LM3556_15 Datasheet, PDF (27/39 Pages) Texas Instruments – Synchronous Boost LED Flash Driver With High-Side Current Source

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8 Application and Implementation
LM3556
SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015
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
1 PH
2.5 V to 5.5 V
10 PF
IN
SW _OUT
10 PF
LED
ENABLE
STROBE
TORCH
TX
SDA
SCL
GND
Flash
LED
TEMP
Figure 41. LM3556 Typical Application
8.2.1 Design Requirements
Example requirements based on default register values are listed in Table 15:
DESIGN PARAMETER
Input Voltage Range
Brightness Control
Output current
LED Configuration
Switching frequency
Table 15. Design Parameters
EXAMPLE VALUE
2.5 V to 5.5 V
I2C Register
1.5 A
1 Flash LED
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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