Datasheet

R
A
=
V
PVIN
- 1.35V
1.35V - I
EN
R
B
R
B
EN
AVIN
R
A
LM21215
Input Power
Supply
V
OUT
R
B
R
FB2
V
OUT
=
R
FB1
R
FB2
+
0.6V
R
FB1
R
FB2
FB
V
OUT
LM21215
0.6V
LM21215
www.ti.com SNVS625E –FEBRUARY 2011–REVISED MARCH 2013
DESIGN GUIDE
OUTPUT VOLTAGE
The first step in designing the LM21215 application is setting the output voltage. This is done by using a voltage
divider between V
OUT
and AGND, with the middle node connected to V
FB
. When operating under steady-state
conditions, the LM21215 will force V
OUT
such that V
FB
is driven to 0.6 V.
Figure 28. Setting V
OUT
A good starting point for the lower feedback resistor, R
FB2
, is 10 kΩ. R
FB1
can then be calculated with the
following equation:
(2)
PRECISION ENABLE
The enable (EN) pin of the LM21215 allows the output to be toggled on and off. This pin is a precision analog
input. When the voltage exceeds 1.35V, the controller will try to regulate the output voltage as long as the input
voltage has exceeded the UVLO voltage of 2.70V. There is an internal current source connected to EN so if
enable is not used, the device will turn on automatically. If EN is not toggled directly the device can be
preprogrammed to turn on at a certain input voltage higher than the UVLO voltage. This can be done with an
external resistor divider from AVIN to EN and EN to AGND as shown below in Figure 29.
Figure 29. Enable Startup Through Vin
The resistor values of R
A
and R
B
can be relatively sized to allow EN to reach the enable threshold voltage
depending on the input supply voltage. With the enable current source accounted for, the equation solving for R
A
is shown below:
(3)
In the above equation, R
A
is the resistor from V
IN
to enable, R
B
is the resistor from enable to ground, I
EN
is the
internal enable pull-up current (2 µA) and 1.35V is the fixed precision enable threshold voltage. Typical values for
R
B
range from 10 kΩ to 100 kΩ.
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