MAX1945SEUI Maxim Integrated Products, MAX1945SEUI Datasheet - Page 13

• Manufacturer Part Number: MAX1945SEUI
• Description: DC/DC Switching Regulators 1MHz 1% 6A Step-Down PWM
• Manufacturer: Maxim Integrated Products

Specifications of MAX1945SEUI

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Use a low-loss inductor with the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
core types are often the best choice for performance.
With any core material the core must be large enough
not to saturate at the peak inductor current (I
Example:
V
V
f
I
LIR = 30%
L = 1µH and I
The key selection parameters for the output capacitor
are capacitance, ESR, ESL, and voltage rating require-
ments. These affect the overall stability, output ripple
voltage, and transient response of the DC-DC convert-
er. The output ripple occurs because of variations in
the charge stored in the output capacitor, the voltage
drop due to the capacitor’s ESR, and the voltage drop
due to the capacitor’s ESL. Calculate the output voltage
ripple due to the output capacitance, ESR, and ESL as:
where the output ripple due to output capacitance,
ESR, and ESL are:
V
The peak inductor current (I
Example:
V
V
f
OSC
OUT(MAX)
OSC
IN
OUT
RIPPLE(C)
IN
OUT
V
V
RIPPLE
= 3.3V
RIPPLE(ESL)
= 3.3V
= 500kHz
= 500kHz
= 1.8V
= 1.8V
I
P-P
= V
= ((V
= 6A
= I
RIPPLE(C)
P-P
PEAK
I
PEAK
IN
= (I
/(8
whichever is greater
- V
______________________________________________________________________________________
P-P
= 6.9A
✕
=
1MHz, 1% Accurate, 6A Internal Switch
Output Capacitor Selection
OUT
C



/t
1
OUT
ON
+ V
+
)/(f
LIR
ESR
)
2
RIPPLE(ESR)
✕
SW
✕
P-P



ESL or (I
f
SW
I
) is:
✕
OUT MAX
L ))
), V
(
RIPPLE(ESR)
✕
P-P
(V
+ V
)
/t
OUT
OFF
RIPPLE(ESL)
PEAK
/V
)
✕
IN
= I
ESL,
)
).
P-P
✕
I
LIR = 30%
L = 1µH
C
ESR
ESL
V
V
V
V
Use these equations for initial capacitor selection.
Determine final values by testing a prototype or an
evaluation circuit. A smaller ripple current results in less
output voltage ripple. Because the inductor ripple cur-
rent is a factor of the inductor value, the output voltage
ripple decreases with a larger inductance. Use ceramic
capacitors for low ESR and low ESL at the switching
frequency of the converter. The low ESL of ceramic
capacitors makes ripple voltages negligible. Load tran-
sient response depends on the selected output. During
a load transient, the output instantly changes by ESR
I
deviates further, depending on the inductor and output
capacitor values. After a short time (see the Transient
Response graphs in the Typical Operating Character-
istics), the controller responds by regulating the output
voltage back to its predetermined value. The controller
response time depends on the closed-loop bandwidth.
A higher bandwidth yields a faster response time, pre-
venting the output from deviating further from its regu-
lating value.
The input capacitor reduces the current peaks drawn
from the input power supply and reduces switching noise
in the IC. The impedance of the input capacitor at the
switching frequency should be less than that of the input
source so that high-frequency switching currents do not
pass through the input source but instead are shunted
through the input capacitor. A high source impedance
requires larger input capacitance. The input capacitor
must meet the ripple current requirement imposed by the
switching currents. The RMS input ripple current is given
by:
where I
OUT(MAX)
LOAD
Step-Down Regulators
RIPPLE(C)
RIPPLE(ESR)
RIPPLE(ESL)
RIPPLE
OUT
(OUTPUT CAPACITOR)
(OUTPUT CAPACITOR)
I
RIPPLE
. Before the controller can respond, the output
= 180µF
RIPPLE
= 51mV
= 6A
= 2mV
=
= 4mV
= 45mV
is the input RMS ripple current.
I
LOAD
×
Input Capacitor Selection




= 2.5nH
V
= 30mΩ
OUT
×
(
V
V
IN
IN
−
V
OUT
)




13
✕