Multiphase, fixed-frequency design procedure – Rainbow Electronics MAX8707 User Manual
Page 30
MAX8707
Multiphase, Fixed-Frequency Controller for
AMD Hammer CPU Core Power Supplies
30
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thermal sensor sets the fault latch and activates the
soft-shutdown sequence. Once the controller ramps
down to the 0V setting, it forces the PWM_ driver out-
puts low. Toggle SHDN or cycle the V
CC
power supply
below 1V to clear the fault latch and reactivate the con-
troller after the junction temperature cools by 15
°C.
Thermal shutdown can be disabled through the no-fault
test mode (see the No-Fault Test Mode section).
No-Fault Test Mode
The latched fault protection features can complicate
the process of debugging prototype breadboards
since there are (at most) a few milliseconds in which to
determine what went wrong. Therefore, a no-fault test
mode is provided to disable the fault protection—over-
voltage protection, undervoltage protection, and ther-
mal shutdown. Additionally, the test mode clears the
fault latch if it has been set. The no-fault test mode is
entered by forcing 11V to 13V on SHDN.
Multiphase, Fixed-Frequency
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 voltages result in better efficiency.
Maximum Load Current: There are two values to con-
sider. The peak load current (I
LOAD(MAX)
) determines
the instantaneous component stresses and filtering
requirements, and thus drives output capacitor selec-
tion, inductor saturation rating, and the design of the
current-limit circuit. The continuous load current (I
LOAD
)
determines the thermal stresses and thus drives the
selection of input capacitors, MOSFETs, and other criti-
cal heat-contributing components. Modern notebook
CPUs generally exhibit I
LOAD
= I
LOAD(MAX)
x 80%.
For multiphase systems, each phase supports a frac-
tion of the load, depending on the current balancing.
When properly balanced, the load current is evenly dis-
tributed among each phase:
where
η
PH
is the total number of active phases.
Switching Frequency: This choice determines the
basic trade-off between size and efficiency. The opti-
mal frequency is largely a function of maximum input
voltage, due to MOSFET switching losses that are pro-
portional to frequency and V
IN
2
. The optimum frequen-
cy is also a moving target, due to rapid improvements
in MOSFET technology that are making higher frequen-
cies more practical.
Inductor Operating Point: This choice provides trade-
offs between size vs. efficiency and transient response
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 induc-
tor 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 usu-
ally 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
PH
is the total number of phases, and f
SW
is the
switching frequency per phase.
Find a low-loss inductor with the lowest possible DC
resistance that fits in the allotted dimensions. If using a
swinging inductor (where the no-load inductance
decreases linearly with increasing current), evaluate
the LIR with properly scaled inductance values. For the
selected inductance value, the actual peak-to-peak
inductor ripple current (
∆I
INDUCTOR
) is defined by:
Ferrite cores are often the best choice, although pow-
dered 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
I
PEAK
LOAD MAX
PH
INDUCTOR
=
+
(
)
η
∆
2
∆I
V
V
V
V
f
L
INDUCTOR
OUT
IN
OUT
IN SW
=
−
(
)
L
V
V
f
I
LIR
V
V
PH
IN
OUT
SW LOAD MAX
OUT
IN
=
−
η
(
)
I
I
LOAD PHASE
LOAD
PH
(
)
=
η