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AN10712

Photo flash LED driver

Rev. 1 — 28 October 2011

Application note

Document information

Info

Keywords

Abstract

Content

Photo flash, dual LED, 500 mA LED current, soft start, SSL3252

Application guidelines for a photo flash driver for mobile applications,

including an application setup.

NXP Semiconductors

AN10712

Photo flash LED driver

Revision history

Rev

v.1

Date

20111028

Description

application note; initial version

Contact information

For more information, please visit:

http://www.nxp.com

For sales office addresses, please send an email to:

salesaddresses@nxp.com

AN10712

All information provided in this document is subject to legal disclaimers.

Application note

Rev. 1 — 28 October 2011

2 of 14

NXP Semiconductors

AN10712

Photo flash LED driver

1. Introduction

The SSL3252 is a photo flash LED driver designed for battery operated mobile devices

such as mobile phones and PDAs. The boost converter delivers high performance and

drives a single or dual high brightness LED at up to 500 mA with over 85 % efficiency. The

driver can be programmed to operate in Flash mode, Torch mode, Assist light mode, or

Indicator mode.

The small silicon size and the high internal switching frequency of 2 MHz minimizes the

size of the application and makes the SSL3252 very suitable for mobile phones where

space is limited, and only requiring three external components. System protection has

been a very important part of the SSL3252 design so a time-out function can be

programmed to prevent overstressing the LED, and the driver itself is protected from

overheating.

2. Application information

2.1 General description

The SSL3252 is a boost converter intended to drive either a single high power flash LED

or two high power flash LEDs in series. The main LED current is controlled by the output

voltage of the boost converter and the integrated linear current source. The SSL3252 has

two interface modes and six operational modes. The Interface mode is selected by the

interface select pin IF_SEL. Depending on the interface mode selected, the device can

either be controlled by an I

2

C-bus interface, or external enable lines.

2.2 Application diagrams

2.2.1 I

2

C control mode

Using the SSL3252 as shown in

Figure 1

is the typical application for the SSL3252 in

I

2

C control mode, and therefore gives the advantage of maximum flexibility of the

operating features of the SSL3252. Apart from setting the driver into the different

operating modes, all of the operating modes can be activated and settings can be altered

to match the behavior of the driver to the application, e.g., adjusting the LED brightness

intensity to meet the required level for a clear picture.

The device cannot enter Shut-down mode when in I

2

C mode. The lowest power

consumption can be achieved in Standby mode. When using I

2

C, the device can still be

put in Shut-down mode by first making all control pins LOW (SDA = SCL = TORCH = 0)

and then going to Direct enable Shut-down mode by making IF_SEL LOW.

AN10712

All information provided in this document is subject to legal disclaimers.

Application note

Rev. 1 — 28 October 2011

3 of 14

NXP Semiconductors

AN10712

Photo flash LED driver

V

bat

battery

Cin

10

μF

L

2.2

μH

PGND

LX

VIN

VO

SSL3252

LED

control

signals

TORCH

STRB/2LED

SDA/EN2

SCL/EN1

IF_SEL

(1)

Cout

10

μF

PGND

flash LEDs

I_IND

PGND

indicator LED

GND

PGND

002aaf151

PGND

(1) The selection of the mode of the IC is done with I

2

C. IF_SEL must be connected to VIN.

Fig 1.

Typical application diagram of SSL3252 using I

2

C control

2.2.2 Direct enable control mode

Using the SSL3252 as shown in

Figure 2

is the typical application for the SSL3252 in

Direct enable control mode. It has the advantage of operating the driver without using I

2

C

communication. This provides a short response time and a less complicated operation,

minimizing flash-ON latency. However, it has less flexible control features for the different

operating modes. The STRB/2LED pin is functioning as 2LED output.

V

bat

battery

Cin

10

μF

L

2.2

μH

PGND

LX

VIN

VO

SSL3252

LED

control

signals

TORCH

STRB/2LED

SDA/EN2

SCL/EN1

IF_SEL

(1)

Cout

10

μF

PGND

flash LEDs

I_IND

PGND

indicator LED

GND

PGND

002aaf152

PGND

(1) The selection of the operational mode of the IC is done with EN1 and EN2. IF_SEL must be

connected to GND.

Fig 2.

Typical application diagram of SSL3252 using Direct enable control

AN10712

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Application note

Rev. 1 — 28 October 2011

4 of 14

NXP Semiconductors

AN10712

Photo flash LED driver

To activate the different modes, EN1, EN2 and TORCH have to be used while IF_SEL

remains LOW. The operating modes are Shut-down, Indicator, Torch, Assist light, and

Flash mode. See

Table 1.

TORCH

0

1

X

Enable definition

EN1

0

1

EN2

0

1

0

1

Operational mode

Shut-down mode

Torch mode

Assist light mode

Indicator mode

Flash mode

LED active

-

main LED

indicator LED

main LED

The result is that the driver uses factory default settings as displayed in the SSL3252

data sheet. In Direct enable control mode the maximum flash time is fixed at 850 ms.

When the maximum flash time is exceeded, it generates a fault condition and the IC will

stop switching.

To minimize the amount of digital outputs used, some of the SSL3252’s inputs can be

hardwired to signals already used by other parts of the mobile application. For instance:

EN1 and EN2 can be hardwired to the camera module or to the double action shutter

button on the application. When the user wants to take a picture, he has to press the

button halfway. EN2 should be hardwired to the signal that activates the camera, which

then also activates the Assist light mode. The user can use the Assist light mode to focus

or to record streaming video. When the user has finished focusing and actually wants to

take a picture, he has to press the shutter button fully. This action should also activate

EN1 while EN2 is still active. This will activate the Flash mode on the SSL3252 and the

‘picture save mode’ on the camera module. When the user was recording streaming

video, he just releases the shutter button to stop recording. This will also deactivate EN1

and EN2 and stop the Flash mode.

2.3 PCB design and component placement

The components suitable for this design are listed in

Table 2.

When designing a PCB

layout for the SSL3252, special attention should be paid to:

•

Component placement

Track width and length

Use of vias

Thermal restrictions

AN10712

All information provided in this document is subject to legal disclaimers.

Application note

Rev. 1 — 28 October 2011

5 of 14

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