ISL6569 Datasheet, PDF (19/22 Pages) Intersil Corporation – Multi-Phase PWM Controller

ISL6569
The first step is to choose the desired bandwidth, f₀, of the
compensated system. Choose a frequency high enough to
assure adequate transient performance but not higher than
1/3 of the switching frequency. The type-III compensator has
an extra high-frequency pole, f_HF. This pole can be used for
added noise rejection or to assure adequate attenuation at
the error-amplifier high-order pole and zero frequencies. A
good general rule is to chose f_HF = 10 f₀, but it can be higher
if desired. Choosing f_HF to be lower than 10 f₀ can cause
problems with too much phase shift below the system
bandwidth.
C₂
R_C ^CC
COMP
C₁
R₁
R_FB
FB
IOUT
VDIFF
FIGURE 14. COMPENSATION CIRCUIT FOR ISL6569 BASED
CONVERTER WITHOUT LOAD-LINE
REGULATION.
In the solutions to the compensation equations, there is a
single degree of freedom. For the solutions presented in
Equations 28, R_FB is selected arbitrarily. The remaining
compensation components are then selected according to
Equations 28.
R₁
=
R_FB
----------C-----⁽--E----S-----R-----⁾---------
LC – C(ESR)
C₁
=
-----L----C-----^–-----C-----⁽--E-----S----R-----⁾
R_FB
C₂
=
-----------------------0---.--7---5----V-----^I-^N-------------------------
(2π)²f₀f_HF LCR_FBV_PP
R_C
=
---V-----^P----^P----^--2----^π---^---²----f--⁰---f--^H----^F----L----C----R-----^F----^B----
0.75 V_{IN }2πf_HF LC–1_
(EQ. 28)
C_C
=
--0----.-7----5----V----^I--^N----^-2----^π----f--^H----^F--------L----C-----^–---1---^---
(2π)²f₀f_HF LCR_FBV_PP
In Equations 28, 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 V_PP is
the peak-to-peak sawtooth signal amplitude as described in
Figure 5 and Electrical Specifications.
Input Supply Voltage Selection
The VCC input of the ISL6569 can be connected to either a
+5V supply directly or through a current limiting resistor to a
+12V supply. An integrated 5.8V shunt regulator maintains
the voltage on the VCC pin when a +12V supply is used. A
300Ω resistor is suggested for limiting the current into the
VCC pin to approximately 20mA.
Switching Frequency
There are a number of variables to consider when choosing
the switching frequency, as there are considerable effects on
the upper-MOSFET loss calculation. These effects are
outlined in MOSFETs, and they establish the upper limit for
the switching frequency. The lower limit is established by the
requirement for fast transient response and small output-
voltage ripple as outlined in Input Supply Voltage Selection.
Choose the lowest switching frequency that allows the
regulator to meet the transient-response requirements.
Switching frequency is determined by the selection of the
frequency-setting resistor, RT (see the figure Typical
Application on page 4). Figure 15 and Equation 29 are
provided to assist in the selecting the correct value for R_T.
R_T = 10^{[11.09 – 1.13log (fS)]}
(EQ. 29)
Input Capacitor Selection
The input capacitors are responsible for sourcing the ac
component of the input current flowing into the upper
MOSFETs. Their RMS current capacity must be sufficient to
handle the ac component of the current drawn by the upper
MOSFETs which is related to duty cycle and the number of
active phases.
1000
100
10
10
100
1000
SWITCHING FREQUENCY (KHZ)
10000
FIGURE 15. R_T VS SWITCHING FREQUENCY
19
FN9085.7
December 29, 2004