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ADD5207ACPZ-RL Datasheet(PDF) 11 Page - Analog Devices |
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ADD5207ACPZ-RL Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page Data Sheet ADD5207 Rev. A | Page 11 of 16 THEORY OF OPERATION CURRENT MODE, STEP-UP SWITCHING REGULATOR OPERATION The ADD5207 uses a current mode PWM boost regulator to generate the minimum voltage needed to drive the LED string at the programmed LED current. The current mode regulation system allows a fast transient response while maintaining a stable output voltage. By selecting the proper resistor-capacitor network from COMP to GND, the regulator response is optimized for a wide range of input voltages, output voltages, and load conditions. The ADD5207 can provide a 36 V maxi- mum output voltage and drive up to 10 LEDs (3.4 V/25 mA type of LEDs) for each channel. INTERNAL 3.3 V REGULATOR The ADD5207 contains a 3.3 V linear regulator that is used for biasing internal circuitry. The internal regulator requires a 1 μF bypass capacitor. Place this bypass capacitor between Pin VDD (Pin 8) and GND, as close as possible to Pin VDD. BOOST CONVERTER SWITCHING FREQUENCY The ADD5207 boost converter switching frequency is user adjustable, between 600 kHz to 1 MHz, by using an external resistor, RF. A frequency of 600 kHz is recommended to optim- ize the regulator for high efficiency, and a frequency of 1 MHz is recommended to minimize the size of external components. See Figure 14 for considerations when selecting a switching frequency and an adjustment resistor (RF). 1000 900 800 700 600 500 400 300 80 100 120 140 160 180 200 220 RF (kΩ) Figure 14. Switching Frequency vs. RF DIMMING FREQUENCY (fPWM) The ADD5207 contains an internal oscillator to generate the PWM dimming signal for LED brightness control. The LED dimming frequency (fPWM) is fixed at 8 kHz internally. CURRENT SOURCE The ADD5207 contains four current sources to provide accu- rate current sinking for each LED string. String-to-string tolerance is kept within ±1.5% at 20 mA. Each LED string current is adjusted up to 25 mA using an external resistor. The ADD5207 contains an LED open fault protection circuit for each channel. If the headroom voltage of the current source remains below 150 mV while the boost converter output reaches the OVP level, the ADD5207 recognizes that the current source has an open-load fault for the current source, and the current source is disabled. If an application requires three LED strings, each LED string should be connected using FB1 to FB3. The unused FB4 pin should be tied to GND. The ADD5207 contains hysteresis to prevent the LED current change that is caused by a ±0.195% jitter of the PWM input. Programming the LED Current As shown in the Figure 2, the ADD5207 has an LED current set pin (ISET). A resistor (RSET) from this pin to ground adjusts the LED current up to 25 mA. LED current level can be set with following equation: ) ( 3600 A R I SET LED = PWM DIMMING MODE The ADD5207 supports 8-bit resolution to control brightness. However, each current source has a minimum on time require- ment for LED current regulation such that the dimming is in the range of 1.5% to 100%. Accordingly, even when the PWM input duty cycle is more than 0% and less than 1.5%, the LED duty cycle is held at 1.5%. Phase Shift PWM Dimming There is a phase delay between each current source channel that is programmed by the number of current sources in operation. If the application requires four separate LED strings, each string has a 90 degree phase delay between channels. If three LED strings are connected at the FB1 to FB3 pins (FB4 = GND), each string has a 120 degree phase delay. SAFETY FEATURES The ADD5207 contains several safety features to provide stable and reliable operation. Soft Start The ADD5207 contains an internal soft start function to reduce inrush current at startup. The soft start time is typically 1.1 ms. Overvoltage Protection (OVP) The ADD5207 contains OVP circuits to prevent boost converter damage if the output voltage becomes excessive for any reason. To keep a safe output level, the integrated OVP circuit monitors |
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