ST AN3981 Application note

AN3981

Application note

LED array drivers

Introduction

The use of LEDs in applications like displays, information and advertising panels, signs, traffic signals, automotive lighting and architectural lighting is becoming more and more common.

LED array drivers, with accurate constant current regulation and embedded serial interfaces, offers a solution that is suitable for most of the above mentioned applications.

This document provides an overview of the STMicroelectronics portfolio of LED array drivers.

According to the application, some specific features may be relevant:

■Brightness adjustment through PWM dimming

■Very high current accuracy

■Diagnostic capability to detect faulty LEDs

■Low power consumption

Moreover, the choice of the most appropriate product is driven also by the array length (number of LED current generators), the LED current, type of LED (single color or RGB), etc.

Figure 1. STMicroelectronics LED array drivers portfolio

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ARRAY DRIVER	ARRAY DRIVER	ARRAY DRIVER		ARRAY DRIVER
		,OW CURRENT	(IGHER
		(IGH ACCURACY	CURRENT
340 #-	340 X0	340 X00Y 340 X0Y		340 $0
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DISPLAYS	AUTOMOTIVE	)NFORMATION ADVERTISEMENTN		DISPLAYS
ARCHITECTURAL SPECIALA	BACKLIGHTING	PANELS		2'"'SOLUTIONS
LIGHTING	WHITETGOODS
				!- V

The wide variety of STMicroelectronics LED array drivers allows the selection of a dedicated solution for a certain application. STMicroelectronics portfolio is summarized in Figure 1. The following sections describe all the product family main features.

Furthermore, the document focuses on the guidelines that ease the use of these products in the final application and also provides some particular application ideas.

January 2012

Doc ID 022217 Rev 1

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www.st.com

Contents	AN3981

Contents

1	STP04CM05 - 4-bit LED array driver . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
	1.1	Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	1.2	Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	1.3	Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	1.4	SPI data sending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	1.5	Current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8

2	8-bit LED array drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 9
	2.1	Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
	2.2	Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
	2.3	Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
	2.4	SPI data sending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
	2.5	Current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	2.6	Error detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12

2.6.1 Correctness of the error detection results . . . . . . . . . . . . . . . . . . . . . . . 15

3	16-bit LED array drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17
	3.1	Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
	3.2	Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	3.3	Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	3.4	SPI data sending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
	3.5	Current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
	3.6	Error detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20

3.6.1 Correctness of the error detection results . . . . . . . . . . . . . . . . . . . . . . . 23

3.7 Power saving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.7.1 Power saving example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.8 Current generators performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4	STP24DP05 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		25
	4.1	Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	25
	4.2	Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	25
	4.3	Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	25

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AN3981	Contents

4.4 SPI data sending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4.5 Current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 4.6 Error detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.6.1 Error detection through the DM pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4.6.2 Error detection through and LE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.6.3 Correctness of the error detection results . . . . . . . . . . . . . . . . . . . . . . . 31

4.7 Flag pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.8 Data flow management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4.9 Gradual output delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5	Common application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			34
	5.1	LED supply voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		34
		5.1.1	Package selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	36
	5.2	Daisy chain connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		41
	5.3	Regulating the current in a group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		42
	5.4	PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		44
	5.5	Short detection between adjacent pins . . . . . . . . . . . . . . . . . . . . . . . . . .		46
	5.6	Higher current requests (outputs in parallel) . . . . . . . . . . . . . . . . . . . . . .		48
	5.7	LED matrix driving solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		48
6	PCB layout guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			51
7	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			53

