Datasheet

LM2678
www.ti.com SNVS029I –MARCH 2000–REVISED APRIL 2013
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 condition. In an application with properly
selected external components, the output will recover smoothly.
Practical values of external components that have been experimentally found to work well under these specific
operating conditions are C
OUT
= 47µF, L = 22µH. It should be noted that even with these components, for a
device’s current limit of I
CLIM
, the maximum load current under which the possibility of the large current limit
hysteresis can be minimized is I
CLIM
/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.
Under extreme over-current or short circuit conditions, the LM267X employs frequency foldback in addition to the
current limit. If the cycle-by-cycle inductor current increases above the current limit threshold (due to short circuit
or inductor saturation for example) the switching frequency will be automatically reduced to protect the IC.
Frequency below 100 KHz is typical for an extreme short circuit condition.
SIMPLE DESIGN PROCEDURE
Using the nomographs and tables in this data sheet (or use the available design software at www.ti.com) a
complete step-down regulator can be designed in a few simple steps.
Step 1: Define the power supply operating conditions:
Required output voltage
Maximum DC input voltage
Maximum output load current
Step 2: Set the output voltage by selecting a fixed output LM2678 (3.3V, 5V or 12V applications) or determine
the required feedback resistors for use with the adjustable LM2678−ADJ
Step 3: Determine the inductor required by using one of the four nomographs, Figure 21 through Figure 24.
Table 1 provides a specific manufacturer and part number for the inductor.
Step 4: Using Table 6 (fixed output voltage) or Table 12 (adjustable output voltage), determine the output
capacitance required for stable operation. Table 3 provides the specific capacitor type from the manufacturer of
choice.
Step 5: Determine an input capacitor from Table 6 for fixed output voltage applications. Use Table 3 to find the
specific capacitor type. For adjustable output circuits select a capacitor from Table 3 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 6: Select a diode from Table 10. The current rating of the diode must be greater than I load max and the
Reverse Voltage rating must be greater than Vin max.
Step 7: Include a 0.01μF/50V capacitor for Cboost in the design.
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 voltage of 13V to 16V. The maximum load current is 4A. Through-hole components are preferred.
Step 1: Operating conditions are:
Vout = 3.3V
Vin max = 16V
Iload max = 4A
Step 2: Select an LM2678T-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 21. The intersection of the 16V horizontal line (V
in
max)
and the 4A vertical line (I
load
max) indicates that L46, a 15μH inductor, is required.
From Table 1, L46 in a through-hole component is available from Renco with part number RL-1283-15-43.
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