LM2623AMMSLASHNOPB datasheet(11/22 Pages) TI

11 / 22 page

LM2623

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SNVS188H – MAY 2004 – REVISED NOVEMBER 2014

Typical Application (continued)

8.2.2.3 Setting the Output Voltage

The output voltage of the step-up regulator can be set by connecting a feedback resistive divider made of RF1

and RF2. The resistor values are selected as follows:

RF1 = RF2 * [(VOUT/ 1.24) −1]

(1)

A value of 50k to 100k is suggested for RF2. Then, RF1 can be selected using Equation 1.

8.2.2.4 VDD Supply

The VDD supply must be between 3 V to 5 V for the LM2623. This voltage can be bootstrapped from a much

lower input voltage by simply connecting the VDD pin to VOUT. In the event that the VDD supply voltage is not a

low ripple voltage source (less than 200 millivolts), it may be advisable to use an RC filter to clean it up.

Excessive ripple on VDD may reduce the efficiency.

8.2.2.5 Setting the Switching Frequency

The switching frequency of the oscillator is selected by choosing an external resistor (R3) connected between VIN

and the FREQ pin. See Figure 3 in the Typical Characteristics section of the data sheet for choosing the R3

value to achieve the desired switching frequency. A high switching frequency allows the use of very small surface

mount inductors and capacitors and results in a very small solution size. A switching frequency between 300 kHz

and 2 MHz is recommended.

8.2.2.6 Output Diode Selection

A Schottky diode should be used for the output diode. The forward current rating of the diode should be higher

than the peak input current, and the reverse voltage rating must be higher than the output voltage. Do not use

ordinary rectifier diodes, since slow switching speeds and long recovery times cause the efficiency and the load

regulation to suffer.

8.2.3 Application Curves

VOUT = 5 V

Figure 10. Efficiency vs Output Current

Figure 11. Output Voltage vs Output Current

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