ST1CC40 application at 5V input driving single high-power LED at 2.2A

[iyer_22]

I am designing a LED driver board for an industrial instrument and am evaluating the ST1CC40 as a constant-current buck driver for the following application:

Application requirements:

• Input voltage: 5V DC (regulated)

• LED: Cree XLamp XP-L2 white LED

• Target forward current: 2.2A

• LED forward voltage: 3V at 2.2A, Tj = 85°C

From the ST1CC40 datasheet, I have confirmed the following:

• VIN range 3V–18V covers our 5V supply 

• Maximum limiting current 5A covers our 2.2A operating point 

• INH threshold VIH = 1.2V min is compatible with our 3.3V PWM signal 

• Duty cycle at operating point: 2.93V / 5V = 58.6% — within range 

• RSENSE = 97mV / 2.2A = 44mΩ (targeting 44mΩ E96 standard value)

I have the following technical questions:

1. With VIN = 5V and VOUT ≈ 2.93V (duty cycle ~58.6%), is there any concern with the internal bootstrap circuit or headroom voltage at this relatively low input voltage/ application?

2. For inductor selection at VIN = 5V, VOUT = 2.93V, ILED = 2.2A, FSW = 850kHz, could you confirm the recommended inductance value and saturation current rating? I intend to use the eDesignSuite tool but would welcome any guidance.

I am designing for an industrial instrument and reliability is a primary concern. Any guidance on derating or thermal recommendations for the SO-8 package at 2.2A continuous would also be appreciated.

[Peter BENSCH]

Welcome @iyer_22, to the community!

1. Your concern about a bootstrap issue is not applicable here: the ST1CC40 is a synchronous buck regulator, so the key question is not bootstrap headroom, but rather dropout margin, peak current, and thermal behaviour.
   At 5V in / 2.93V out, the duty cycle of about 58.6% is well within a normal operating range and would lead to an (estimated) peak current of 2.2A/0.586 = 3.75A.
2. For the inductor, your concern is valid: if this estimated peak current is around 3.75A, then an inductor with only 3.6A saturation current is too tight for an industrial design. I would recommend using a part with more headroom, ideally with saturation current comfortably (20...30%) above the expected peak current.

For the SO-8 thermal design at 2.2A continuous, good PCB copper area, thermal vias, and a conservative derating approach are strongly recommended. Also, the sense resistor dissipation is non-negligible and should be included in the thermal budget. Please use the EVAL boards STEVAL-ILL046V1 and STEVAL-ILL046V2 as a template and, depending on the maximum ambient temperature, feel free to equip them with larger cooling surfaces.

Hope that helps?

Regards
/Peter