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LM3552 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM3552 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 15 page 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 = 0.165 Ω) 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 1.2M Ω 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 1.2M 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 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 0.98 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 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 www.national.com 9 |
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