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ADD5203 データシート(PDF) 11 Page - Analog Devices |
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ADD5203 データシート(HTML) 11 Page - Analog Devices |
11 / 24 page ADD5203 Rev. 0 | Page 11 of 24 THEORY OF OPERATION CURRENT-MODE, STEP-UP SWITCHING REGULATOR OPERATION The ADD5203 uses a current-mode PWM boost regulator to provide the minimal voltage needed to enable the LED string at the programmed LED current. The current-mode regulation system allows fast transient response while maintaining a stable output voltage. By selecting the proper resistor-capacitor network from COMP to GND, the regulator response can be optimized for a wide range of input voltages, output voltages, and load conditions. The ADD5203 can provide a 45 V maximum output voltage and drive up to 13 LEDs (3.4 V/30 mA type of LEDs) for each channel. INTERNAL 3.3 V REGULATOR The ADD5203 contains a 3.3 V linear regulator. The regulator is used for biasing internal circuitry. The internal regulator requires a 1 μF bypass capacitor. Place this bypass capacitor between VDDIO (Pin 4) and GND, as close as possible to Pin VDDIO. BOOST CONVERTER SWITCHING FREQUENCY The ADD5203 boost converter switching frequency is user adjustable, between 350 kHz to 1 MHz, by using an external resistor, RF. A frequency of 350 kHz is recommended to optimize the regulator for high efficiency, and a frequency of 1 MHz is recommended for small external components. See Figure 14 for considerations when selecting a switching frequency and an adjustment resistor (RF). 1100 1000 900 800 700 600 500 400 300 120 170 220 270 320 370 420 470 RF (kΩ) Figure 14. Switching Frequency vs. RF DIMMING FREQUENCY (fPWM) The ADD5203 contains an internal oscillator to generate the PWM dimming signal for LED brightness control. The LED dimming frequency (fPWM) is adjustable, in the fPWM range of 200 Hz to 10 kHz, by using an external resistor (RFPWM) and capacitor (CFPWM). The RFPWM should be in the range of 13 kΩ to 110 kΩ, and the CFPWM should be in the range of 20 pF to 390 pF. Table 8. RFPWM and CFPWM Recommendation Dimming Freq (fPWM) RFPWM (kΩ) CFPWM (pF) 200 Hz 110 390 500 Hz 75 200 1 kHz 50 150 5 kHz 18 47 10 kHz 13 20 CURRENT SOURCE The ADD5203 contains eight current sources to provide accurate 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 30 mA by an external resistor. The ADD5203 contains an LED open and short fault protection circuit for each channel. If the headroom voltage of the current source remains below 200 mV while the boost converter output reaches the OVP level, the ADD5203 recognizes that the current source has an open load fault for the current source, and the current source is disabled. If the headroom voltage of the current source goes above 7.2 V, the current source is disabled for short protection. If an application requires four LED strings, each LED string should be connected using FB1 to FB4. Tie the unused FB pins (FB5 to FB8) to GND. The ADD5203 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 Figure 2, the ADD5203 has an LED current set pin (ISET). A resistor (RSET) from this pin to ground adjusts the LED current up to 30 mA. LED current level can be set with the following equation: ) ( 2831 A R I SET LED = DIMMING CONTROL INTERFACE The ADD5203 dimming control interface method is selectable between the SMBus serial input and/or the external PWM input. The LED dimming modes supported by ADD5203 can be controlled externally through these dimming control interfaces. The SEL1 and SEL2 pins should be set based on the application conditions (see Table 9). Table 9. Brightness Control Mode Selection SEL1 SEL2 Dimming Mode Interface High High Fixed delay PWM SMBus Low No delay PWM SMBus Open High Fixed delay PWM PWM Low No delay PWM PWM Low High DC current SMBus Open DC current PWM Low Direct PWM PWM |
同様の部品番号 - ADD5203 |
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同様の説明 - ADD5203 |
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