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APW7136BCITRG Datasheet(PDF) 9 Page - Anpec Electronics Coropration |
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APW7136BCITRG Datasheet(HTML) 9 Page - Anpec Electronics Coropration |
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9 / 15 page  Copyright © ANPEC Electronics Corp. Rev. A.2 - Jan., 2008 APW7136A/B/C www.anpec.com.tw 9 Function Descriptions Main Control Loop The APW 7136 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 COMP node, which is the output of the error amplifier (EAMP). An external resistive divider connected between V OUT and ground al- lows the EAMP to receive an output feedback voltage V FB at FB pin. When the load current increases, it causes a slightly decrease in V FB relative to the 0.25V reference, which in turn causes the COMP voltage to increase until the average inductor current matches the new load current. VIN Under-Voltage Lockout (UVLO) The Under-Voltage Lockout (UVLO) circuit compares the input voltage at VIN with the UVLO threshold (2.2V, 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. When the input voltage falls below the UVLO falling threshold, the controller turns off the converter. Soft-Start The APW7136 has a built-in soft-start to control the N- channel MOSFET current rise during start-up. During soft- start, an internal ramp, connected to one of the inverting inputs, raise up to replace the output voltage of error am- plifier until the ramp voltage reaches the V COMP. Current-Limit Protection The APW 7136 monitors the inductor current, flowing through the N-channel MOSFET, and limits the current peak at current-limit level to prevent loads and the APW7136 from damages during overload conditions. Over-Temperature Protection (OTP) The over-temperature circuit limits the junction tempera- ture of the APW7136. When the junction temperature ex- ceeds 150οC, a thermal sensor turns off the power MOSFET, allowing the devices to cool. The thermal sen- sor allows the converters to start a soft-start process and regulate the output voltage again after the junction tem- perature cools by 40οC. The OTP designed with a 40οC hysteresis lowers the average Junction Temperature (T J) during continuous thermal overload conditions, increas- ing the lifetime of the device. Enable/Shutdown Driving EN to ground places the APW7136 in shutdown mode. When in shutdown, the internal power MOSFET turns off, all internal circuitry shuts down and the quiescnet supply current reduces to 1 µA maximum. 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 a open-loop operation. Therefore, an over- voltage protection (OVP), monitoring the output voltage via OVP pin, is integrated into the chip to prevent the LX and the output voltages from exceeding their maximum voltage ratings. When the voltage on the OVP pin rises above the OVP threshold, the converter stops switching and prevents the output voltage from rising. The converter can work again when the OVP voltage falls below the falling of OVP voltage threshold. This pin also could be used as a digital input allowing brightness control using a PWM signal from 100Hz to 100KHz. The 0% duty cycle of PWM signal corresponds to zero LEDs current and 100% corresponds to full one. |
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