lm21215mhx National Semiconductor Corporation, lm21215mhx Datasheet - Page 14

• Manufacturer Part Number: lm21215mhx
• Description: 15a High Efficiency Synchronous Buck Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
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
V
LM21215 will force V
A good starting point for the lower feedback resistor, R
10 kΩ. R
tion:
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 de-
vice 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
The resistor values of R
allow EN to reach the enable threshold voltage depending on
the input supply voltage. With the enable current source ac-
counted for, the equation solving for R
FB
. When operating under steady-state conditions, the
FB1
FIGURE 5. Enable Startup Through Vin
OUT
can then be calculated with the following equa-
and AGND, with the middle node connected to
FIGURE 4. Setting V
Figure
OUT
A
such that V
5.
and R
B
can be relatively sized to
FB
30103704
A
is driven to 0.6 V.
OUT
is shown below:
30103728
FB2
, is
14
In the above equation, R
R
enable pull-up current (2 µA) and 1.35V is the fixed precision
enable threshold voltage. Typical values for R
kΩ to 100 kΩ.
SOFT START
When EN has exceeded 1.35V, and both PVIN and AVIN
have exceeded the UVLO threshold, the LM21215 will begin
charging the output linearly to the voltage level dictated by the
feedback resistor network. The LM21215 employs a user ad-
justable soft start circuit to lengthen the charging time of the
output set by a capacitor from the soft start pin to ground. After
enable exceeds 1.35V, an internal 2 µA current source begins
to charge the soft start capacitor. This allows the user to limit
inrush currents due to a high output capacitance and not
cause an over current condition. Adding a soft-start capacitor
can also reduce the stress on the input rail. Larger capacitor
values will result in longer start up times. Use the equation
below to approximate the size of the soft-start capacitor:
where I
If V
the soft start sequence will begin. There is a small delay be-
tween enable transitioning high and the beginning of the soft
start sequence. This delay allows the LM21215 to initialize its
internal circuitry. Once the output has charged to 90% of the
nominal output voltage the power good flag will transition
high. This behavior is illustrated in
As shown above, the size of the capacitor is influenced by the
nominal feedback voltage level 0.6V, the soft-start charging
current I
start capacitor is used then the LM21215 defaults to a mini-
mum startup time of 500 µs. The LM21215 will not startup
faster than 500µs. When enable is cycled or the device enters
UVLO, the charge developed on the soft-start capacitor is
discharged to reset the startup process. This also happens
when the device enters short circuit mode from an over-cur-
rent event.
B
IN
is the resistor from enable to ground, I
is higher than the UVLO level and enable is toggled high
SS
SS
is nominally 2 µA and t
(2 µA), and the desired soft start time. If no soft-
FIGURE 6. Soft Start Timing
A
is the resistor from V
SS
is the desired startup time.
Figure
6.
EN
B
is the internal
range from 10
IN
30103731
to enable,