LM3551 Datasheet, PDF (9/15 Pages) National Semiconductor (TI) – 1A White LED Driver with Flash Timeout Protection

Block Diagram
20151203
Circuit Description
OVERVIEW
The LM3551/2 are high power white LED flash drivers ca-
pable 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 lock-
out.
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 con-
verter utilizes a current-mode controlled, constant frequency
(1.25MHz.), PWM architecture. This architecture creates a
predictable noise spectrum that allows for easy filtering and
low noise. A very low on-resistance power NFET(R_DSON =
0.165Ω) and high value current limit (2.2A typ.) help effi-
ciently 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 shut-
down pin (SD) that turns on the part when a voltage less than
0.3V is applied. An internal 1.2MΩ pull-up to V_IN is provided
to place the LM3551 into shutdown when no control signal is
provided. The LM3552 utilizes a enable pin (EN) that turns
on the part when a voltage greater than 1.2V is applied. An
internal 1.2MΩ 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 mecha-
nism 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 0.98Ω 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 0.36Ω and is ca-
pable 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 EQUA-
TIONS in the Application Informations section for more infor-
mation regarding setting LED current .
In shutdown mode, the LM3551/2 provide a true load dis-
connect 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
9
www.national.com