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List of figures	AN3981

List of figures

Figure 1.	STMicroelectronics LED array drivers portfolio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 1
Figure 2.	STP04CM05 typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 6
Figure 3.	STP04CM05 block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 7
Figure 4.	Typical timing diagram for a 4-bit LED array driver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
Figure 5.	STP04CM05 output current vs. REXT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
Figure 6.	Typical application circuit for STP08xP05 devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
Figure 7.	8-bit LED array driver block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
Figure 8.	Typical timing diagram for an 8-bit LED array driver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
Figure 9.	STP08CP05 and STP08DP05 output current vs. REXT . . . . . . . . . . . . . . . . . . . . . . . . . .	12
Figure 10.	Fault conditions detected by error detection mode: a) shorted LED; b) open LED;
	c) short-circuit from current generator to VLED; d) open channel; e) short-circuit from
	current generator to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
Figure 11.	Error status code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
Figure 12.	Digital key for entering error detection mode in 8-bit LED drivers . . . . . . . . . . . . . . . . . . .	14
Figure 13.	Digital key for exiting error detection mode in 8-bit LED drivers . . . . . . . . . . . . . . . . . . . . .	14
Figure 14. Example of error detection with OUT7 and OUT2 good and all other outputs faulty . . . . .		15
Figure 15.	Error detection results interpretation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
Figure 16.	Failed error detection due to wrongly entered digital key . . . . . . . . . . . . . . . . . . . . . . . . . .	16
Figure 17.	Typical application circuit for STP16xPyy05 devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
Figure 18.	16-bit LED array driver block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
Figure 19.	Typical timing diagram of a 16-bit LED array driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
Figure 20.	Output current vs. REXT for a 16-bit LED array driver . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
Figure 21.	Fault conditions detected by error detection mode: a) shorted LED; b) open LED;
	c) short-circuit from current generator to VLED; d) open channel; e) short-circuit from
	current generator to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
Figure 22.	Error status code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
Figure 23.	Digital key for entering error detection mode in 16-bit LED drivers . . . . . . . . . . . . . . . . . .	22
Figure 24.	Digital key for exiting error detection mode in 16-bit LED drivers. . . . . . . . . . . . . . . . . . . .	22
Figure 25.	Example of error detection with OUT15 and OUT0 faulty and all other outputs good . . . .	23
Figure 26.	Error detection results interpretation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	23
Figure 27.	Typical application circuit for the STP24DP05 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	25
Figure 28.	STP24DP05 block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	26
Figure 29.	Typical timing diagram of the STP24DP05. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27
Figure 30.	STP24DP05 output current vs. REXT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28
Figure 31.	Fault conditions detected by error detection mode: a) shorted LED; b) open LED;
	c) short-circuit from current generator to VLED; d) open channel; e) short-circuit from
	current generator to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28
Figure 32.	Error status code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	29
Figure 33.	Error detection through the DM pin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	30
Figure 34.	Digital key for entering error detection mode in 32-bit LED drivers . . . . . . . . . . . . . . . . . .	31
Figure 35.	Digital key for exiting error detection mode in 32-bit LED drivers. . . . . . . . . . . . . . . . . . . .	31
Figure 36.	Example of error detection with the STP24DP05 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	31
Figure 37.	Error detection results interpretation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	32
Figure 38.	Error flag and overtemperature flag pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	32
Figure 39.	Data flow management. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	33
Figure 40.	Gradual output delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	33
Figure 41.	Solution with single supply voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	35
Figure 42.	Solution with separated supply voltage for red LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	36

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Figure 43. Maximum power dissipation vs. ambient temperature for different packages for 4-bit
	LED drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	37
Figure 44. Maximum power dissipation vs. ambient temperature for different packages for 8-bit
	LED drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	38
Figure 45. Maximum power dissipation vs. ambient temperature for different packages for 16-bit
	LED drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	38
Figure 46. Maximum power dissipation vs. ambient temperature for 24-bit LED drivers (TQFP-48). .		39
Figure 47. Maximum power curves for different packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		40
Figure 48. Thermal performance of the TSSOP24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		41
Figure 49. Thermal performance of the HTSSOP - 24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		41
Figure 50. LED drivers connected in daisy chain configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		41
Figure 51. Example of data loading for devices connected in daisy chain . . . . . . . . . . . . . . . . . . . . .		42
Figure 52. Example of current regulation for more devices at the same time . . . . . . . . . . . . . . . . . . .		43
Figure 53. Channel current vs. RREG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		43
Figure 54.	PWM dimming using the pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	44
Figure 55. Example showing how to perform a PWM dimming separately for each channel . . . . . . .		45
Figure 56. Description of shorted adjacent pin detection strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . .		47
Figure 57. Possible implementation of the shorted adjacent pin detection algorithm . . . . . . . . . . . . .		47
Figure 58. High current capability solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		48
Figure 59. LED matrix panel driving solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		49
Figure 60. LED matrix timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		50
Figure 61. LED matrix driving example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		50
Figure 62. Example of propagation delay of serial data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		51
Figure 63.	Digital signal quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	52

