LM3551, LM3552

SNVS371C – AUGUST 2005 – REVISED MARCH 2011

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Circuit Description

OVERVIEW

The LM3551/2 are high power white LED flash drivers capable of delivering up to 1A of output current. The

devices utilize a highly efficient inductive DC/DC boost converter to achieve the required output voltage. A

current-mode PWM control scheme regulates the output current over a wide input voltage range. Both the

LM3551 and the LM3552 have two low-side load disconnect FET's allowing for a continuous low-power Torch-

mode and a high-power, short duration Flash-mode.

Several application specific safety features are integrated into the LM3551/2 design. A flash timeout circuit is

present on-chip to prevent a failure in the flash LED caused by a timing violation. Over-Voltage Protection

protects the output capacitor, inductor and main power switch in the event of an open circuit condition. Other

safety features include inductor current limit, thermal shutdown, and an undervoltage lockout.

CIRCUIT COMPONENTS

Inductive DC/DC Boost Converter

In order to achieve the output voltages required to power high power white LEDs, the LM3551 and LM3552

utilize a highly efficient inductive DC/DC converter. The boost converter utilizes a current-mode controlled,

constant frequency (1.25MHz.), PWM architecture. This architecture creates a predictable noise spectrum that

current limit (2.2A typ.) help efficiently provide a high power output (700mA@5V) over the entire lithium-ion

voltage range. The feedback voltage for both the LM3551 and LM3552 is tightly regulated to 1.265V.

SD/EN Pin

The LM3551 and LM3552 provide two different options in regards to turn-on control logic. The LM3551 utilizes a

shutdown pin (SD) that turns on the part when a voltage less than 0.3V is applied. An internal 1.2M

Ω pull-up to

enable pin (EN) that turns on the part when a voltage greater than 1.2V is applied. An internal 1.2M

Ω pull-down

to GND is provided to place the LM3552 into shutdown when no control signal is provided.

Low-side Load Disconnect FETs

The LM3551 and LM3552 have two low-side load disconnect NFETs (FET-T and FET-F) that provide the

physical mechanism of Torch Mode and Flash Mode. In Torch Mode, FET-T is enabled allowing current to flow

through it. FET-T has an on-resistance of 0.98

Ω and is capable of handling currents up to 200mA. In Flash

Mode, both FET-T and FET-F are enabled. FET-F has an on-resistance of 0.36

Ω and is capable of handling

currents up to 500mA. The total Flash current is equal to the sum total of the current flowing through FET-T and

FET-F. See the CURRENT SET EQUATIONS in the Application Informations section for more information

regarding setting LED current .

In shutdown mode, the LM3551/2 provide a true load disconnect helping to keep the total shutdown current to a

minimum.

Over-Voltage Protection (OVP)

The over voltage protection (OVP) is engaged when a failure mode occurs (FB pin grounded, Flash LED

becomes open or disconnected, etc.). In the event of a failure, OVP prevents the output voltage from exceeding

12.4V (typ). When the OVP level is reached, the switch FET shuts off preventing the output voltage from climbing

higher. Once the FET has shut off, the output will droop at a rate determined by the value of the output capacitor

and current leakage through the OVP pin and any other leakage path. When the output voltage drops to

10.6V(typ), switching will resume. The LM3551 and LM3552 will go back into OVP if the failure is still present

resulting in a pulsed output condition.

NOTE

To disable OVP, ground to OVP pin. CAUTION: The LM3551 and LM3552 may be

damaged if an OVP condition occurs and OVP is disabled.

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