LM2621

SNVS033D – MAY 2004 – REVISED NOVEMBER 2015

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8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

The LM2621 is primarily used as a Boost type step-up converter. The following section provides information

regarding connection and component choices to build a successful boost converter. Examples of typical

applications are also provided including a SEPIC step-up/step-down topology. More details on designing a

SEPIC converter can be found here: SLYT309.

8.2 Typical Applications

8.2.1 Step-Up DC-DC Converter Typical Application Using LM2621

Figure 13. Typical Circuit

8.2.1.1 Design Requirements

In order to successfully build an application, the designer should have the following parameters:

•

Output voltage to set the feedback voltage divider and to assess the source for biasing the VDD pin.

•

Input voltage range (min and max) to ensure safe operation within absolute max. rating of the IC.

•

Output current to ensure that the system will not hit the internal peak current limit of the IC (2.85 A typical)

during normal operation.

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Setting the Output Voltage

The output voltage of the step-up regulator can be set between 1.24 V and 14 V by connecting a feedback

resistive divider made of RF1 and RF2. The resistor values are selected as follows:

RF2 = RF1 /[(VOUT/ 1.24) −1]

(1)

A value of 150 k

Ω is suggested for RF1. Then, RF2 can be selected using the above equation. A 39-pF capacitor

(CF1) connected across RF1 helps in feeding back most of the AC ripple at VOUT to the FB pin. This helps reduce

the peak-to-peak output voltage ripple as well as improve the efficiency of the step-up regulator, because a set

hysteresis of 30 mV at the FB pin is used for the gated oscillator control scheme.

10

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