Multiphase quick-pwm design procedure – Rainbow Electronics MAX8760 User Manual
Page 31
MAX8760
Dual-Phase, Quick-PWM Controller for AMD
Mobile Turion 64 CPU Core Power Supplies
______________________________________________________________________________________
31
Power-on reset (POR) occurs when V
CC
rises above
approximately 2V, resetting the fault latch, activating
boot mode, and preparing the PWM for operation. V
CC
undervoltage lockout (UVLO) circuitry inhibits switch-
ing, and forces the DL gate driver high (to enforce out-
put overvoltage protection). When V
CC
rises above
4.25V, the DAC inputs are sampled and the output volt-
age begins to slew to the target voltage.
For automatic startup, the battery voltage should be
present before V
CC
. If the quick-PWM controller
attempts to bring the output into regulation without the
battery voltage present, the fault latch trips. Toggle the
SHDN pin to reset the fault latch.
Input Undervoltage Lockout
During startup, the V
CC
UVLO circuitry forces the DL
gate driver high and the DH gate driver low, inhibiting
switching until an adequate supply voltage is reached.
Once V
CC
rises above 4.25V, valid transitions detected
at the trigger input initiate a corresponding on-time
pulse (see the On-Time One-Shot (TON) section). If the
V
CC
voltage drops below 4.25V, it is assumed that
there is not enough supply voltage to make valid deci-
sions. To protect the output from overvoltage faults, the
controller activates the shutdown sequence.
Multiphase Quick-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 switch-
ing frequency and inductor operating point, and the fol-
lowing four factors dictate the rest of the design:
•
Input voltage range: The maximum value
(V
IN(MAX)
) must accommodate the worst-case high
AC adapter 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 volt-
ages result in better efficiency.
•
Maximum load current: There are two values to
consider. The peak load current (I
LOAD(MAX)
) deter-
mines the instantaneous component stresses and fil-
tering requirements, and thus drives output capacitor
selection, inductor saturation rating, and the design
of the current-limit circuit. The continuous load cur-
rent (I
LOAD
) determines the thermal stresses and
thus drives the selection of input capacitors,
MOSFETs, and other critical heat-contributing com-
ponents. Modern notebook CPUs generally exhibit
I
LOAD
= I
LOAD(MAX)
x 80%.
For multiphase systems, each phase supports a
fraction of the load, depending on the current bal-
ancing. When properly balanced, the load current is
evenly distributed among each phase:
where
η
TOTAL
is the total number of active phases.
•
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
IN
2
. The opti-
mum frequency is also a moving target, due to rapid
improvements in MOSFET technology that are mak-
ing higher frequencies more practical.
•
Inductor operating point: This choice provides
trade-offs between size vs. efficiency and transient
response vs. output noise. Low inductor values pro-
vide better transient response and smaller physical
size, but also result in lower efficiency and higher out-
put noise due to increased ripple current. The mini-
mum 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 found between
20% and 50% ripple current.
Inductor Selection
The switching frequency and operating point (% ripple
current or LIR) determine the inductor value as follows:
where
η
TOTAL
is the total number of phases.
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, although powdered
iron is inexpensive and can work well at 200kHz. The
core must be large enough not to saturate at the peak
inductor current (I
PEAK
):
I
I
LIR
PEAK
LOAD MAX
TOTAL
=
+
(
)
η
1
2
L
V
V
f
I
LIR
V
V
TOTAL
IN
OUT
SW LOAD MAX
OUT
IN
(
)
=
−
η
I
I
LOAD PHASE
LOAD
TOTAL
(
)
=
η