Datasheet

LM2677
www.ti.com SNVS077I –MAY 2004–REVISED JUNE 2012
Step 1: Operating conditions are:
Vout = 3.3V
Vin max = 16V
Iload max = 2.5A
Step 2: Select an LM2677T-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 2.5A vertical line (I
load
max) indicates that L33, a 22μH inductor, is required.
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.
Step 4: Use Table 6 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 capacitors to be paralleled and an
identifying capacitor code given. Table 3 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)
Step 5: Use Table 7 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
I
load
max). Again using Table 3 for specific component characteristics the following choices are suitable:
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 8 a 3A Schottky diode must be selected. For through-hole components 20V rated diodes are
sufficient and 2 part types are suitable:
1N5820
SR302
Step 7: A 0.01μF capacitor will be used for Cboost.
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 alternator 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.
Step 1: Operating conditions are:
Vout = 14.8V
Vin max = 28V
Iload max = 2A
Step 2: Select an LM2677S-ADJ. To set the output voltage to 14.9V two resistors need to be chosen (R1 and R2
in Figure 20). For the adjustable device the output voltage is set by the following relationship:
where
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