DN1157

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.
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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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