MAX1956ETI Maxim Integrated Products, MAX1956ETI Datasheet - Page 14

• Manufacturer Part Number: MAX1956ETI
• Description: DC/DC Switching Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1956ETI

• Number Of Outputs: 2
• Output Voltage: 0.8 V to 4.95 V
• Input Voltage: 1.6 V to 5.5 V
• Package / Case: TQFN EP-28
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

1.6V to 5.5V Input, 0.5% Accurate, Dual
180° Out-of-Phase Step-Down Controllers
Output voltage is set with a resistor-divider, as shown in
Figure 4. The output voltage can be set to as low as
0.8V. The maximum output voltage is limited by maxi-
mum duty cycle and external component selection.
Select R
and 10kΩ, and calculate R
Three key inductor parameters must be specified for
operation with the MAX1955/MAX1956: inductance
value (L), peak inductor current (I
tance (R
efficiency is to set the inductor peak-to-peak ripple cur-
rent equal to 30% of maximum load current, thus LIR =
0.3. The switching frequency, input voltage, output
voltage, and selected LIR determine the inductor value
as follows:
where f
The exact inductor value is not critical and can be
adjusted in order to make trade-offs among size, cost,
and efficiency. Lower inductor values minimize size and
cost, and also improve transient response, but reduce
efficiency and increase output voltage ripple because of
higher peak currents. Higher inductance increases effi-
ciency by reducing the RMS current. However, resistive
losses because of extra wire turns could exceed the
benefit gained from lower AC current levels, especially
when the inductance is increased without also allowing
larger inductor dimensions.
Find a low-loss inductor with the lowest possible DC
resistance that fits in the allotted dimensions. The
inductor’s saturation current rating must exceed the
peak inductor current at the maximum defined load
current (I
14
______________________________________________________________________________________
SW
I
X
PEAK
DC
LOAD(MAX)
(the resistor from FB to GND) between 8kΩ
is the switching frequency (typically 600kHz).
). A good compromise between size and
L
=
=
R
I
V
OUT MAX
Y
IN
=
):
Setting the Output Voltage
×
(
R
V
f
SW
OUT IN
X
×
)
Y
×
Design Procedure



+
(
from:
V
I
V
OUT MAX



OUT
. 0 8
LIR
Inductor Selection
2
-
V
(
OUT

 ×

PEAK
-
1
I
)
OUT MAX



)
), and DC resis-
×
LIR
(
)
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 must meet the ripple current
requirement (I
defined by the following equation:
The key selection parameters for the output capacitor
are the actual capacitance value, the ESR, the ESL,
and the voltage-rating requirements, which affect the
overall stability, output ripple voltage, and transient
response.
The output ripple has three components: variations in
the charge stored in the output capacitor, the voltage
drop across the capacitor’s ESR, and the voltage drop
across the capacitor’s ESL caused by the current into
and out of the capacitor:
The output voltage ripple from the ESR is:
The output voltage ripple because of the output capaci-
tance is:
Figure 4. Feedback Divider Network and Compensation
Circuitry
I
RMS
V
RIPPLE
=
V
V
OUT_
1
IN
=
R
R
Y
X
(
×
V
I
OUT
RMS
RIPPLE ESR
V
V
V
RIPPLE C
RIPPLE(ESR)
OUT
1
) imposed by the switching currents
)
2
2
Output Capacitor Selection
(
×
( )
×
Input Capacitor Selection
FB_
V
(
OUT
V
)
=
IN
+
8
V
-
1
= I
RIPPLE C
V
×
COMP_
×
OUT
C
P-P
(
V
OUT
IN
I
P P
2
✕
-
( )
)
-
ESR
V
×
C
OUT
+
F
R
f
SW
C
V
RIPPLE ESL
1
)
+
(
I
C
OUT
c
(
2
)
)
2