Datasheet

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
PEAK
).
Example:
V
IN
= 3.3V
V
OUT
= 1.8V
f
OSC
= 500kHz
I
OUT(MAX)
= 6A
LIR = 30%
L = 1µH and I
PEAK
= 6.9A
Output Capacitor Selection
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:
V
RIPPLE
= V
RIPPLE(C)
+ V
RIPPLE(ESR)
+ V
RIPPLE(ESL)
where the output ripple due to output capacitance,
ESR, and ESL are:
V
RIPPLE(C)
= I
P-P
/(8 ✕ C
OUT
✕ f
SW
), V
RIPPLE(ESR)
= I
P-P
✕
ESR
V
RIPPLE(ESL)
= (I
P-P
/t
ON
) ✕ ESL or (I
P-P
/t
OFF
) ✕ ESL,
whichever is greater
The peak inductor current (I
P-P
) is:
I
P-P
= ((V
IN
- V
OUT
)/(f
SW
✕ L )) ✕ (V
OUT
/V
IN
)
Example:
V
IN
= 3.3V
V
OUT
= 1.8V
f
OSC
= 500kHz
I
OUT(MAX)
= 6A
LIR = 30%
L = 1µH
C
OUT
= 180µF
ESR
(OUTPUT CAPACITOR)
= 30mΩ
ESL
(OUTPUT CAPACITOR)
= 2.5nH
V
RIPPLE(C)
= 2mV
V
RIPPLE(ESR)
= 45mV
V
RIPPLE(ESL)
= 4mV
V
RIPPLE
= 51mV
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
LOAD
. Before the controller can respond, the output
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.
Input Capacitor Selection
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
RIPPLE
is the input RMS ripple current.
II
VVV
V
RIPPLE LOAD
OUT IN OUT
IN
=×
×
()








−
I
LIR
I
PEAK OUT MAX
=+






1
2
()
MAX1945R/MAX1945S
1MHz, 1% Accurate, 6A Internal Switch
Step-Down Regulators
______________________________________________________________________________________ 13