Datasheet

MAX1970/MAX1971/MAX1972
Design Procedure
Output Voltage Selection
Both output voltages can be selected in three different
ways as indicated by Table 1. Each output has two pre-
set voltages that can be set using FBSEL_ and it can
also be set to any voltage from 0.8V to V
IN
by using an
external resistor voltage-divider.
To use a resistor-divider to set the output voltage to
1.2V or higher (Figure 5), connect a resistor from FB_ to
OUT_ (R_
a
), and connect a resistor from FB_ to GND
(R_
b
). Select the value of R_
b
, between 10kΩ and
30kΩ. Then R_
a
is calculated by:
A resistor-divider can also be used to set the voltage of
one output from 0.8V to 1.2V. To do this, the other out-
put must be above 1.2V. Figure 6 shows an example of
this where OUT1 is set to 1V. To set the output voltage to
less than 1.2V, connect a resistor from FB1 to OUT1 (R1),
and from FB1 to OUT2 (R2). Select values of R1 and R2
such that current flowing through R1 and R2 is about
100µA and following equation is satisfied:
Each output is capable of continuously sourcing up to
750mA of current as long as the following condition is
met:
Inductor Value
A 3.3µH to 6.8µH inductor with a saturation current of
800mA (min) is recommended for most applications.
For best efficiency, the inductor’s DC resistance should
be less than 100mΩ, and saturation current should be
greater than 1A. See Table 2 for recommended induc-
tors and manufacturers.
VIVI
V
A
OUT OUT OUT OUT
IN
1122
105
×+ ×
≤ .
RR
V
V
OUT
OUT
12
12
12
1
2
.
.
=
−
−
RR
V
ab
OUT
__
.
=×
⎡
⎣
⎢
⎤
⎦
⎥
−
12
1
Dual, 180° Out-of-Phase, 1.4MHz, 750mA Step-
Down Regulator with POR and RSI/PFO
14 ______________________________________________________________________________________
Figure 2. Timing Diagram
V
OUT
T
D
RSI
~1V PKMAX
V
IN
PFO
4.04V
3.94V
T
RESET
= 1µs MIN
T
D
POR