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STP04CM05 - 4-bit LED array driver	AN3981

1 STP04CM05 - 4-bit LED array driver

1.1Main features

●Four constant current output channels

●Adjustable output current from 80 mA to 400 mA through an external resistor

●20 V current generators rated voltage

●Supply voltage from 3.3 V to 5.5 V.

Figure 2. STP04CM05 typical application circuit

,%$$COMMON RAILIVOLTAGE

3UPPLY VOLTAGE

6$$

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/54

/54 /54

3$)

$ATAALOADED

#,+

THROUGH SERIAL

340 #-

,%

INTERFACE

2 %84

/%

#URRENT SETTING

RESISTOR

!-V

1.2Applications

The possibility to set a current up to 400 mA per channel makes this product suitable for those applications where high power LEDs are the preferred solution.

Very high brightness display and architectural and special lighting (mood lighting) are common examples of the use of the STP04CM05.

1.3Block diagram

Figure 3 represents the block diagram of the STP04CM05.

The 4-bit shift register contains the digital data loaded through the SDI pin. The “Latch and Enable” block contains the digital word loaded from the 4-bit shift register once the LE pulse has been detected.

The “Analog stage” represents the circuitry to drive each current generator consistently with the digital word contained in the “Latch and Enable” block, setting the correct current in accordance with the resistor connected to the REXT pin. This block is enabled by the OE pin.

The Thermal shutdown block monitors the junction temperature and switches the current generators off if this temperature goes over the thermal shutdown threshold (typically

170 °C), independent to the actual status of the generators. The generators go back to the

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AN3981	STP04CM05 - 4-bit LED array driver

previous status if the junction temperature falls, typically 15-20 °C, below the thermal shutdown threshold.

Figure 3. STP04CM05 block diagram

	287	287	287
7KHUPDO	$QDORJ VWDJH		5 (;7
VKXWGRZQ
2( '0
/( '0	/DWFK DQG (QDEOH
6',	ELW VKLIW UHJLVWHU		6'2

&/.

!- V

1.4SPI data sending

Figure 4 shows a typical timing diagram of a 4-bit LED driver.

The LE pulse latches the last 4 bits loaded into the shift register through the SDI pin and sampled by the last 4 clock rising edges.

As long as the OE pin is high, all current generators are not active. Once OE is set low, the current generators are enabled and their status is determined by the data latched by the last LE pulse.

Figure 4. Typical timing diagram for a 4-bit LED array driver

#,+

3$)	PREVIOUS	DATA

,% PULSE LATCHES THE LAST BIT

,%

CLOCK

CYCLES

BEFORE

ETHEE,%

PULSE

LOADEDEINTOTTHE SHIFTTREGISTER

/% LOW ENABLES THE CURRENTTGENERATORS

/%

(

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/&&

,

(

/54

/.

,

(

/54

/.

,

/54

/&&

(

,

3$/

PREVIOUS DATA

!- V

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STP04CM05 - 4-bit LED array driver	AN3981

1.5Current setting

The output current of all channels can be programmed through one external resistor in the range 80-400 mA.

The choice of the resistor value can be made using the curve below (Figure 5) or using the following formula:

Equation 1

REXT = VREF 64

IOUT

where VREF = 1.25 V.

