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MAX16974 Datasheet(PDF) 14 Page - Maxim Integrated Products

Part No. MAX16974
Description  High-Voltage, 2.2MHz, 2A Automotive Step- Down Converter with Low Operating Current
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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MAX16974 Datasheet(HTML) 14 Page - Maxim Integrated Products

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High-Voltage, 2.2MHz, 2A Automotive Step-
Down Converter with Low Operating Current
14_ _ ______________________________________________________________________________________
Table 1 shows a comparison between small and large
inductor sizes.
The inductor value must be chosen so the maximum induc-
tor current does not reach the minimum current limit of
the device. The optimum operating point is usually found
between 10% and 30% ripple current. When pulse skip-
ping (light loads), the inductor value also determines the
load-current value at which PFM/PWM switchover occurs.
Find a low-loss inductor having the lowest possible
DC resistance that fits in the allotted dimensions. Most
inductor manufacturers provide inductors in standard
values, such as 1.0FH, 1.5FH, 2.2FH, 3.3FH, etc. Also
look for nonstandard values, which can provide a bet-
ter 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. For
the selected inductance value, the actual peak-to-peak
inductor ripple current (DIINDUCTOR) is defined by:
OUT SUP
OUT
INDUCTOR
SUP
SW
V
(V
V
)
I
V
f
L
=
×
×
where DIINDUCTOR is in A, L is in H, and fSW is in Hz.
Ferrite cores are often the best choices, although pow-
dered iron is inexpensive and can work well at 220kHz.
The core must be large enough not to saturate at the
peak inductor current (IPEAK):
INDUCTOR
PEAK
LOAD(MAX)
I
I
I
2
=
+
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor RMS current requirement (IRMS) is
defined by the following equation:
OUT SUP
OUT
RMS
LOAD(MAX)
SUP
V
(V
V
)
I
I
V
=
IRMS has a maximum value when the input voltage
equals twice the output voltage (VSUP = 2VOUT), so
IRMS(MAX) = ILOAD(MAX)/2.
Choose an input capacitor that exhibits less than +10NC
self-heating temperature rise at the RMS input current for
optimal long-term reliability.
The input-voltage ripple is comprised of DVQ (caused
by the capacitor discharge) and DVESR (caused by the
equivalent series resistance (ESR) of the capacitor). Use
low-ESR ceramic capacitors with high ripple-current
capability at the input. Assume the contribution from the
ESR and capacitor discharge equal to 50%. Calculate
the input capacitance and ESR required for a specified
input-voltage ripple using the following equations:
ESR
IN
L
OUT
V
ESR
I
I
2
=
+
where
SUP
OUT
OUT
L
SUP
SW
(V
V
) V
I
V
f
L
×
∆ =
×
×
and
OUT
OUT
IN
Q
SW
SUPSW
I
D(1 D)
V
C
and D
V
f
V
×
=
=
×
where IOUT is the maximum output current, and D is the
duty cycle.
Output Capacitor
The output filter capacitor must have low enough ESR to
meet output ripple and load-transient requirements, yet
have high enough ESR to satisfy stability requirements.
The output capacitance must be high enough to absorb
the inductor energy while transitioning from full-load
to no-load conditions without tripping the overvoltage
fault protection. When using high-capacitance, low-ESR
capacitors, the filter capacitor’s ESR dominates the
output voltage ripple. So the size of the output capaci-
tor depends on the maximum ESR required to meet the
output voltage ripple (VRIPPLE(P-P)) specifications:
RIPPLE(P P)
LOAD(MAX)
V
ESR I
LIR
=
×
×
The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as
to the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value.
Table_1._Inductor_Size_Comparison
INDUCTOR_SIZE
SMALLER
LARGER
Lower price
Smaller ripple
Smaller form factor
Higher efficiency
Faster load response
Larger fixed-frequency
range in skip mode


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