MAX15569 Datasheet, PDF (34/41 Pages) Maxim Integrated Products – 2-Phase/1-Phase QuickTune-PWM Controller with Serial I2C Interface

MAX15569
2-Phase/1-Phase QuickTune-PWM Controller with
Serial I²C Interface
Multiphase QuickTune-PWM Design
Procedure
Firmly establish the input voltage range and maximum
load current before choosing a switching frequency
and inductor operating point (ripple-current ratio). The
primary design trade-off lies in choosing a good switching
frequency and inductor operating point, and the following
four factors dictate the rest of the design:
1) Input Voltage Range: The maximum value (V_IN(MAX))
must accommodate the worst-case high AC adapt-
er voltage. The minimum value (V_IN(MIN)) must
account for the lowest input voltage after drops due to
connectors, fuses, and battery selector switches. If
there is a choice at all, lower input voltages result in
better efficiency.
2) Maximum Load Current: There are two values to
consider. The peak load current (I_LOAD(MAX)) deter-
mines the instantaneous component stresses and
filtering requirements, and drives output-capacitor
selection, inductor saturation rating, and the design of
the current-limit circuit. The continuous-load current
(I_LOAD) determines the thermal stresses and drives
the selection of the input capacitors, MOSFETs, and
other critical heat-contributing components. Modern
notebook CPUs generally exhibit I_LOAD = 0.8 x
I_LOAD(MAX). For multiphase systems, each phase
supports a fraction of the load, depending on the
current balancing. When properly balanced, the load
current is evenly distributed among phases:
I_LOAD(PHASE)
=
I_LOAD
N_PH
where N_PH is the total number of active phases.
3) Switching Frequency: This choice determines the
basic trade-off between size and efficiency. The
optimal frequency is largely a function of maximum
input voltage due to MOSFET switching losses
that are proportional to frequency and V_IN². The
optimum frequency is also a moving target due to rapid
improvements in MOSFET technology that are making
higher frequencies more practical.
4) Inductor Operation Point: This choice provides trade-
offs between size vs. efficiency and transient respons-
es vs. output noise. Low inductor values provide better
transient response and smaller physical size, but also
result in lower efficiency and higher output noise due
to increased ripple current. The minimum practical
inductor value is one that causes the circuit to operate
at the edge of critical conduction (where the inductor
current just touches zero with every cycle at maximum
load). Inductor values lower than this grant no further
size-reduction benefit. The optimum operating point is
usually between 30% and 50% ripple current. For a
multiphase core regulator, select an LIR value of ~0.4.
Inductor Selection
The switching frequency and operating point (% ripple
current or LIR) determine the inductor value as follows:
L
=

N_PH
fSW
V_IN - V_OUT
× ILOAD(MAX)
×
LIR


V_OUT
VIN



where N_PH is the total number of phases. Find a low-loss
inductor having the lowest possible DC resistance that fits
in the allotted dimensions. The core must not saturate at
the peak-inductor current (I_PEAK):
= I_PEAK


I_LOAD(MAX)
N_PH
₁
+
LIR
2


Output Capacitor Selection
Output capacitor selection is determined by the controller
stability and the transient soar and sag requirements of
the application.
Output Capacitor ESR
The output filter capacitor must have low enough
effective series resistance (ESR) to meet output-ripple
and load-transient requirements, yet have high enough
ESR to satisfy stability requirements. In CPU V_CORE
converters and other applications where the output is
subject to large-load transients, the size of the output
capacitor typically depends on how much ESR is needed
to prevent the output from dipping too low under a load
transient. Ignoring the sag due to finite capacitance:
(R _ESR
+
R_PCB )
≤
V_STEP
DI_LOAD(MAX)
The output-voltage ripple of a step-down controller equals
the total inductor ripple current multiplied by the output
capacitor’s ESR. When operating multiphase out-of-
phase systems, the peak inductor currents of each phase
are staggered, resulting in lower output ripple voltage by
reducing the total inductor ripple current. For multiphase
operation, the maximum ESR to meet ripple requirements
is given in the following equation:
R _ESR
≤

_(VIN
−
V_IN × f_SW × L
(N_PH × V_OUT ))V_OUT

 ^VRIPPLE

where NPH is the total number of active phases and f_SW
is the switching frequency per phase.
www.maximintegrated.com
Maxim Integrated │  34