lm21215a-1 National Semiconductor Corporation, lm21215a-1 Datasheet - Page 14

• Manufacturer Part Number: lm21215a-1
• Description: 15a High Efficiency Synchronous Buck Regulator With Frequency Synchronization
• Manufacturer: National Semiconductor Corporation

www.national.com
Design Guide
OUTPUT VOLTAGE
The first step in designing the LM21215A-1 application is set-
ting the output voltage. This is done by using a voltage divider
between V
V
LM21215A-1 will force V
A good starting point for the lower feedback resistor, R
10kΩ. R
PRECISION ENABLE
The enable (EN) pin of the LM21215A-1 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 reg-
ulate 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
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 6. Enable Startup Through Vin
OUT
can then be calculated the following equation:
and AGND, with the middle node connected to
FIGURE 5. Setting V
Figure
OUT
A
6.
and R
such that V
B
can be relatively sized to
30152104
A
OUT
FB
is shown below:
is driven to 0.6 V.
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FB2
, is
14
In the above equation, R
R
enable pull-up current (2µA) and 1.35V is the fixed precision
enable threshold voltage. Typical values for R
10kΩ to 100kΩ.
SOFT START
When EN has exceeded 1.35V, and both PVIN and AVIN
have exceeded the UVLO threshold, the LM21215A-1 will be-
gin charging the output linearly to the voltage level dictated
by the feedback resistor network. The LM21215A-1 employs
a user adjustable 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 capaci-
tance 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 startup 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 LM21215A-1 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 LM21215A-1 defaults to a
minimum startup time of 500 µs. The LM21215A-1 will not
startup faster than 500 µs. When enable is cycled or the de-
vice 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-current 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 7. Soft Start Timing
A
is the resistor from V
SS
is the desired startup time.
Figure
7.
EN
is the internal
B
IN
range from
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to enable,