ISL6569A Datasheet, PDF (19/22 Pages) Intersil Corporation – Multi-Phase PWM Controller

C₂
R_C ^CC
COMP
C₁
R₁
R_FB
FB
IOUT
VDIFF
ISL6569A
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.
1000
FIGURE 14. COMPENSATION CIRCUIT FOR ISL6569A 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
(EQ. 28)
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_
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 6 and Electrical Specifications.
Input Supply Voltage Selection
The VCC input of the ISL6569A 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.
100
10
10
100
1000
SWITCHING FREQUENCY (kHz)
10000
FIGURE 15. R_T vs SWITCHING FREQUENCY
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.
For a two phase design, use Figure 16 to determine the
input-capacitor RMS current requirement given the duty
cycle, maximum sustained output current (I_O), and the ratio
of the combined peak-to-peak inductor current (I_C,PP) to I_O.
Select a bulk capacitor with a ripple current rating which will
minimize the total number of input capacitors required to
support the RMS current calculated. The voltage rating of
the capacitors should also be at least 1.25 times greater
than the maximum input voltage.
19
FN9092.2
December 29, 2004