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LP5520

SNVS440B – MAY 2007 – REVISED MARCH 2016

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Typical Applications (continued)

8.2.1.1 Design Requirements

Table 14. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Input voltage range

2.9 V to 5.5 V

Maximum output voltage

20 V

LED configuration

3 strings with 6 LEDs in series

LED current

Maximum 60 mA per string

Brightness control

Operation mode

Automatic/normal

Input capacitor

10 µF, 6.3 V

Output capacitors

2 × 4.7 µF, 25 V

Inductor

4.7 µH

Temperature sensor

LM20

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Recommended External Components

8.2.1.2.1.1

choice. Capacitor voltage rating must be sufficient; TI recommends 25 V or greater. Examples of suitable

capacitors are: TDK C3216X5R1E475K, Panasonic ECJ3YB1E475K, and Panasonic ECJ4YB1E475K.

Some ceramic capacitors, especially those in small packages, exhibit a strong capacitance reduction with the

increased applied voltage (DC bias effect). The capacitance value can fall below half of the nominal capacitance.

Output capacitance that is too low can make the boost converter unstable. Output capacitor value reduction due

to DC bias must be less than 70% at 20 V (minimum 3 µF of real capacitance remaining).

8.2.1.2.1.2

8.2.1.2.1.3

A schottky diode must be used for the output diode. To maintain high efficiency the average current rating of the

Schottky diode must be greater than the peak inductor current (1 A). Schottky diodes with a low forward drop and

fast switching speeds are ideal for increasing efficiency in portable applications. Choose a reverse breakdown

voltage of the schottky diode significantly larger (approximately 30 V) than the output voltage. Do not use

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

load regulation to suffer. A schottky diode with low parasitic capacitance helps in reducing EMI noise. Examples

of suitable diodes are Central Semiconductor CMMSH1-40 and Infineon BAS52-02V.

8.2.1.2.1.4

ringing. These components must be as near as possible to the SW pin to ensure reliable operation. High

50V or greater voltage rating is recommended for the capacitors. The ferrite bead DC resistance must be less

than 0.1

Ω and current rating 1 A or above. The impedance at 100 MHz must be from 30 Ω to 300 Ω. Examples

of suitable types are TDK MPZ1608S101A and Taiyo-Yuden FBMH 1608HM600-T.