Datasheet

f
SW
R
ADJ
=
54680
- 13.15
R
EN1
=
- 1
V
TO
1.35
x R
EN2
R
FB1
=
- 1
V
OUT
0.6
x R
FB2
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7.6 Feedback Resistors: R
FB1
, R
FB2
, and R
AC
The resistors labeled R
FB1
and R
FB2
create a voltage divider from V
OUT
to the feedback pin that is used to
set the output of the voltage regulator. Nominally, the output of the LM21212-2 evaluation board is set to
1.2V, giving resistor values of R
FB1
= R
FB2
= 10kΩ. If a different output voltage is required, the value of R
FB2
can be adjusted according to Equation 12:
(12)
R
FB1
does not need to be changed from its value of 10kΩ. Resistor R
AC
has a value of 49.9Ω and is
provided as an injection point for loop stability measurements, as well as, a way to further tweak the
output voltage accuracy to account for resistor tolerance values differing from ideal calculated values. The
jumper is used to short out R
AC
when not needed.
7.7 Programmable UVLO: R
EN1
and R
EN2
The resistors labeled R
EN1
and R
EN2
create a voltage divider from V
IN
to the enable pin that can be used to
enable the device above a programmed V
IN
, effectively creating a programmable UVLO voltage above the
device's internal UVLO (nominally 2.7V). To allow evaluation of the device down to 2.95V, these
components are not installed. To change the turn-on threshold of the device a 10 kΩ resistor is
recommended for R
EN1
and the value of R
EN2
can be calculated using Equation 13:
(13)
where, V
TO
is the desired V
IN
voltage at which the device will enable.
7.8 Resistor-Adjustable Frequency
The frequency adjust (FADJ) pin allows the LM21212-2 to be programmed to a predetermined switching
frequency between 300 kHz to 1.55 MHz by connecting a resistor between FADJ and AGND. To
determine the resistor (R
ADJ
) value for a desired frequency by using Equation 14:
(14)
where, R
ADJ
is resistance in kΩ, and f
SW
is frequency in kHz. The desired frequency must fall within the
operational frequency range, 300 kHz to 1550 kHz, and a corresponding resistor must be used for normal
operation.
10
AN-2140 LM21212-2 Evaluation Board SNVA480A–May 2011–Revised May 2013
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