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NCV3066MNTXG Datasheet(PDF) 8 Page - ON Semiconductor |
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NCV3066MNTXG Datasheet(HTML) 8 Page - ON Semiconductor |
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8 / 20 page  NCP3066, NCV3066 http://onsemi.com 8 Peak Current Sense Comparator Under normal conditions, the output switch conduction is initiated by the Voltage Feedback comparator and terminated by the oscillator. Abnormal operating conditions occur when the converter output is overloaded or when feedback voltage sensing is lost. Under these conditions, the Ipk Current Sense comparator will protect the Darlington output Switch. The switch current is converted to a voltage by inserting a fractional ohm resistor, RSense, in series with VCC and Darlington output switch. The voltage drop across RSense is monitored by the Current Sense comparator. If the voltage drop exceeds 200 mV (nom) with respect to VCC, the comparator will set the latch and terminate the output switch conduction on a cycle−by−cycle basis. Real Vturn−off on Rs Resistor t_delay I1 Io di/dt slope I through the Darlington Switch Vipk(sense) Figure 13. Current Sense Waveform The VIPK(Sense) Current Limit Sense Voltage threshold is specified at static conditions. In dynamic operation the sensed current turn−off value depends on comparator response time and di/dt current slope. Real Vturn−off on Rsc resistor Vturn_off = Vipk(sense) + RSense*(tdelay*di/dt) Typical Ipk comparator response time tdelay is 350 ns. The di/dt current slope is dependent on the voltage difference across the inductor and the value of the inductor. Increasing the value of the inductor will reduce the di/dt slope. It is recommended to verify the actual peak current in the application at worst conditions to be sure that the max peak current will never get over the 1.5 A Darlington Switch Current max rating. Thermal Shutdown Internal thermal shutdown circuitry is provided to protect the IC in the event that the maximum junction temperature is exceeded. When activated, typically at 160 °C, the Darlington Output Switch is disabled. The temperature sensing circuit is designed with some hysteresis. The Darlington Switch is enabled again when the chip temperature decreases under the low threshold. This feature is provided to prevent catastrophic failures from accidental device overheating. It is not intended to be used as a replacement for proper heatsinking. Output Switch The output switch is designed in Darlington configuration. This allows the application designer to operate at all conditions at high switching speed and low voltage drop. The Darlington Output Switch is designed to switch a maximum of 40 V collector to emitter voltage and current up to 1.5 A. ON/OFF Function The ON/OFF function provides interruption of switching and puts the circuitry into the low consumption mode. This feature is applicable for digital dimming of the LEDs as well. The ON/OFF signal inhibits switching of the regulator and reduces the average current through the LEDs. The frequency of this pulse width−modulated signal with the duty cycle can range from less than 1% to 100% is limited by the value of 1 kHz. Pulling this pin below 0.8 V or leaving it opened turns the regulator off. In this state the consumption of the device is reduced below 100 uA. Pulling this pin above 2.4 V (up to max. 25 V) allows the regulator running in normal state. If the ON/OFF feature is not needed, the ON/OFF pin can be wired to VCC, provided this voltage does not exceed 25 V. No Output Capacitor Operation A traditional buck topology includes an inductor followed by an output capacitor which filters the ripple. The capacitor is placed in parallel with the LED or array of LEDs to lower the ripple current. A constant current buck regulator such as the NCP3066 focuses on the control of the current through the load, not the voltage across it. The switching frequency of the NCP3066 is in the range of 100−250 kHz which is much higher than the human eye can detect. By configuring the NCP3066 in a continuous conduction buck configuration with low peak to peak ripple the output filter capacitor can be eliminated. The important design parameter is to keep the peak current below the maximum current rating of the LED. Using 15−40% peak to peak ripple results in a good compromise between achieving max average output current without exceeding the maximum limit. This saves space and reduces part count for applications. |
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