No Boost Inductor Design with LM364x for Long-Time Focusing in Camera Module

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1 Introduction

1.1 Key Specification of LM3644

2.5 V to 5.5 V Input Voltage Range
Dual Independent 1.5-A LED Current Source Up to 1500 mA per channel in Flash mode Programmability
Accurate and Programmable LED Current Range from 1.4 mA to 1.5 A
Torch Currents up to 360 mA (LM3644TT)
Flash Current up to 1500 mA
Support Flash Timeout function
Support Input Voltage Flash Monitor (IVFM)
Size: 1.69mm × 1.31mm, DSBGA (12)

1.2 LM3644 Simplified Block Diagram

LM3644 can support most of hero LED vender products in the market so as to make LED flash driving design more easily. It can support up to 2 channel LED flashing or torching with 1.5-A maximum flash current and 360 mA (LM3644TT) torch current per each channel. LM3644 also supports flash timeout and IVFM to keep flash LED and system always being in safe.

Figure 1-1 LM3644 LED Flash Driver

2 No Boost Inductor Flash Driving Design with LM3644

2.1 Why Try to Not Use Boost Inductor Solution?

In most smart-phone applications, the UVP threshold of system is more than 3.6 V, and most of LED forward voltage-drops at 1 A or 1.5 A flash current condition is 3 V around, at this condition, flash driver will work in linear mode or bypass mode, it means boost inductor will not participate, at this point, boost inductor prefers a normal copper wire. For designers, they can remove boost inductor to save more PCB area and solution cost.

2.2 No Boost Inductor Schematic of LM3644 for Two LED

Figure 2-1 LM3644 Dual Channel Without Boost Inductor Simplified Schematic for Two LEDs

2.3 No Boost Inductor Schematic of LM3644 for Single LED

Figure 2-2 LM3644 Dual Channel without Boost Inductor Simplified Schematic for Single LED