ST AN2333 Application note

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AN2333

Application note

White LED power supply for large display backlight

Introduction

This application note is dedicated to the STLD40D, it's a boost converter that operates from

3.0 V to 5.5 V dc and can provide an output voltage as high as 37 V 10 white LEDs in series.

Figure 1. Package

and can drive up to

Figure 2. Pin configuration

June 2011 Doc ID 12202 Rev 3 1/9

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Contents AN2333

Contents

1 Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Selection of external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 Input and output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3 LED selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1 PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2/9 Doc ID 12202 Rev 3

AN2333 Schematic description

1 Schematic description

The converter is a PFM (pulse frequency modulation) inductor switcher and can work in discontinuous (DCM) as well as continuous (CCM) Mode operation. The output current capability is 20 mA with an output voltage of 37Vdc.The regulation is done by sensing the LED current through resistor R enable signal pin EN. By applying a low frequency PWM signal the WLEDs can be dimmed.

.The device can be turned ON/OFF through the logic

LED

The maximum peak inductor current can be programmed by connecting a resistor R pin R

SET

1.1 Application schematic

Figure 3. Typical application schematic

SET

Doc ID 12202 Rev 3 3/9

Selection of external components AN2333

2 Selection of external components

2.1 Input and output capacitor selection

For input and output capacitor it is recommended to use a ceramic capacitor with low ESR. For good stability of the device supplied by a low input voltage of 3.0 V at maximum ratings of output power, it is recommended to use 2.2 µF / 6.3 V as a minimum value of input capacitor and 4.7 µF / 40 V as a minimum value of output capacitor.

2.2 Inductor selection

Shielded thick inductor with low DC series resistance of wiring is recommended for this application. For good efficiency ti is recommended to use inductor with series DC resistance R

= RD/10, [Ω; 1, Ω] where RD is the dynamic resistance of the LED [Ω; 1, Ω]. For

DCL

nominal operation, the peak inductor current can be calculated by this formula:

Equation 1

OUT

------------ V n

------------------------------------------------------- -

2L• F• V

------------------------------------------------ -

–()+

OUT

•()

Where:

● I

● n Efficiency of the STLD40D

● V

●

● F Switching frequency

Peak inductor current

PEAK

Current sourced at the V

OUT

Output voltage at pin V

OUT

Input voltage at pin V

L Inductance value of the inductor

2.3 LED selection

All LEDs with forward voltage from 2.7 V to 5 V are feasible for use with device STLD40D. The LED forward voltage must include the voltage spread of this value. The current in the LED can be set through the sensing resistor value.

OUT

4/9 Doc ID 12202 Rev 3

AN2333 Selection of external components

Table 1. Recommended components

Val ue

Symbol Description Parameter

VRRM 40 V

Boost schottky diode

at IF = 300 mA, TJ = 25 °C 0.5 V

at VR = 10 V, TJ = 25 °C 30 µA

VRRM 40 V

STPS1L40M

Feedback LED current

LED

regulation resistor

Input ceramic capacitor,

low ESR

at IF = 1 A, TJ = 25 °C 0.46 V

at VR = 20 V, TJ = 25 °C 21 µA

ILED=20mA 8 W

Ceramic type 2.2 µF

Capacitance 4.7 µF

Output ceramic

out

capacitor, low ESR

Volt age 40 V

ESR 1.6 W

Min. Typ. Max

Unit

Boost inductor (height<2mm)

Inductance 4.7 µF

DCR 1 W

=GND 1 A

satRSET

Doc ID 12202 Rev 3 5/9

Selection of external components AN2333

Figure 4. I

Figure 6. γ vs. V

LED

vs. R

LED

Figure 5. I

Figure 7. I

vs. R

LIMIT

vs. duty cycle EN pin

LED

SET

(dimming)

6/9 Doc ID 12202 Rev 3

AN2333 PCB design

3 PCB design

3.1 PCB design rules

STLD40D is a powerful switched device. The PCB must be designed in line with the rules for designing switched supplies. The power windings must be as short as possible and wide, because of large current. It is recommended to use a dual layer PCB minimal. Place all external components close to the STLD40D. Also switched high-energy loops should be as small as possible to reduce EMI.

Figure 8. PCB layout - Top overlay view

Doc ID 12202 Rev 3 7/9

Revision history AN2333

4 Revision history

Table 2. Document revision history

Date Revision Changes

20-Mar-2006 1 Initial release

19-May-2006 2 Minor text changes

01-Jun-2011 3 Modified: Table 1 on page 5

8/9 Doc ID 12202 Rev 3

AN2333

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