t
SS: 50mS
Application Hints (Continued)
Step 7: Include a 0.01µF/50V capacitor for Cboost in the
design and then determine the value of a softstart capacitor
if desired.
VSST: 0.63V
VOUT: 3.3V
VSCHOTTKY: 0.5V
VIN: 16V
Step 8: Define a value for RADJ to set the peak switch
current limit to be at least 20% greater than Iout max to allow
for at least 30% inductor ripple current ( 15% of Iout). For
designs that must operate over the full temperature range
the switch current limit should be set to at least 50% greater
than Iout max (1.5 x Iout max).
Using Vin max ensures that the softstart delay time will be at
least the desired 50mS.
Using the formula for Css a value of 0.148µF is determined
to be required. Use of a standard value 0.22µF capacitor will
produce more than sufficient softstart delay.
FIXED OUTPUT VOLTAGE DESIGN EXAMPLE
Step 8: Determine a value for RADJ to provide a peak switch
current limit of at least 2.5A plus 50% or 3.75A.
A system logic power supply bus of 3.3V is to be generated
from a wall adapter which provides an unregulated DC volt-
age of 13V to 16V. The maximum load current is 2.5A. A
softstart delay time of 50mS is desired. Through-hole com-
ponents are preferred.
Use a value of 10KΩ.
ADJUSTABLE OUTPUT DESIGN EXAMPLE
Step 1: Operating conditions are:
Vout = 3.3V
In this example it is desired to convert the voltage from a two
battery automotive power supply (voltage range of 20V to
28V, typical in large truck applications) to the 14.8VDC alter-
nator supply typically used to power electronic equipment
from single battery 12V vehicle systems. The load current
required is 2A maximum. It is also desired to implement the
power supply with all surface mount components. Softstart is
not required.
Vin max = 16V
Iload max = 2.5A
Step 2: Select an LM2673T-3.3. The output voltage will have
a tolerance of
2% at room temperature and 3% over the full operating
temperature range.
Step 3: Use the nomograph for the 3.3V device ,Figure 3.
The intersection of the 16V horizontal line (Vin max) and the
2.5A vertical line (Iload max) indicates that L33, a 22µH
inductor, is required.
Step 1: Operating conditions are:
Vout = 14.8V
Vin max = 28V
From Table 1, L33 in a through-hole component is available
from Renco with part number RL-1283-22-43 or part number
PE-53933 from Pulse Engineering.
Iload max = 2A
Step 2: Select an LM2673S-ADJ. To set the output voltage
to 14.9V two resistors need to be chosen (R1 and R2 in
Figure 2). For the adjustable device the output voltage is set
by the following relationship:
Step 4: Use Table 3 to determine an output capacitor. With a
3.3V output and a 33µH inductor there are four through-hole
output capacitor solutions with the number of same type
capacitors to be paralleled and an identifying capacitor code
given. Table 2 provides the actual capacitor characteristics.
Any of the following choices will work in the circuit:
1 x 220µF/10V Sanyo OS-CON (code C5)
1 x 1000µF/35V Sanyo MV-GX (code C10)
1 x 2200µF/10V Nichicon PL (code C5)
1 x 1000µF/35V Panasonic HFQ (code C7)
Where VFB is the feedback voltage of typically 1.21V.
A recommended value to use for R1 is 1K. In this example
then R2 is determined to be:
Step 5: Use Table 4 to select an input capacitor. With 3.3V
output and 22µH there are three through-hole solutions.
These capacitors provide a sufficient voltage rating and an
rms current rating greater than 1.25A (1/2 Iload max). Again
using Table 2 for specific component characteristics the
following choices are suitable:
R2 = 11.23KΩ
The closest standard 1% tolerance value to use is 11.3KΩ
This will set the nominal output voltage to 14.88V which is
within 0.5% of the target value.
1 x 1000µF/63V Sanyo MV-GX (code C14)
1 x 820µF/63V Nichicon PL (code C24)
1 x 560µF/50V Panasonic HFQ (code C13)
Step 3: To use the nomograph for the adjustable device,
Figure 6, requires
a
calculation of the inductor
Step 6: From Table 5 a 3A or more Schottky diode must be
selected. The 20V rated diodes are sufficient for the appli-
cation and for through-hole components two part types are
suitable:
Volt•microsecond constant (E•T expressed in V•µS) from
the following formula:
1N5820
SR302
where VSAT is the voltage drop across the internal power
switch which is Rds(ON) times Iload. In this example this would
be typically 0.15Ω x 2A or 0.3V and VD is the voltage drop
across the forward bisased Schottky diode, typically 0.5V.
Step 7: A 0.01µF capacitor will be used for Cboost. For the
50mS softstart delay the following parameters are to be
used:
ISST: 3.7µA
13
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