Dimming car lights using digital voltage divider

In automobiles, as well as other lighting applications, it is often necessary to adjust interior lighting. Using a mechanical voltage divider to do this wastes power, is relatively inefficient, and has durability issues.

The design in this application note takes advantage of the user's familiarity with the functions of the up/down control keys in cellular phones and uses a low-cost digital potentiometer (digital voltage divider) to achieve intensity dimming control. The use of digital voltage dividers not only avoids the inefficiency and mechanical durability problems of mechanical voltage dividers, but also greatly saves power consumption and improves efficiency.

Car light dimming solution

This design uses a non-volatile digital voltage divider to establish the dimming threshold and a separate pseudo-sawtooth oscillator for pulse-width modulation (PWM) lighting control. The MAX5475 (U1) is a 32-tap nonvolatile digital voltage divider with 100kΩ end-to-end resistance. This design uses a double-pole double-throw (DPDT) switch (SW1) to control the digital voltage divider. One end controls the U/D pin of the MAX5475, and the other end controls the INC pin. Therefore, pressing the switch in the "up" direction produces a high-to-low transition on INC, incrementing the digital voltage divider; pressing the switch in the "down" direction decrements the digital voltage divider. The nonvolatile nature of the MAX5475 brush position allows dimming settings to be maintained even when power is removed.

The digital voltage divider wiper (VWIPER) is fed into the inverting input of the LMX358 (U2) dual op amp. Then and the pseudo sawtooth generated by the other input of the dual operational amplifier

Waves are compared. Basically, the sawtooth ramp creates a PWM drive for FET Q1. Increasing VWIPER increases the duty cycle, FET on time, and lamp brightness. (In practice, since the lamp includes an RC charging network formed by R1 and C1, it is non-linear. However, the waveform is well suited for low-cost duty cycle control during the charging interval). Logic-level n-channel enhancement-mode FETs drive the lamps. Q1 should be selected based on the load requirements of the lamp.

To generate a sawtooth oscillator, the LMX358 should be configured as shown in Figure 1. Resistors R2, R3 and R4 create oscillation hysteresis. With a 5V input, the amplifier IN2+ input triggers between 0.41V and 4.55V. Note that reducing the value of R2 will increase the rising range; increasing the value of R2 will reduce the rising range. This design uses a 10k R2 to achieve its rising range within the accuracy of the MAX5475 digital voltage divider. When capacitor C1 charges and discharges through resistor R1 between the 0.41 and 4.55V thresholds, a rising ramp is generated.

The oscillation period should be relatively slow to reduce the effects of FET turn-on and turn-off losses. In order to calculate the oscillation period (TPERIOD), TDISCHARGE and TCHARGE must first be calculated, where VIN2+(high) = 4.55V, and VIN2+(low) = 0.41V.

In this case, TPERIOD = 12kHz.

Summarize

This application note describes how to use a low-cost digital voltage divider to implement PWM dimming control. The design uses up/down control keys similar to those found in cellular phones, taking advantage of familiar up/down control functions without requiring a microprocessor, reducing costs. While using a mechanical voltage divider for car light dimming is not only inefficient but also has durability issues, this design uses a digital voltage divider to improve efficiency and save power consumption.

2008-1-29 21:35:58