IN

OUT

TEMP

SW

ENABLE

TORCH

TX

SDA

SCL

GND

1 PH

10 PF

10 PF

STROBE

LED

Flash

LED

2.5 V to 5.5 V

LM3556

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SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015

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 LM3556 can drive one flash LED at currents up to 1.5 A. The 4-MHz DC-DC boost regulator allows for the

use of small-value discrete external components.

8.2 Typical Application

Figure 41. LM3556 Typical Application

8.2.1 Design Requirements

Example requirements based on default register values are listed in Table 15:

Table 15. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Input Voltage Range

2.5 V to 5.5 V

Brightness Control

I2C Register

Output current

1.5 A

LED Configuration

1 Flash LED

Switching frequency

4 MHz (typical)

8.2.2 Detailed Design Procedure

8.2.2.1 Output Capacitor Selection

The LM3556 is designed to operate with a ceramic output capacitor of at least 10 µF. When the boost converter

is running, the output capacitor supplies the load current during the boost converter's on-time. When the NMOS

switch turns off, the inductor energy is discharged through the internal PMOS switch, supplying power to the load

and restoring charge to the output capacitor. This causes a sag in the output voltage during the on-time and a

rise in the output voltage during the off-time. The output capacitor is therefore chosen to limit the output ripple to

an acceptable level depending on load current and input/output voltage differentials and also to ensure the

converter remains stable.

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