Datasheet

LM26400Y

SNVS457C –FEBRUARY 2007–REVISED APRIL 2013

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Figure 35. Programming Output Voltage

It is recommended that the lower feedback resistor R2 always be 5.9kΩ. This simplifies the selection of the C

value (For an explanation of C

, please refer to the section LOAD STEP RESPONSE). The 5.9kΩ is also a

suitable R2 value in applications that need to increase the output voltage on the fly by paralleling another resistor

with R2. Since the FB pin is 0.6V during normal operation, the current through the feedback resistors is normally

0.6V / 5.9kΩ = 0.1mA and the power dissipation in R2 is 0.6V x 0.6V / 5.9kΩ = 61µW - low enough for 0402 size

or smaller resistors.

Use the following equation to determine the upper feedback resistor R1.

(15)

To determine the maximum allowed resistor tolerance, use the following equation:

(16)

where TOL is the set point accuracy of the regulator, Φ is the tolerance of V

Example:

OUT

= 1.2V, with a set point accuracy of +/-3.5%.

(17)

Choose 1% resistors. R2 = 5.90kΩ.

(18)

INDUCTOR SELECTION

An inductance value that gives a peak-to-peak ripple current of 0.4A to 0.8A is recommended. Too large a ripple

current can reduce the maximum achievable DC load current because the peak current of the switch is limited to

a typical of 3A. Too small a ripple current can cause the regulator to oscillate due to the lack of inductor current

ramp signal, especially under high input voltages. Use the following equation to determine inductance:

(19)

where V

IN_MAX

is the maximum input voltage of the application.

The rated current of the inductor should be higher than the maximum DC load current. Generally speaking, the

lower the DC resistance of the inductor winding, the higher the overall regulator efficiency.

Product Folder Links: LM26400Y