How to implement a PWM dimming in a LED DC-DC Driver Module without microcontroller?
The input voltage VIN_LDM is the battery voltage modulated by a high side switch.
VIN-LDM is shaped by the reverse protection device and PI filter before supplying the DC/DC controller (see Figure 1). EN/PWMI pin (EN/PWMI/PWMA pin on TLD5099EP) and the input voltage of the DC/DC controller are connected together.
In the low duty cycle scenario, when the input voltage falls to 0 V the DC-DC input voltage VIN‑DC/DC discharges slowly and EN/PWMI pin detects delayed the falling edge of the PWM signal. When power supply has high duty cycle, VIN-DC/DC does not reach 0 V. In this scenario, EN/PWMI pin does not stop the DC/DC activity at the right time. As a result, the output current generated by the LDM does not fit the request of BCM sent by PWM through the power line.
Figure 1: Shape of input voltage after PI filter plus rectifier
A solution to increase the timing performance is depicted in Figure 2. The proposed circuit has EN/PWMI pin connected to VIN-LDM. This connection grants the EN/PWMI pin to detect also fast pulses and then PWM with high duty cycle are processed without degradation of performances. Moreover, VIN-DC/DC shows almost no ripple, because the capacitors of PI filter are not discharged. Two scenarios of PWM duty cycle are sketched where the small ripple on VIN-DC/DC is highlighted.
No extra protection is needed for EN/PWMI pin against reverse polarity because this pin has maximum absolute rating from -40 V up to 45 V. The resistor REN is a pull-down device to discharge the capacitance of EN/PWMI pin; 1 to 10 kΩ is a good value for this application.
Figure 2: Simplified schematic on connection of ENABLE pin
For further information check the LITIX Power webpage, while more information on how to design a proper solution can be found in the application note Z8F67392382 multitopology controllers with PWM dimming