DN1157 Electronic Cigarette (eCig) Design Note Background The electronic cigarette (eCig) industry has grown dramatically over the last few years and the growth is expected to continue. Higher power eCigs require voltage and current control from an MCU and power switching using MOSFETs and gate drivers. In an eCig driver, the voltage may need to be stepped up (boost) or stepped down (buck). Figure 1 shows a typical boost arrangement that may be used in an eCig. When step up is needed, the MCU applies PWM to the boost circuit (Figure 1, to IN of the DGD2104A) and the output voltage (to buck) is higher than the battery voltage. When step down is needed, the Q1 is on 100% duty cycle providing the battery voltage to the buck (not shown) and the MCU provides PWM to the buck. . Figure 1: Typical Boost Schematic for eCig Driver Gate Driver Performance and UVLO Figure 1 shows a typical synchronous boost arrangement that could be used in an eCig application. Considering Figure 1, when Q1 is on at 100% duty cycle, VS is at battery voltage; the Li-ion battery in an eCig is 3.7V typical (min of 3.2V and max 4.2V). That means VS = 3.7V typically when the Q1 is on in the 100% duty cycle state (and even at start up).VBS is the supply for the high side driver for DGD2104A (Figure 1). To improve the safety of the full system during a fault condition or unexpected power down, many Diode’s Gate Driver ICs have VBS UVLO (Under Voltage Lock Out). The DGD2104A has a VBS UVLO+ (positive going threshold) of 8.9V typical and a VBS UVLO- (negative going threshold) of 8.2V typical. Increasing Vcc In the 100% duty state, VBS is supplied by VCC through the boot strap diode (DB) or a MOSFET. Hence to function in this type of circuit, VCC must be at a level to maintain the optimal supply conditions for the high side. The suggested value of VCC can be calculated as follows: VCC = (VBS UVLO+) + (VD) + (BAT (max)) Where VD is the voltage drop of the bootstrap diode, and BAT (max) is maximum value of the Li-ion battery. DN1157 – Rev 1 1 of 2 www.diodes.com January 2016 © Diodes Incorporated DN1157 As an example, considering the VBS UVLO+ = 8.9V o f the DGD2104A, a n d BAT (max) = 4.2V and VD = 0.6V, you would require a VCC = 13.7V. Hence for eCig applications, with requirements like those above, we recommend a VCC = 14V. If a Schottky diode or a MOSFET are used in the VCC bootstrap line, then there would be a smaller voltage drop, and a lower VCC could be used in the application. Best selection of VCC will be determined by the application and part selection. 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Copyright © 2016, Diodes Incorporated www.diodes.com DN1157 – Rev 1 2 of 2 www.diodes.com January 2016 © Diodes Incorporated