For example, if a current of 100 mA per channel must be programmed, according to Equation 1, the external resistor value is:

Equation 2

			1.25
	REXT			64	= 800Ω
			0.1

The STP04CM05 is characterized by a high current accuracy between channels (typically ±1%, maximum ±1.5%). The current accuracy between chips is maximum ±6%.

Figure 5. STP04CM05 output current vs. REXT

			7HPS	&
			9GG 9
			9GURS	IURP 9 WR 9
			,VHW P$P$P$P$P$P$
5H[W 2KP
			2XSXW &XUUQW P$

!-V

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AN3981	8-bit LED array drivers

2 8-bit LED array drivers

2.1Main features

The 8-bit LED array driver family is composed of two products: the STP08CP05 and STP08DP05.

●Eight constant current output channels

●Adjustable output current from 5 to 100 mA through an external resistor

●Short and open output error detection (only for the STP08DP05)

●20 V current generators rated voltage

●Supply voltage from 3.3 V to 5.5 V.

Figure 6. Typical application circuit for STP08xP05 devices

					,%$$COMMON RAILIVOLTAGE
	3UPPLY VOLTAGE	/54	/54	/54
	6$$				3$)	$ATA LOADEDD
					#,+	THROUGH SERIAL
			340 X0		,%	INTERFACE
	2 %84				/%
	#URRENT SETTING
	RESISTOR

!-V

2.2Applications

This family is suitable for several applications in different market segments, such as gaming machines, white goods, LED signage, and automotive LED lighting.

2.3Block diagram

Figure 7 represents the block diagram valid for the 8-bit LED array driver family.

The 8-bit shift register contains the digital data loaded through the SDI pin. The “Latch and Enable” block contains the digital word loaded from the 8-bit shift register once the LE pulse has been detected.

The “Analog stage” represents the circuitry to drive each current generator consistently with the digital word contained in the “Latch and Enable” block, setting the correct current in accordance with the resistor connected to the REXT pin. This block is enabled by the OE pin.

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8-bit LED array drivers	AN3981

The “Control logic” block decodes the signal sequence provided to the LE and OE pin for entering and ending the error detection, performed by the block “Open/Short-Circuit Detector”.

The Thermal shutdown block monitors the junction temperature and switches the current generators off if this temperature goes over the thermal shutdown threshold (typically

170 °C), independent to the actual status of the generators. The generators go back to the previous status if the junction temperature falls, typically 15-20 °C, below the thermal shutdown threshold.

Figure 7. 8-bit LED array driver block diagram

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2.4SPI data sending

Figure 8 shows a typical timing diagram of an 8-bit LED driver.

The LE pulse latches the last 8 bits loaded into the shift register through the SDI pin and sampled by the last 8 clock rising edges.

As long as the OE pin is high, all current generators are not active. Once OE is set low, the current generators are enabled and their status is determined by the data latched by the last LE pulse.

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8-bit LED array drivers

Figure 8. Typical timing diagram for an 8-bit LED array driver

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2.5Current setting

The output current of all channels can be programmed through one external resistor in the range 5-100 mA. The choice of the resistor value can be made using the curve below (Figure 9) or using the following formula:

Equation 3

REXT = VREF 16

IOUT

where VREF = 1.25 V.

For example, if a current of 20 mA per channel must be programmed, according to Equation 1, the external resistor value is:

Equation 4

			1.25
	REXT				16	= 1kΩ
			0.02

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8-bit LED array drivers	AN3981

Figure 9. STP08CP05 and STP08DP05 output current vs. REXT

	1800
	1600
	1400
	1200
(Ω)	1000
Rext	800
	600
	400
	200
	0
	0	50	100	150	200	250	300	350	400	450
					Output current (mA)

AM10625v1

The current accuracy of both the STP08CP05 and STP08DP05 is typically ±1.5% between channels and a maximum of ±3% chip-to-chip.

