MAX17020ETJ+T Maxim Integrated Products, MAX17020ETJ+T Datasheet - Page 26

• Manufacturer Part Number: MAX17020ETJ+T
• Description: IC CTLR PWM DUAL STEP DN 32-TQFN
• Manufacturer: Maxim Integrated Products
• Series: Quick-PWM™r

Specifications of MAX17020ETJ+T

• Applications: Power Supplies
• Current - Supply: 1mA
• Voltage - Supply: 6 V ~ 24 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The switching frequency and inductor operating point
determine the inductor value as follows:
For example: I
2.5V, f
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
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.
The inductor ripple current also impacts transient-
response performance, especially at low V
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:
Dual Quick-PWM Step-Down Controller
with Low-Power LDO, RTC Regulator
26
V
SAG
lower than this grant no further size-reduction bene-
fit. The optimum operating point is usually found
between 20% and 50% ripple current. When pulse
skipping (SKIP low and light loads), the inductor
value also determines the load-current value at
which PFM/PWM switchover occurs.
______________________________________________________________________________________
SW
=
2
L
= 355kHz, 30% ripple current or LIR = 0.3:
×
=
L
C
L
×
12
I
PEAK
OUT
=
(
LOAD(MAX)
Δ
V
2 5
V
I
LOAD MAX
.
IN
×
PEAK
×
V
V
=
355
V
×
RIPPLE
I
OUT
×
LOAD MAX
f
SW
(
(
kHz
):
12
⎡
⎢
⎢
⎣
×
V
⎛
⎜
⎝
(
)
×
I
×
(
)
LOAD MAX
= 4A, V
−
2
V
(
4
IN
2 5
V
×
Transient Response
A
IN
.
)
⎡
⎢
⎣
−
Inductor Selection
×
V
×
⎛
⎝ ⎜
(
−
V
V
V
)
0 3
⎛
⎝ ⎜
OUT
IN
OUT
V
.
1
V
OUT
IN
IN
+
)
=
LIR
)
K
×
= 12V, V
2
×
4 65
)
⎞
⎠ ⎟
LIR
.
K
+
⎞
⎠ ⎟
⎞
⎟ −
⎠
IN
t
μH
OFF M
- V
t t
OFF MIN
(
OUT
OUT2
I I N
(
)
⎤
⎥
⎦
dif-
)
⎤
⎥
⎥
⎦
=
where t
Electrical Characteristics table) and K is from Table 3.
The amount of overshoot during a full-load to no-load tran-
sient due to stored inductor energy can be calculated as:
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
half the ripple current; therefore:
where I
threshold voltage divided by the current-sense resis-
tance (R
minimum valley current-limit threshold is 40mV.
Connect a resistor between ILIM_ and analog ground
(AGND) 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 inter-
nal 5μA current source. The 40kΩ 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.
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:
In applications without large and fast load transients,
the output capacitor’s size often depends on how much
ESR is needed to maintain an acceptable level of out-
put voltage ripple. The output ripple voltage of a step-
down controller equals the total inductor ripple current
I
LIM VAL
LIM(VAL)
OFF(MIN)
SENSE
(
V
)
). When using a 100kΩ ILIM resistor, the
SOAR
>
equals the minimum valley current-limit
I
LOAD MAX
R
is the minimum off-time (see the
ESR
Output Capacitor Selection
≈
(
(
≤
Setting the Current Limit
Δ
2
I
Δ
LOAD MAX
×
I
LOAD MAX
C
)
V
−
OUT
STEP
⎛
⎜
⎝
(
I
LOAD MAX
(
×
V
)
)
OUT
2
(
)
LOAD(MAX)
×
2
L
)
×
LIR
⎞
⎟
⎠
minus