Max8720 – Rainbow Electronics MAX8720 User Manual
Page 24
MAX8720
value also determines the load-current 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)
= 15A, V
IN
= 12V, V
OUT
=
1.25V, f
SW
= 300kHz, 30% ripple current or LIR = 0.3
Find a low-loss inductor with 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 total output-voltage sag is the sum of the voltage
sag while the inductor is ramping up and the voltage
sag before the next pulse can occur.
where t
OFF(MIN)
is the minimum off-time (see the
Electrical Characteristics) and K is from Table 3.
The amount of overshoot during a full-load to no-load
transient due to stored inductor energy can be calculat-
ed 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
peak inductor current occurs at I
LOAD(MAX)
plus half
the ripple current; therefore:
where I
LIMIT(LOW)
equals the minimum current-limit
threshold voltage divided by the R
DS(ON)
of N
L
. For the
100mV default setting, the minimum current-limit
threshold is 90mV.
Connect ILIM to V
CC
for a default 100mV current-limit
threshold. For an adjustable threshold, connect a resis-
tor-divider from REF to GND, with ILIM connected to the
center tap. The external adjustment range of 0.5V to 2.0V
corresponds to a current-limit threshold of 50mV to
200mV. When adjusting the current limit, use 1% toler-
ance resistors and a 10µA 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 equiv-
alent series resistance (ESR) to meet output ripple and
load-transient requirements, yet have high enough ESR
to satisfy stability requirements. The output capaci-
tance must be high enough to absorb the inductor
energy while transitioning from full-load to no-load con-
ditions without tripping the overvoltage fault protection.
When using high-capacitance, low-ESR capacitors (see
the Output-Capacitor Stability Requirements section),
the filter capacitor’s ESR dominates the output voltage
ripple. Thus, the output capacitor’s size depends on
the maximum ESR required to meet the output-voltage-
ripple (V
RIPPLE(P-P)
) specifications:
In CPU V
CORE
converters and other applications where
the output is subject to violent load transients, the out-
put capacitor’s size typically depends on how much
ESR is needed to prevent the output from dipping too
V
R
I
LIR
RIPPLE P P
ESR ILOAD MAX
(
)
(
)
−
=
I
I
LIR
LIMIT LOW
LOAD MAX
(
)
(
)
>
−
1
2
V
I
L
C
V
SOAR
LOAD MAX
OUT
OUT
≈
(
)
(
)
∆
2
2
V
L
I
K
V
V
t
C
V
K
V
V
V
t
SAG
LOAD MAX
OUT
IN
OFF MIN
OUT
OUT
IN
OUT
IN
OFF MIN
=
+
−
−
(
)
(
)
(
)
(
)
∆
2
I
I
LIR
PEAK
LOAD MAX
=
+
(
)
1
2
L
V
V
V
V
kHz
A
H
=
Ч
−
Ч
Ч
Ч
=
1 25
12
1 25
12
300
15
0 3
0 83
.
(
.
)
.
.
µ
L
V
V
V
V
f
I
LIR
OUT
IN
OUT
IN SW LOAD MAX
=
−
(
)
(
)
Dynamically Adjustable 6-Bit VID
Step-Down Controller
24
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