Datasheet

582.4
R
ILIM
(kÖ) =
I
HSMAX
- 14.2
ÂI
LMAX
I
HSMAX
=
2
+ I
DCMAX
(V
INMAX
± V
OUTMIN
) V
OUTMIN
ÂI
LMAX
=
FOR D > 0.5
ÂI
LMAX
=
FOR D < 0.5
L
MIN
f
SWMIN
V
INMAX
(V
INMIN
± V
OUTMAX
) V
OUTMAX
L
MIN
f
SWMIN
V
INMIN
R
EN1
=
- 1
V
TO
1.35
x R
EN2
Component Selection www.ti.com
5.7 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 the equation:
(13)
where V
TO
is the desired V
IN
voltage at which the device will enable.
5.8 Programmable Current Limit: R
ILIM
The resistor R
ILIM
will set the internal current limit on the LM21215. A 0Ω resistor is used on this board,
allowing the maximum current capability of the device. If a user desires a lower current limit, it should be
programmed such that the peak inductor current (I
L
) does not trigger the current limit in normal operation.
This requires setting the resistor R
ILIM
to the appropriate value to allow the maximum ripple current plus the
DC output current through the high-side FET during normal operation. The maximum ripple current can be
described as:
(14)
where V
INMAX
, V
INMIN
, V
OUTMAX
, V
OUTMIN
, L
MIN
and F
SWMIN
are the respective maximum and minimum conditions
of the system as defined by the component tolerance and device variation. From this, the maximum
allowable current through the high-side FET (I
HSMAX
) of device can be described as:
(15)
where I
OUTMAX
is the maximum defined DC output current, up to 15 A. Once the I
HSMAX
value has been
determined, a nominal value of the R
ILIM
resistor can be calculated as follows:
(16)
where the R
ILIM
value is the nominal resistance necessary for the given I
HSMAX
value. A conservative design
should also take into account the device variation over V
IN
and temperature, which can be seen in the
Electrical Characteristics table for the I
CLR
parameter and the typical performance characteristics in 15A
High Efficiency Point of Load Synchronous Buck Regulator (SNVS625). These variations can cause the
I
HSMAX
value to increase, depending on the range of the input voltage and junction temperature.
6
AN-2130 LM21215 Evaluation Board SNVA476A–March 2011–Revised May 2013
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