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CS5304 Datasheet(PDF) 12 Page - ON Semiconductor

Part No. CS5304
Description  Three-Phase Buck Controller
Download  19 Pages
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

CS5304 Datasheet(HTML) 12 Page - ON Semiconductor

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CS5304
http://onsemi.com
12
APPLICATIONS INFORMATION
FIXED FREQUENCY MULTI−PHASE CONTROL
In a multi−phase converter, multiple converters are
connected in parallel and are switched on at different times.
This reduces output current from the individual converters
and increases the apparent ripple frequency. Because several
converters are connected in parallel, output current can ramp
up or down faster than a single converter (with the same
value output inductor) and heat is spread among multiple
components.
The CS5304 uses a three−phase, fixed frequency,
Enhanced V2 architecture. Each phase is delayed 120
° from
the previous phase. Normally GATE(H) transitions high at
the beginning of each oscillator cycle. Inductor current
ramps up until the combination of the current sense signal
and the output ripple trip the PWM comparator and bring
GATE(H) low. Once GATE(H) goes low, it will remain low
until the beginning of the next oscillator cycle. While
GATE(H) is high, the enhanced V2 loop will respond to line
and load transients. Once GATE(H) is low, the loop will not
respond again until the beginning of the next cycle.
Therefore, constant frequency Enhanced V2 will typically
respond within 1/3 of the off−time for a three−phase
converter.
The Enhanced V2 architecture measures and adjusts
current in each phase. An additional input (CSx) for inductor
current information has been added to the V2 loop for each
phase as shown in Figure 9.
Figure 9. Enhanced V2 Feedback and Current
Sense Scheme
CSREF
VOUT
SWNODE
CSX
VFB
L
RL
RS
COMP
DACOUT
+
+
+
+
E.A.
+
+
+
+
OFFSET
CSA
PWM-
COMP
The inductor current is measured across RS, amplified by
CSA and summed with the OFFSET and Output Voltage at
the non−inverting input of the PWM comparator. The
inductor current provides the PWM ramp and as inductor
current increases the voltage on the positive pin of the PWM
comparator rises and terminates the PWM cycle. If the
inductor starts the cycle with a higher current, the PWM
cycle will terminate earlier providing negative feedback.
The CS5304 provides a CSx input for each phase, but the
CSREF, VFB and COMP inputs are common to all phases.
Current sharing is accomplished by referencing all phases to
the same VFB and COMP pins, so that a phase with a larger
current signal will turn off earlier than phases with a smaller
current signal.
Including both current and voltage information in the
feedback signal allows the open loop output impedance of
the power stage to be controlled. If the COMP pin is held
steady and the inductor current changes, there must also be
a change in the output voltage. Or, in a closed loop
configuration when the output current changes, the COMP
pin must move to keep the same output voltage. The required
change in the output voltage or COMP pin depends on the
scaling of the current feedback signal and is calculated as
DV + RS
CSA Gain
DI
The single−phase power stage output impedance is:
Single Stage Impedance + DV DI + RS
CSA Gain.
The multi−phase power stage output impedance is the
single−phase output impedance divided by the number of
phases. The output impedance of the power stage determines
how the converter will respond during the first few
ms of a
transient before the feedback loop has repositioned the
COMP pin.
The peak output current of each phase can also be
calculated from;
Ipkout (per phase) +
VCOMP * VFB * VOFFSET
RS
CSA Gain
Figure 10 shows the step response of a single phase with the
COMP pin at a fixed level. Before T1 the converter is in
normal steady state operation. The inductor current provides
the PWM ramp through the Current Share Amplifier. The
PWM cycle ends when the sum of the current signal, voltage
signal and OFFSET exceed the level of the COMP pin. At T1
the output current increases and the output voltage sags. The
next PWM cycle begins and the cycle continues longer than
previously while the current signal increases enough to make
up for the lower voltage at the VFB pin and the cycle ends at
T2. After T2 the output voltage remains lower than at light
load and the current signal level is raised so that the sum of
the current and voltage signal is the same as with the original
load. In a closed loop system the COMP pin would move
higher to restore the output voltage to the original level.


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