Rainbow Electronics MAX17031 User Manual
Page 20
MAX17031
Inductor Operating Point: This choice provides
trade-offs between size vs. efficiency and transient
response vs. output ripple. Low inductor values pro-
vide better transient response and smaller physical
size, but also result in lower efficiency and higher
output ripple due to increased ripple currents. The
minimum practical inductor value is one that causes
the circuit to operate at the edge of critical conduc-
tion (where the inductor current just touches zero
with every cycle at maximum load). Inductor values
lower than this grant no further size-reduction bene-
fit. The optimum operating point is usually found
between 20% and 50% value at which PFM/PWM
switchover occurs.
Inductor Selection
The switching frequency and inductor operating point
determine the inductor value as follows:
For example: I
LOAD(MAX)
= 4A, V
IN
= 12V, V
OUT2
=
2.5V, f
SW
= 355kHz, 30% ripple current or LIR = 0.3:
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
):
Most inductor manufacturers provide inductors in stan-
dard values, such as 1.0µH, 1.5µH, 2.2µH, 3.3µH, etc.
Also look for nonstandard values, which can provide a
better compromise in LIR across the input voltage
range. If using a swinging inductor (where the no-load
inductance decreases linearly with increasing current),
evaluate the LIR with properly scaled inductance values.
Transient Response
The inductor ripple current also impacts transient-
response performance, especially at low V
IN
- V
OUT
dif-
ferentials. 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 maxi-
mum duty factor, which can be calculated from the on-
time and minimum off-time:
where t
OFF(MIN)
is the minimum off-time (see the
Electrical Characteristics
table).
The amount of overshoot during a full-load to no-load tran-
sient due to stored inductor energy can be calculated as:
Setting the Current Limit
The minimum current-limit threshold must be great
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The val-
ley of the inductor current occurs at I
LOAD(MAX)
minus
half the ripple current; therefore:
where I
LIM(VAL)
equals the minimum valley current-limit
threshold voltage divided by the current-sense resis-
tance (R
SENSE
). When using a 100k
Ω ILIM resistor, the
minimum valley current-limit threshold is 40mV.
Connect a resistor between ILIM_ and analog ground to
set the adjustable current-limit threshold. The valley
current-limit threshold is approximately 1/10 the ILIM
voltage formed by the external resistance and internal
5µA current source. The 40k
Ω to 400kΩ adjustment
range corresponds to a 20mV to 200mV valley current-
limit threshold. When adjusting the current limit, use 1%
tolerance resistors to prevent significant inaccuracy in
the valley current-limit tolerance.
Output Capacitor Selection
The output filter capacitor must have low enough equiv-
alent series resistance (ESR) to meet output ripple and
load-transient requirements, yet have high enough ESR
to satisfy stability requirements.
For processor core voltage converters and other appli-
cations where the output is subject to violent load tran-
sients, the output capacitor’s size 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:
I
I
I
LIR
LIM VAL
LOAD MAX
LOAD MAX
(
)
(
)
(
)
>
−
⎛
⎝⎜
⎞
⎠⎟
2
V
I
L
C
V
SOAR
LOAD MAX
OUT OUT
≈
(
)
∆
(
)
2
2
V
L
I
V
K
V
t
SAG
LOAD MAX
OUT
IN
OFF MIN
=
(
)
⎛
⎝⎜
⎞
⎠⎟
+
∆
(
)
(
2
))
(
⎡
⎣
⎢
⎤
⎦
⎥
−
(
)
⎛
⎝⎜
⎞
⎠⎟
−
2C
V
V
V
K
V
t
OUT OUT
IN
OUT
IN
OFF M
MIN)
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
I
I
LIR
PEAK
LOAD MAX
=
+
⎛
⎝⎜
⎞
⎠⎟
(
)
1
2
L
V
V
V
V
kHz
A
H
=
Ч
−
(
)
Ч
Ч
Ч
=
2 5
12
2 5
12
355
4
0 3
4 65
.
.
.
.
µ
L
V
V
V
V f
I
LIR
OUT
IN
OUT
IN SW LOAD MAX
=
−
(
)
(
)
Dual Quick-PWM Step-Down Controller with Low-
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20
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