Value process speeds design of automotive interior lighting: Part 2 - The switch mode power supply solution

Referring to the first figure in Part 1, the parallel constant current drive scheme for the LEDs dictate a buck topology. The partial schematic of the figure below shows one power converter and 1 LED.

The operation of this circuit is as follows: The NCV8842 buck converter operating at approximately 170 kHz constant frequency provides a large step ratio for V_out/V_in by varying the duty cycle of the device's integrated 1.5A main switch. The switch action at pin 1 of U1 at the start of a cycle causes the current in inductor L1 to rise. An internal generated voltage ramp is generated with an internal clock. A DC component and instantaneous component of the output signal is used at pin 9, the inverting input of the error amplifier. This double feedback loop makes for fast transient response and may be beneficial if PWM (pulse width modulation) dimming is required.

These two fast and slow components are then used by the PWM comparator to determine the switch's duty cycle. The switch, commanded to turn off D3, allows a recirculation path for the peak inductor current while the load current is provided by C1. At the beginning of the next cycle then, the switch is turned back on and the process repeats at the internal clock frequency. The NCV8842, in the event of an open load condition, will remain in regulation due to the internal pulse skip mode at a light or no load condition.

References 1 and 2 can be used for efficiency evaluation. The load current and V_input voltage place a burden on C2, the input capacitor, because the input current in a buck converter is chopped or pulsed, causing large RMS currents to flow in C2. Therefore care should be used in derating the input capacitor. C2 should also be chosen for low ESR (equivalent series resistance) because the pulsed current across the parasitic ESR will cause a ripple voltage on the input power line feed. Depending on test conditions, such as conducted emissions onto power lines testing, an additional inductor/choke may be needed in series between D1 and C2.

Although the NCV8842 can be synchronized to an external clock, this pin is left unused for the application. The output voltage is programmed to be 5V with R1 and R2 and internal 1.270V reference. Compensation is made easy with a 0.1 μF ceramic on the Vc pin to ground. The device can be shut down via Q2, and consume less than 1 μA.

An established regulated output voltage (5V at C1) would then feed the anode of the LED D5 with a constant current . The constant current is established with the low side current sink U2A and Q1. A divided down reference voltage via R4 and R5 feed the non-inverting input of the op amp with approximately 1V. The inverting input maintains the 1V reference across R6 to set a constant current sink delivered from the 5V supply.

I_LED = 1V/6.6 ohms = 150 ma