LP8863-Q1 Automatic String Detection Deep Dive

The LP8863-Q1 device has a LED string configuration auto-detection feature. Every time powered on, the device automatically detects the number of connected LED strings so as to select the correct phase shift.

This application report firstly introduces the mechanism of LP8863-Q1 string auto-detection, gives the detection flow step by step. Then, it analyzes two common application issues: the first case is using invalid LED string configuration, the second case is powering off then powering on quickly, both could affect the proper string configuration auto-detection, leading to a wrong phase shift and fault misreporting. Lastly the article gives a software method to solve the problem.

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1 Introduction

The LP8863-Q1 device is an automotive 6-channel backlighting LED driver, which defaults to phase shift mode. When the device is powered on, the string configuration is automatically detected and the phases of each channel configured. For example, if four strings are connected, the LED outputs are phase shifted by 90° (= 360 / 4); if 6 strings are connected, the LED outputs are phase shifted by 60° (=360/6). The LED strings are always appropriately phase shifted for their string configuration. This reduces the ripple of the boost output, and increases the load frequency which can move potential capacitor noise above the audible band.

The string configurations in Table 1 are valid configurations for auto detection. Any invalid configurations are NOT allowed to be used. The detected configuration can be read from AUTO_LED_STRING_CFG in AUTO_DETECT_DIAGNOSTICS register.

Table 1. LP8863-Q1 Valid LED String Configurations

Configuration	LED0	LED1	LED2	LED3	LED4	LED5	Detected LED String Configuration (AUTO_LED_STRING_CFG)	Phase Shift
6 channels	150 mA	150 mA	150 mA	150 mA	150 mA	150 mA	0h = 6 separate strings	60°
5 channels	150 mA	150 mA	150 mA	150 mA	150 mA	GND	1h = 5 separate strings	72°
4 channels	150 mA	150 mA	150 mA	150 mA	GND	GND	2h = 4 separate strings	90°
3 channels	150 mA	150 mA	150 mA	GND	GND	GND	3h = 3 separate strings	120°
2 channels	150 mA	150 mA	GND	GND	GND	GND	4h = 2 separate strings	180°
3 channels	300 mA		300 mA		300 mA		5h = outputs connected in groups of 3 to drive 2 strings	120°
2 channels	450 mA			450 mA			6h = outputs connected in groups of 2 to drive 3 strings	180°
1 channel	900 mA						7h = all outputs connected together to drive one string	None

2 LP8863-Q1 LED String Auto-Detection Mechanism

Figure 1 shows the LP8863-Q1 string auto-detection flow.

Figure 1. LP8863-Q1 LED String Auto-Detection Flow

When VDD is powered up, the device automatically injects 1 mA current on LED0 pin in order to check whether using multi-string configuration.

If only LED0 and LED1 are detected beyond 1 V, which indicates LED0 and LED1 pins are tied together, the device will set AUTO_LED_STRING_CFG = 5h (outputs connected in groups of 2 to drive 3 strings)
If only LED0, LED1 and LED2 are detected beyond 1 V, which indicates LED0, LED1 and LED2 pins are tied together, the device will set AUTO_LED_STRING_CFG = 6h (outputs connected in groups of 3 to drive 2 strings)
If all LEDx are detected beyond 1 V, which indicates all LEDx pins are tied together, the device will set AUTO_LED_STRING_CFG = 7h (all outputs connected together to drive one string)

If none of above situations meets, the device automatically injects 1 mA current on every LEDx pin in order to check whether using separate-string configuration.

If only LED0 and LED1 are detected beyond 1 V, which indicates LED2 to approximately LED5 pins are tied to ground, the device will set AUTO_LED_STRING_CFG = 4h (2 separate strings)
If LED0, LED1 and LED2 are detected beyond 1V, which indicates LED3 ~ LED5 pins are tied to ground, the device will set AUTO_LED_STRING_CFG = 3h (3 separate strings)
If only LED0, LED1, LED2 and LED3 are detected beyond 1 V, which indicates LED4 and LED5 pins are tied to ground, the device will set AUTO_LED_STRING_CFG = 2h (4 separate strings)
If only LED0, LED1, LED2, LED3 and LED4 are detected beyond 1 V, which indicates LED5 pin is tied to ground, the device will set AUTO_LED_STRING_CFG = 1h (5 separate strings)
If all LEDx are detected beyond 1 V, which indicates no LEDx pin is tied to ground, the device will set AUTO_LED_STRING_CFG = 0h (6 separate strings)

If none of above situations meets, which means the string detection does not match one of 8 valid configurations, the device automatically defaults to AUTO_LED_STRING_CFG = 0h (6 separate strings) and alarms invalid LED string fault.

Figure 2(a) demonstrates a normal start-up of LP8863-Q1. Figure 2(b) shows an enlarged portion of LED string detection of Figure 2(a). Figure 2(c) displays the phase shift of each LEDx.

(a) Start-Up

(b) LED String Auto-Detection

Figure 2. LP8863-Q1 Normal Start-up Using 6 Separate Strings