Under current limiting conditions, the LM267x is designed to
respond in the following manner:
Step 6: Select a diode from Table 5. The current rating of the
diode must be greater than I load max and the Reverse Volt-
age rating must be greater than Vin max.
1. At the moment when the inductor current reaches the
current limit threshold, the ON-pulse is immediately
terminated. This happens for any application condition.
Step 7: Include a 0.01μF/50V capacitor for Cboost in the de-
sign.
2. However, the current limit block is also designed to
momentarily reduce the duty cycle to below 50% to avoid
subharmonic oscillations, which could cause the inductor
to saturate.
FIXED OUTPUT VOLTAGE DESIGN EXAMPLE
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.
Through-hole components are preferred.
3. Thereafter, once the inductor current falls below the
current limit threshold, there is a small relaxation time
during which the duty cycle progressively rises back
above 50% to the value required to achieve regulation.
Step 1: Operating conditions are:
Vout = 3.3V
If the output capacitance is sufficiently ‘large’, it may be pos-
sible that as the output tries to recover, the output capacitor
charging current is large enough to repeatedly re-trigger the
current limit circuit before the output has fully settled. This
condition is exacerbated with higher output voltage settings
because the energy requirement of the output capacitor
varies as the square of the output voltage (½CV2), thus re-
quiring an increased charging current.
Vin max = 16V
Iload max = 2.5A
Step 2: Select an LM2676T-3.3EP. 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.
A simple test to determine if this condition might exist for a
suspect application is to apply a short circuit across the output
of the converter, and then remove the shorted output condi-
tion. In an application with properly selected external compo-
nents, the output will recover smoothly.
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.
Practical values of external components that have been ex-
perimentally found to work well under these specific operating
conditions are COUT = 47µF, L = 22µH. It should be noted that
even with these components, for a device’s current limit of
ICLIM, the maximum load current under which the possibility of
the large current limit hysteresis can be minimized is ICLIM/2.
For example, if the input is 24V and the set output voltage is
18V, then for a desired maximum current of 1.5A, the current
limit of the chosen switcher must be confirmed to be at least
3A.
Step 4: Use Table 3 to determine an output capacitor. With a
3.3V output and a 22μH inductor there are four through-hole
output capacitor solutions with the number of same type ca-
pacitors 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)
SIMPLE DESIGN PROCEDURE
Using the nomographs and tables in this data sheet (or use
the available design software at http://www.national.com) a
complete step-down regulator can be designed in a few sim-
ple steps.
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 fol-
lowing choices are suitable:
Step 1: Define the power supply operating conditions:
Required output voltage
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 6: From Table 5 a 3A Schottky diode must be selected.
For through-hole components 20V rated diodes are sufficient
and 2 part types are suitable:
Maximum DC input voltage
Maximum output load current
Step 2: Set the output voltage by selecting a fixed output
LM2676EP (3.3V, 5V or 12V applications) or determine the
required feedback resistors for use with the adjustable
LM2676−ADJEP
1N5820
Step 3: Determine the inductor required by using one of the
four nomographs, Figure 3 through Figure 6. Table 1 provides
a specific manufacturer and part number for the inductor.
SR302
Step 7: A 0.01μF capacitor will be used for Cboost.
Step 4: Using Table 3 (fixed output voltage) or Table 6 (ad-
justable output voltage), determine the output capacitance
required for stable operation. Table 2 provides the specific
capacitor type from the manufacturer of choice.
ADJUSTABLE OUTPUT DESIGN EXAMPLE
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 re-
quired is 2A maximum. It is also desired to implement the
power supply with all surface mount components.
Step 5: Determine an input capacitor from Table 4 for fixed
output voltage applications. Use Table 2 to find the specific
capacitor type. For adjustable output circuits select a capac-
itor from Table 2 with a sufficient working voltage (WV) rating
greater than Vin max, and an rms current rating greater than
one-half the maximum load current (2 or more capacitors in
parallel may be required).
Step 1: Operating conditions are:
Vout = 14.8V
13
200996 Version 2 Revision 3 Print Date/Time: 2011/03/24 16:39:24
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