1. TPS92691 SEPIC LED DRIVER SIMULATION
Requirement:
1. To drive 3 LEDs (SFH4725AS) in Series and total Forward Voltage 12.7V.
2. To drive minimum Current 200mA and maximum current 3A
3. Battery minimum voltage 7.4V, Nominal Voltage 12V and Maximum Voltage 18.5V

2. TPS92691 Sepic LED Driver Schematics
No Soft start Capacitor
PWM:

3. 1. VIADJ:2.1V for ILED 3A 1st Plot : PWM 2nd Plot: Output LED Current
3rd Plot: Coupled inductor Primary current , Secondary current
4thPlot: Gate Voltage
Observations:
I. 11A ripple (20uS) on L2(Primary side) is coming during 1st PWM ON
3.25A is output current ramp up and than Stable for 3A.
Output current ramp up after 600us of PWM
Gate is switching on as soon as first PWM ON so 11A ripple is coming

4. VIADJ:2.1V for ILED 3A Cont.…..
1st Plot : PWM
2nd Plot: Output LED Current
3rd Plot: Gate Current
4thPlot: Gate Voltage
1st Plot : Gate Voltage
2nd Plot: Coupled Inductor Primary and Secondary

5. 2. VIADJ:1.05V for ILED 1.5A 1st : PWM 2nd : Output LED Current
3rd : Gate Voltage
4th : Coupled inductor Primary current , Secondary current
Observations:
9.5A ripple ion L2 (Primary side) s coming during 1st PWM ON
1.85A is output current ramp up and than Stable for 1.5A
Output current ramp up after 1ms in 1st PWM..

6. 3. VIADJ: 0.14 V for ILED 200mA 1st : LED Forward Voltage 2nd : PWM
3rd : Output LED Current
4th : Coupled inductor Primary current , Secondary current
Observations:
8A ripple on L2 (Primary side) is coming during 1st PWM ON
350mA is output current ramp up and than Stable for 200mA
No output current for 2 PWM pulses, it will ramp up after 200us of 3rd PWM

7. VIADJ:0.14V for ILED 200mA Cont.…..
1st Plot : PWM
2nd Plot: Output LED Current
3rd Plot: Gate Voltage
4thPlot: Coupled Inductor Primary current
1st Plot : MOSFET Q1 current
2nd Plot: Coupled Inductor Primary and Secondary

8. VIADJ vs. ILED VIADJ ILED Time delay to ramp up Remarks 2.1 V 3A
600us in 1st PWM
ILED is sync with 2nd PWM onwards
1.05 V
1.5A
1ms in 1st PWM
0.14 V
200mA
200us in 3rd PWM
ILED is sync with 4th PWM onwards
As per simulation we need Coupled inductor 10uH with 12A DC current ,
N-CH MOSFET 15A

9. Queries
No Sepic calculation formulas in datasheet, Please specify which document to consider for Design calculations.
2. We have used “AN-1484 Designing A SEPIC Converter” for design calculations, please confirm whether is this correct document?
3. Gate is switching ON as soon as PWM is ON, due to this 11A ripple is coming in 20us. Please let us know how to avoid this.
4. As per calculation the Primary and Secondary current is 6.2A+3.6A=9.8A but we have observed in 11A in primary side itself, Please advise how to reduce this.
5. From Output high Current 3A to Low Current 200mA, Output LED current ramp up is getting delay. 3A – 600us in 1st PWM, 1.5A – 1ms in 1st WPM and 200mA -2 PWM and 200us in 3rd PWM. Please advise how to reduce this delay?

10. THANK YOU