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APW7215 Datasheet(PDF) 10 Page - Anpec Electronics Coropration |
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APW7215 Datasheet(HTML) 10 Page - Anpec Electronics Coropration |
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10 / 17 page  Copyright © ANPEC Electronics Corp. Rev. A.2 - Oct., 2012 www.anpec.com.tw 10 APW7215 Function Description Main Control Loop The APW 7215 is a constant frequency current-mode switching regulator. During normal operation, the inter- nal N-channel power MOSFET is turned on each cycle when the oscillator sets an internal RS latch and turned off when an internal comparator (ICMP) resets the latch. The peak inductor current at which ICMP resets the RS latch is controlled by the voltage on the internal COMP node, which is the output of the error amplifier (EAMP). An external current-sense resistor connected between cath- ode of the lowest LED and ground allows the EAMP to receive a current feedback voltage V FB at FB pin. When the LEDs voltage decreases to cause the LEDs current to decrease, it causes a slightly decrease in V FB relative to the reference voltage, which in turn causes the internal COMP voltage to increase until the LEDs current reaches the set point. VIN Under-Voltage Lockout (UVLO) The Under-Voltage Lockout (UVLO) circuit compares the input voltage at VIN with the UVLO threshold (2.4V rising, typical) to ensure the input voltage is high enough for reliable operation. The 100mV (typ) hysteresis prevents supply transients from causing a restart. Once the input voltage exceeds the UVLO rising threshold, startup begins. W hen the input voltage falls below the UVLO falling threshold, the controller turns off the converter. Soft-Start The APW7215 has a built-in soft-start to control the N chan- nel MOSFET current raises during start-up. During soft- start, an internal ramp voltage connected to one of the inverting inputs of the current limit comparator. The in- ductor current limit is proportional to the voltage. When the threshold voltage of the internal soft-start comparator is reached, the full current limit is released. Current-Limit Protection The APW 7215 monitors the inductor current flowing through the N-channel MOSFET, and limits the current peak at current-limit level to prevent loads and the device from damages in overload conditions. Over-Temperature Protection (OTP) The over-temperature circuit limits the junction tempera- ture of the APW7215. When the junction temperature ex- ceeds 150oC, a thermal sensor turns off the power MOSFET, allowing the device to cool. The thermal sen- sor allows the converter to start a soft-start process and regulate the LEDs current again after the junction tem- perature cools by 40oC. The OTP is designed with a 40oC hysteresis to lower the average Junction Temperature (T J) during continuous thermal overload conditions, in- creasing the lifetime of the device. Enable/Shutdown Driving EN to ground places the APW7215 in shutdown mode. When in shutdown, the internal power MOSFET turns off, all internal circuitry shuts down and the quies- cent supply current reduces to 1 µA maximum. This pin also could be used as a digital input allowing brightness controlled by using a PWM signal with frequency from 5kHz to 100kHz. The 0% duty cycle of PWM signal corre- sponds to zero LEDs current and 100% corresponds to full one. If use EN Pin to enable the device, suggestion dimmimg duty range is from 15% to 100% at 100kHz dimmimg frequency. Open-LED Protection In driving LED applications, the feedback voltage on FB pin falls down if one of the LEDs, in series, is failed. Meanwhile, the converter unceasingly boosts the output voltage like an open-loop operation. Therefore, an over- voltage protection monitoring the output voltage via LX pin prevents the LX and the output voltages from exceed- ing their maximum voltage ratings. Once the voltage on the LX pin rises above the OVP threshold, the converter stops switching and prevents the output voltage from rising. The converter can work again when the LX voltage falls below the falling of OVP voltage threshold. |
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