SLUSE50 November 2023 TPS92642-Q1
PRODUCTION DATA
The TPS92642-Q1 features an internal analog circuit to impose a limit on the on-time, tPWN(ON) and off-time, tPWM(OFF), of the output current, ILED. As illustrated in Figure 6-7, the device is controlled by external UDIM pulses with widths less than tPWM_ON(LMT). Any external UDIM pulses that are longer than tPWM(LMT) are truncated to maintain a maximum fixed duty ratio of DPLMT. The device also rejects any spurious pulses that can be present on the external UDIM input by blanking the UDIM signal during the PLMT off time. The off time is a function of the period, tPLMT, set by the external capacitor, CPLMT. This mechanism is designed to help limit overexposure to infrared lights due to error conditions in DMS applications. The relationship between tPLMT, tPWM_ON(LMT) and DPLMT is given in Equation 10.
The desired pulse period, tPLMT, is set by external capacitor, CPLMT.
The maximum pulse on-duration, tPWM_ON(LMT), and minimum off-duration, tPWM_OFF(LMT), is given in Equation 12 and Equation 13.
For LED current set point greater than 2.5 A, the maximum pulse-duration is impacted due to the switching noise in the system. The maximum pulse-duration as a function of LED current is given in Equation 14. Because the off-duration, tOFF, does not change, the duty cycle ratio reduces with increase in LED current.
As an alternative, the PLMT pin can be externally driven by an external pulse generator or microcontroller to set the maximum on-duration and minimum off-duration, independent of the external UDIM input. In this case, the circuit functionality remains unchanged, however ,the pulse parameters are set by external pulse signal driving PLMT pin. The relationship between external PLMT signal and UDIM signal is shown in the following figure.
The pulse duty cycle limit function is disabled by connecting PLMT pin to GND. The LED current is controlled directly by the PWM signal connected to UDIM input.