AN2708 datasheet(18/42 Pages) STMICROELECTRONICS

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High-frequency ballast

AN2708

18/42

of the VIPer12A-E power switch in order to deliver the energy from the input to the output by

means of an inductor. PWM driver, power switch, thermal and overcurrent protection are

integrated in the same silicon chip ensuring minimum size and good performances at very

low cost.

Thanks to this implementation strategy the microcontroller is always supplied, allowing the

lamps to turn on, even when L6562 and L6574 are in a latched shutdown state.

The startup procedure is very important in an application that contains two different

sections. The ballast section starts before the PFC, avoiding any extra voltage at the PFC

section output, and consequently the L6562 OVP activation. This behavior is guaranteed

under all conditions because the VS turn-on threshold of L6574 is lower than that of the

L6562. The turn-on threshold is reached by a resistor chosen in order to ensure the startup

current of both the L6562 and the L6574. When the ballast section is running, the charge

pump (C11, R14, D3 and DZ1) supplies both the devices and the filter R17-C10 allows to

reduce the noise at Vcc.

2.2.3

Lamp turn-on and lamp turn-off

To get low-power consumption (less than 0.6 W) during the lamps’ turnoff state, both the

half-bridge and the PFC have to be disabled, even in the presence of the mains at the

ballast input. To manage this standby condition the L6574 control section and the L6562

ZCD pin are interfaced with the DALI slave microcontroller.

Short pulses (> 200 nsec) at the EN1 and EN2 inputs are recognized by the L6574. In

particular, EN1 high (> 0.6 V) stops all the half-bridge functions and puts the L6574 in a

latched shutdown state. At the same time, by forcing externally the ZCD pin to a voltage

below 150 mV, the L6562 is stopped. To cancel this status, in order to turn on the lamps, a

pulse (>0.6 V) is sent by the microcontroller to the L6574 second control pin EN2 and the

ZCD pin external pull down is removed. The half-bridge driver restarts the preheating and

ignition procedure, and the L6562 performs again its operation. The controls timing diagram

is shown in Figure 8.

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