Rainbow Electronics MAX1545 User Manual
Page 32
MAX1519/MAX1545
Dual-Phase, Quick-PWM Controllers for
Programmable CPU Core Power Supplies
32
______________________________________________________________________________________
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
):
Transient Response
The inductor ripple current impacts transient-response
performance, especially at low V
IN
- V
OUT
differentials.
Low inductor values allow the inductor current to slew
faster, replenishing charge removed from the output filter
capacitors by a sudden load step. The amount of output
sag is also a function of the maximum duty factor, which
can be calculated from the on-time and minimum off-
time. For a dual-phase controller, the worst-case output
sag voltage can be determined by:
where t
OFF(MIN)
is the minimum off-time (see the
Electrical Characteristics) and K is from Table 6.
The amount of overshoot due to stored inductor energy
can be calculated as:
where
η
TOTAL
is the total number of active phases.
Setting the Current Limit
The minimum current-limit threshold must be high
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The valley
of the inductor current occurs at I
LOAD(MAX)
minus half
the ripple current; therefore:
where
η
TOTAL
is the total number of active phases, and
I
LIMIT(LOW)
equals the minimum current-limit threshold
voltage divided by the current-sense resistor (R
SENSE
).
For the 30mV default setting, the minimum current-limit
threshold is 28mV.
Connect ILIM to V
CC
for the default current-limit thresh-
old (see the Electrical Characteristics). In adjustable
mode, the current-limit threshold is precisely 1/20 the
voltage seen at ILIM. For an adjustable threshold, con-
nect a resistive divider from REF to GND with ILIM con-
nected to the center tap. When adjusting the current
limit, use 1% tolerance resistors with approximately 10µA
of divider current to prevent a significant increase of
errors in the current-limit tolerance.
Output Capacitor Selection
The output filter capacitor must have low enough effec-
tive 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 output
capacitor’s size 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:
In non-CPU applications, the output capacitor’s size
often depends on how much ESR is needed to maintain
an acceptable level of output ripple voltage. The output
ripple voltage of a step-down controller equals the total
inductor ripple current multiplied by the output capaci-
tor’s ESR. When operating multiphase systems out-of-
R
V
I
ESR
STEP
LOAD MAX
≤
∆
(
)
I
I
LIR
LIMIT LOW
LOAD MAX
TOTAL
(
)
(
)
>
−
η
1
2
V
I
L
C
V
SOAR
LOAD MAX
TOTAL
OUT OUT
≈
(
)
(
)
∆
2
2
η
V
L
I
V
K
V
t
C
V
V
V
K
V
t
I
C
V
K
V
t
SAG
LOAD MAX
OUT
IN
OFF MIN
OUT OUT
IN
OUT
IN
OFF MIN
LOAD MAX
OUT
OUT
IN
OFF MIN
=
−
−
+
+
+
(
)
(
)
(
)
(
)
(
)
(
)
(
)
∆
∆
2
2
2
2
2
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
(
)