LM3488MM datasheet(18/33 Pages) TI

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VOUT = 1.26

(1+

RF1

RF2

2fSL

'iL =

DVIN

LM3488

LM3488-Q1

SNVS089M – JULY 2000 – REVISED MARCH 2013

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The important quantities in determining a proper inductance value are IL (the average inductor current) and ΔiL

(the inductor current ripple). If

ΔiL is larger than IL, the inductor current will drop to zero for a portion of the cycle

and the converter will operate in discontinuous conduction mode. If

ΔiL is smaller than IL, the inductor current will

stay above zero and the converter will operate in continuous conduction mode. All the analysis in this datasheet

assumes operation in continuous conduction mode. To operate in continuous conduction mode, the following

conditions must be met:

IL > ΔiL

(9)

(10)

(11)

Choose the minimum IOUT to determine the minimum L. A common choice is to set ΔiL to 30% of IL. Choosing an

appropriate core size for the inductor involves calculating the average and peak currents expected through the

inductor. In a boost converter,

(12)

and IL_peak = IL(max) + ΔiL(max),

where

(13)

A core size with ratings higher than these values should be chosen. If the core is not properly rated, saturation

will dramatically reduce overall efficiency.

The LM3488 can be set to switch at very high frequencies. When the switching frequency is high, the converter

can be operated with very small inductor values. With a small inductor value, the peak inductor current can be

extremely higher than the output currents, especially under light load conditions.

The LM3488 senses the peak current through the switch. The peak current through the switch is the same as the

peak current calculated above.

PROGRAMMING THE OUTPUT VOLTAGE

The output voltage can be programmed using a resistor divider between the output and the feedback pins, as

shown in Figure 34. The resistors are selected such that the voltage at the feedback pin is 1.26V. RF1 and RF2

can be selected using the equation,

(14)

A 100pF capacitor may be connected between the feedback and ground pins to reduce noise.

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