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LM3402MR Datasheet(PDF) 11 Page - Texas Instruments |
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LM3402MR Datasheet(HTML) 11 Page - Texas Instruments |
11 / 38 page fSW = VO 1.34 x 10 -10 x RON VO = n x VF + 200 mV LM3402/02HV CS VSNS IF RSNS IF + - VREF + - One-shot CS Comparator VO VF VF LED 1 LED n tON = 1.34 x 10 -10 x RON VIN LM3402, LM3402HV www.ti.com SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013 APPLICATION INFORMATION THEORY OF OPERATION The LM3402 and LM3402HV are buck regulators with a wide input voltage range, low voltage reference, and a fast output enable/disable function. These features combine to make them ideal for use as a constant current source for LEDs with forward currents as high as 500 mA. The controlled on-time (COT) architecture is a combination of hysteretic mode control and a one-shot on-timer that varies inversely with input voltage. Hysteretic operation eliminates the need for small-signal control loop compensation. When the converter runs in continuous conduction mode (CCM) the controlled on-time maintains a constant switching frequency over the range of input voltage. Fast transient response, PWM dimming, a low power shutdown mode, and simple output overvoltage protection round out the functions of the LM3402/02HV. CONTROLLED ON-TIME OVERVIEW Figure 19 shows the feedback system used to control the current through an array of LEDs. A voltage signal, VSNS, is created as the LED current flows through the current setting resistor, RSNS, to ground. VSNS is fed back to the CS pin, where it is compared against a 200 mV reference, VREF. The on-comparator turns on the power MOSFET when VSNS falls below VREF. The power MOSFET conducts for a controlled on-time, tON, set by an external resistor, RON, and by the input voltage, VIN. On-time is governed by the following equation: (1) At the conclusion of tON the power MOSFET turns off for a minimum off-time, tOFF-MIN, of 300 ns. Once tOFF-MIN is complete the CS comparator compares VSNS and VREF again, waiting to begin the next cycle. Figure 19. Comparator and One-Shot The LM3402/02HV regulators should be operated in continuous conduction mode (CCM), where inductor current stays positive throughout the switching cycle. During steady-state operationin the CCM, the converter maintains a constant switching frequency, which can be selected using the following equation: (2) VF = forward voltage of each LED, n = number of LEDs in series (3) Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3402 LM3402HV |
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