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744770933 Scheda tecnica(PDF) 11 Page - Diodes Incorporated |
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744770933 Scheda tecnica(HTML) 11 Page - Diodes Incorporated |
11 / 33 page ZXLD1370 ZXLD1370 Document number: DS32165 Rev. 2 - 2 11 of 33 www.diodes.com May 2010 © Diodes Incorporated A Product Line of Diodes Incorporated b) Boost and Buck-Boost modes Control in Boost and Buck-boost mode is achieved by sensing the coil current in the series resistor Rs, connected between the two inputs of a current monitor within the control loop block. An output from the control loop drives the input of a comparator which drives the gate of the external NMOS switch transistor M1 via the internal Gate Driver. In boost and buck-boost modes, when the switch is on, current flows from VIN, via Rs, coil and switch to ground. This current ramps up until an upper threshold value is reached. At this point GATE goes low, the switch is turned off and the current flows via Rs, coil, D1 and LED back to VIN (Buck-boost mode), or GND (Boost mode). When the coil current has ramped down to a lower threshold value, GATE goes high, the switch is turned on again and the cycle of events repeats, resulting in continuous oscillation. The average current in the coil is equal to the average of the maximum and minimum threshold currents and the ripple current (hysteresis) is equal to the difference between the thresholds. Figure 15: Boost and Buck-Boost configuration The average current in the LED is always less than the average current in the coil and the ratio between these currents is set by the values of external resistors RGI1 and RGI2. The peak LED current is equal to the peak coil current. The control loop maintains the average LED current at the set level by adjusting the thresholds and the hysteresis continuously to force the average current in the coil to the value demanded by the voltage on the ADJ and GI pins. This minimises variation in output current with changes in operating conditions. Loop compensation is achieved by a single external capacitor C2, connected between SHP and SGND. Figure 16 - Operating waveforms (Boost and Buck-boost modes) Note: In Boost and Buck-boost modes, average ILED= average ICOIL x RGI1/(RGI1+RGI2) For more detailed descriptions of device operation and for choosing external components, please refer to the application circuits and descriptions in the later sections of this specification. ~15V 0V VVIN VVIN -225mV 225mV/Rs 0A Gate Voltage ISM Voltage Coil current 225mV/Rs 0A LED current Average LED current tOFF tON |
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