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CPC9909 Datasheet(PDF) 7 Page - IXYS Corporation |
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CPC9909 Datasheet(HTML) 7 Page - IXYS Corporation |
7 / 12 page ![]() INTEGRATED CIRCUITS DIVISION CPC9909 R03 www.ixysic.com 7 2.3 Input Voltage Regulator The CPC9909 has an internal voltage regulator that can work with input voltages ranging from 12VDC to 550VDC. When a DC voltage greater than 12V is applied at the VIN pin, the internal voltage regulator regulates the voltage down to a typical 7.8V. The VDD pin is the internal voltage regulator output pin and must be bypassed by a low-ESR capacitor to provide a low impedance path for high frequency switching noise. The CPC9909 driver does not require the bulky start-up resistors typically needed for off-line controllers. The internal voltage regulator provides sufficient voltage and current to power internal IC circuits. This voltage is also available at the VDD pin, and can be used as a bias voltage for external circuitry. The internal voltage regulator can be bypassed by applying an external DC voltage to the VDD pin that is slightly higher than the internally generated regulator voltage. This reduces the power dissipation of the integrated circuit, and it is more suitable in isolated applications where an auxiliary winding can be used to drive the VDD pin. The total input current drawn from the VIN pin is equal to the quiescent current drawn by the internal circuitry (which is specified at 0.6mA maximum) plus the gate driver current. See “Shut-Down Mode Supply Current” in Section 1.5 “Electrical Characteristics” on page 5. The current draw of the gate driver depends on the switching frequency and the gate charge of the external power MOSFET. The total input current can be calculated by: Where QGATE is the total gate charge of the MOSFET and fS is the oscillator external frequency. 2.4 Current Sense Resistor The peak LED current is set by an external sense resistor (RSENSE) connected from the CS pin to ground. The value of the current sense resistor is calculated based on the average LED current desired, the current sense threshold, and the inductor ripple current. The peak-to-peak difference in the inductor current waveform is referred to as inductor ripple current (the inductor is typically selected to be large enough to keep this ripple within 30% of the average). Factor in the ripple current when calculating the sense resistor. The current sense resistor value can be found by: Where: • V CS(high) = current sense threshold =0.25V (or VLD) • I LED = average LED/inductor current • I L = inductor ripple current = 0.3*ILED Combining terms: 2.5 Current Sense Blanking The CPC9909 has an internal current sense blanking circuit. When the power MOSFET is turned on, the external inductor can cause an undesired spike at the current sense pin, initiating a premature termination of the gate pulse. To avoid this condition, a typical 400ns internal leading edge blanking time is implemented, thereby eliminating the need for external RC filtering, and simplifying the design. During the current sense blanking time, the current limit comparator is disabled, preventing the gate-drive circuit from terminating the gate-drive signal. 2.6 Enable/Disable Function Connecting the PWMD pin to VDD enables the gate driver. Connecting PWMD to GND disables the gate driver and sets the device in standby mode. In standby mode, the quiescent current is 0.6mA maximum. 2.7 Minimum Off-Time One-Shot The CPC9909 uses a fixed off-time control scheme. The minimum off-time is set by an external resistor connected between the RT and GND terminals. The off-time can be determined by: Off-time selection indirectly determines the switching frequency of the LED driver. IIN 0.6mA QGATE fS + RSENSE VCS high ILED 0.5IL + ---------------------------------- = RSENSE VCS high 1.15 ILED ---------------------------- = toff s RT k 66 0.8 + = |
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