AN2946 datasheet(13/38 Pages) STMICROELECTRONICS

13 / 38 page

AN2946

Hardware design

Doc ID 15473 Rev 2

13/38

3.1.3

LED lamp driving circuit

The LED lamp driver is designed with flyback topology. No isolation is required in this

application. Flyback is suitable for a wide ratio range of output voltage to input voltage. The

battery voltage is 11 V ~ 14.3 V while for the LED lamp, which is connected in a 3*7 matrix

(3 LED lamps in series and 7 strings in parallel), the maximum LED voltage is defined as

12 V. The flyback converter keeps the LED current constant in the above-mentioned battery

voltage range. In Figure 12, T1 is the flyback transformer and Q4 is the power MOSFET. D5

and D9 clamp the maximum voltage across Q4 in the off-state. D10 acts as the output

rectifier and C30 and C33 are the output capacitors. R46, R47 and U9 are used to sense the

LED lamp current and feedback to the MCU for constant current regulation. The PWM signal

from the MCU is converted from TTL level to CMOS level via U2 and amplified by Q7 and

Q8 to drive Q4. When OCP and/or OVP activate, Q3 and Q9 are used to guarantee a single

turn-on within each switching cycle. The MOSFET current is sensed by resistors R30, R33

and R54 and amplified by U5B. The output of U5B is used to achieve OCP. There are two

levels of OCP implemented in the LED lamp driver. The MOSFET current is sensed and

transferred to comparators U7A and U7B. U7B sets the first current limit which is activated

cycle by cycle. R37 and R36 form a voltage divider and set the threshold at the negative

input of U7B. Overcurrent in the primary circuit of T1 results in high logic output of U7B, thus

pulling down the voltage of the 'LED_Protection' node at the Q5 collector. Consequently,

comparator U2 outputs low voltage and forces Q4 to shut down. In case a heavy overload or

short-circuit occurs, such as a physical short-circuit of T1 or D10 which might exist for some

time, a second level OCP is needed to protect the driver. R41 and R43 form another voltage

divider and set a higher threshold. A high current spike from Q4 triggers the threshold and

U7A generates 'LED_Fault' interrupt to the MCU. After receiving continuous interrupts, the

MCU stops outputting the 'PWM_Driver' signal and waits for a certain time for the next try

(refer to Section 4.4). Such burst mode operation definitely lowers the voltage stress and

current stress on power components. The OVP of the LED lamp is achieved by D12. The

principal is similar to that of the first level of OCP.

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