Equation 38, RFB is selected arbitrarily. The remaining
FIGURE 19. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6336B CIRCUIT
ISL6336B
COMP
CC
RC
RFB
FB
VDIFF
-
+
VDROOP
C2 (OPTIONAL)
1
2
 LC
------------------- f0
>
RC
RFB
2
f
0VP-P LC
0.75VIN
--------------------------------------
=
CC
0.75VIN
2
V
P-PRFBf0
-------------------------------------
=
Case 1:
1
2
 LC
------------------- f0
1
2
C ESR

------------------------------
<
RC
RFB
VP-P 2
2 f
0
2 LC
0.75 VIN
----------------------------------------------
=
CC
0.75VIN
2
2 f
0
2 V
P-PRFB LC
--------------------------------------------------------------
=
Case 2:
(EQ. 37)
f0
1
2
C ESR

------------------------------
>
RC
RFB
2
 f
0VP-PL
0.75 VIN ESR

------------------------------------------
=
CC
0.75VIN ESR
 C
2
V
P-PRFBf0 L
-------------------------------------------------
=
Case 3:
FIGURE 20. COMPENSATION CIRCUIT FOR ISL6336B BASED
CONVERTER WITHOUT LOAD-LINE
REGULATION
ISL6336B
COMP
CC
RC
RFB
FB
VDIFF
C2
C1
R1


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