TPS92513HV: LED current drive

Hello Hideki,

1)  Iadj will dim in an analog method.  If Iadj is reduced the DC value of current in the LEDs will reduce, see Figure 5 on page 7 of the TPS92513 datasheet.  PWM dimming turns the drive stage off and on, see 9.5 on page 20 for waveforms.  If PWM is high the output is driving current, if PWM is low the current goes to zero.  A 50% PWM at 200 Hz will cause the output current to be on 50% of the time and off 50% of the time at a rate of 200 Hz.  Iadj has a 10:1 dimming range, PWM dimming has a 100:1 dimming range.  The two dimming methods can be used at the same time.

2)  The TPS92513 is a current driver.  The current to the LEDs will remain constant if there is drop in the cable harness as long as the overall Vout doesn't get very near Vin.  Example  30V input 18V LED string and a 2V drop on the harness will work fine.  Note that the efficiency will be lower due to the power loss in the harness.

3)  Vstart and Vstop I'm guessing is the turn-on and turn-off thresholds for Vin.  Section 8.3.1 of the datasheet, page 9 and 10, shows the calculation for this.  There are two resistors use to set the Input voltage turn-on and turn-off levels.  Section 9.3 shows a design example, 9.4.2 shows the UVLO resistor calculations for the design example.

Best Regards,

Hello Hideki,

1)  The limitation will be the rise and fall time of the inductor current.  This depends on the PWM duty cycle as well as the PWM frequency.  Lower PWM frequency will allow lower PWM duty cycles.  The input voltage, LED voltage and inductor value also affect PWM dimming range.

3)  For input UVLO generally the hysteresis is setup to prevent turn-off/turn-on oscillation.  The input power source to the pwm driver will have some impedance.  If the driver turns on the input voltage will drop.  Without hysteresis the converter will cycle on and off at the UVLO threshold.  If there isn't a lot of input capacitance the input current will also be present as ripple voltage on the input which can also cause turn-off/turn-on oscillation.

4)  Yes, at the operating point of consideration.  The dynamic impedance, or Rled in equation 24, is the slope of the Vf/If curve times the number of LEDs in the string.  An example would be a 1 ampere LED that will be run at 500 mA maximum.  On the Vf/If curve the slope of the line where Vf intersects at 500 mA is Rled.  Another way to calculate for this example would be to take Vf at 400 mA and Vf at 600 mA and calculate the slope of those two points, (Vf600mA - Vf400 mA)/(600 mA - 400 mA) times the number of LEDs in the LED string.  If Vf600 mA is 3.05V, Vf400 mA is 2.95V and there are 8 LEDs, Rled = ((3.05V - 2.95V)/(600 mA - 400 mA)) * 8 = 4 ohms.

The highest ripple will be at the highest operating current and where Rled should be calculated.  The TPS92513 is a buck converter so the ripple will stay fairly constant at different operating currents provided it is running in continuous conduction mode.  Rled decreases as operating current increases which means the ripple current will increase as operating current increases.

Best Regards,