LM2679 Datasheet, PDF (22/37 Pages) National Semiconductor (TI) – SIMPLE SWITCHER 5A Step-Down Voltage Regulator with Adjustable Current Limit

LM2679
SNVS026O – MARCH 2000 – REVISED JUNE 2016
www.ti.com
Step 1: Operating conditions are:
• V_OUT = 14.8 V
• V_IN max = 28 V
• I_LOAD max = 3.5 A
Step 2: Select an LM2679S-ADJ. To set the output voltage to 14.9 V, two resistors need to be chosen (R1 and
R2 in Figure 19). For the adjustable device, the output voltage is set by Equation 3.
where
• V_FB is the feedback voltage of typically 1.21 V
(3)
A recommended value to use for R1 is 1 k. In this example, R2 is determined with Equation 4.
(4)
R2 = 11.23 kΩ
The closest standard 1% tolerance value to use is 11.3 kΩ
This sets the nominal output voltage to 14.88 V which is within 0.5% of the target value.
Step 3: To use the nomograph for the adjustable device, Figure 17, requires a calculation of the inductor Volt •
microsecond constant (E • T expressed in V • μS) from Equation 5.
where
• V_SAT is the voltage drop across the internal power switch which is R_ds(ON) times I_LOAD
(5)
In this example, this is typically 0.12 Ω × 3.5 A or 0.42 V and V_D is the voltage drop across the forward biased
Schottky diode, typically 0.5 V. The switching frequency of 260 kHz is the nominal value to use to estimate the
ON time of the switch during which energy is stored in the inductor.
For this example E • T is found with Equation 6 and Equation 7.
(6)
(7)
Using Figure 17, the intersection of 27 V • μS horizontally and the 3.5 A vertical line (I_LOAD max) indicates that
L48 , a 47-μH inductor, or L49, a 33-μH inductor could be used. Either inductor is suitable, but for this example
selecting the larger inductance results in lower ripple current.
From Table 3, L48 in a surface mount component is available from Pulse Engineering with part number P0848.
Step 4: Use Table 9 and Table 10 to determine an output capacitor. With a 14.8-V output the 12.5 to 15 V row is
used and with a 47-μH inductor there are three surface mount output capacitor solutions. Table 1 and Table 2
provide the actual capacitor characteristics based on the C Code number. Any of the following choices can be
used:
• 1 × 33-μF, 20-V AVX TPS (code C6)
• 1 × 47-μF, 20-V Sprague 594 (code C8)
• 1 × 47-μF, 20-V Kemet T495 (code C8)
NOTE
When using the adjustable device in low voltage applications (less than 3-V output), if the
nomograph, Figure 17, selects an inductance of 22 μH or less, Table 9 and Table 10 do
not provide an output capacitor solution. With these conditions the number of output
capacitors required for stable operation becomes impractical. It is recommended to use
either a 33-μH or 47-μH inductor and the output capacitors from Table 9 and Table 10.
22
Submit Documentation Feedback
Product Folder Links: LM2679
Copyright © 2000–2016, Texas Instruments Incorporated