LP8556: BL flicker issue

Jeff Yang76

Intellectual 490 points

Other Parts Discussed in Thread: LP8556

art Number: LP8556

Hi,

We have meet LP8556 BL flicker issue, as initial verify result, seems it caused by output current ripple not stable.

Even when Vin= 4.4V or 3.6V, current ripple ~15mA.

And when Vin under ~3.8V, PWM = low, Vout force raise up to Vboost_max, seems trigger OVP condition.

Is there any register setting is incorrect?

Below is our LP8556 application:

Vin=3.4~4.4V(1 series cell system), 4S3P LED, per LED forward voltage typ=5.9V. Iout = 60mA, using PWM direct control.

0x98 = 0x9f, 0x9E = 0x22, 0xA0 = 0xff, 0xA1 = 0xef, 0xA2 = 0x2f, 0xA3 = 0x2, 0xA4 = 0x72, 0xA5 = 0xf4, 0xA6 = 0x80, 0xA7 = 0xf5, 0xA9 = 0xe0,

0xAA = 0xf, 0xAB = 0x0, 0xAC = 0x0, 0xAD = 0x0, 0xAE = 0xf, 0xAF = 0x1, 0x00 = 0x0, 0x01 = 0x81, 0x02 = 0x30, 0x03 = 0x1, 0x04 = 0x0, 0x16 = 0x3f

Schematic:

Waveform:

Vin= 4.4V

CH1: Vout

CH4: LED1+LED2 Iout

Vin= 3.6V

CH1: Vout

CH4: LED1+LED2 Iout

Waveform:

Vin= 4.4V

CH1: PWM

CH4: LED1+LED2 Iout

Waveform:

Vin= 3.6V

CH1: PWM

CH4: LED1+LED2 Iout

over 7 years ago

0 Caryss Li over 7 years ago

TI__Expert 4550 points

Hi Jeff,
A few changes to register.
0xA2 ->0x29
0xA7 -> 0xF6
0xAE -> 0x0E

I will close the post, since I have been reached by email. We will discuss by emails.

0 Jeff Yang76 over 7 years ago in reply to Caryss Li

Intellectual 490 points

Hi Caryss,

We found it should be trigger input current limit or inductor saturation,

Because we raise up input voltage to Vin=12V, it can solve output current ripple not stable and trigger OVP when PWM signal is low.

But I still have some question as below.

A7h bit[1:0] IBOOST_LIM, 2.6A is the maximum setting? / IBOOST_LIM_2X = 1 and IBOOST_LIM= 11 , not permitted means no limit or can set this value?

Because I measure input peak power consumption about~10W, means when Vin= 3.3V input current will raise to ~3A. It will trigger input current limit.

Thanks!

Jeff

0 Jeff Yang76 over 7 years ago in reply to Jeff Yang76

Intellectual 490 points

Hi Caryss,

Any update for these questions? thanks!

Jeff

0 Caryss Li over 7 years ago in reply to Jeff Yang76

TI__Expert 4550 points

Hi Jeff,
2.6A is the max current limit, IBOOST_LIM_2X = 1 and IBOOST_LIM= 11 cannot be set, we cannot predict the behavior.

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LP8556: BL flicker issue