2.6Error detection

The meaning of “error detection” is the detection of the faulty conditions that may occur on each channel due to:

●the LED, which can be damaged and behaves abnormally as a short or open circuit

●any undesired event that can prevent the LED from working correctly (interruption of

the PCB - printed circuit board - trace connecting the LED to VLED or to the current generator, short-circuit from current generator to ground or VLED).

All these possible events are summarized in Figure 10.

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Figure 10. Fault conditions detected by error detection mode: a) shorted LED; b) open LED; c) short-circuit from current generator to VLED; d) open channel; e) short-circuit from current generator to GND

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The diagnostic circuitry embedded in the STP08DP05 can detect these faulty conditions (in accordance with the detection thresholds of Table 1) and provide the results of the detection as an “Error status code” coming out of the SDO pin (see Figure 11), with “0” logic level indicating a faulty channel, whereas a “1” logic level indicates a “good” channel.

Table 1.		Detection conditions
Detection conditions				Detection result
I	ODEC	(1) = 0.5·I (2)		Open channel or output short to GND detected
		O
	IODEC = 0.5·IO			No error detected
	VO = 2.5 V			LED short-circuit or LED shorted to supply voltage
	VO = 2.2 V			No error detected

1.IODEC is the detected output current in detection mode.

2.IO is the programmed output current.

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8-bit LED array drivers	AN3981

Figure 11. Error status code

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The error detection process can be divided into three phases:

1.Entering detection mode. From “normal mode” the device can enter “error mode” through a logic key (Figure 12), which is a logic sequence of the OE ⁄ DM2 and LE/DM1 signals over five CLK cycles. After these five CLK cycles, the device is in “error detection mode” and ready for sampling data on the SDI pin

2.Error detection. Once the device has entered “error detection mode”, an 8-bit string should be loaded into the shift register through the SDI pin in order to set the outputs in accordance with the diagnostic demands (only the outputs that are on are checked by the error detection process). Therefore, if the user wants to check all the outputs, a

string with all “1” must be sent. After that the outputs are ready for the detection process, which starts when OE ⁄ DM2 is set low. The device drives LEDs in order to analyze if an open or short condition has occurred. The OE ⁄ DM2 must remain low for at least 1 µs to complete the detection process. As shown in Figure 14, during the

detection process, at least three clock pulses are necessary: two pulses as soon as the OE ⁄ DM2 is set low, and one clock pulse before ending the detection process. Once the OE ⁄ DM2 is set high, the error status code is available at the SDO pin. To download the complete error string, at least 8 clock pulses are necessary

3.Resuming normal mode. The device quits detection mode and returns to normal mode through an exit logic key (Figure 13), a logic sequence over five clock pulses.

Figure 12. Digital key for entering error detection mode in 8-bit LED drivers

Figure 13. Digital key for exiting error detection mode in 8-bit LED drivers

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For proper device operation, the entering sequence must be followed by a resuming sequence as it is not possible to insert consecutive equal sequences.

Figure 14 shows an example of error detection where OUT7 and OUT2 are good, whereas all other outputs are considered faulty.

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Figure 14. Example of error detection with OUT7 and OUT2 good and all other outputs faulty

8 CLK pulse to load data into the shift register

logic key to quit the Error Detection Mode

CLK

LE

at least 1μs

OE

SDO

logic key to enter the Error Detection Mode

Detection result: OUT7 and OUT2 are GOOD,
all other outputs are NOT GOOD	AM10059v1

2.6.1Correctness of the error detection results

It is important to underline that the result of the diagnostic process must be compared with the data loaded into the shift register.

In fact, if a channel is not switched on, the result of the detection is “0” anyway.

It is recommended to follow the approach shown in Figure 15.

Figure 15. Error detection results interpretation

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The correct interpretation of the detection results can be affected by any setup/hold time problem. For this reason it is recommended to synchronize all signals with the falling edge of the CLK signal.

If this is not possible, it is advisable to start the OE signal (for example) typically 20 ns after the rising edge of the CLK signal.

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8-bit LED array drivers	AN3981

Figure 16 shows a typical problem in error detection mode due to the wrong digital key: the device does not recognize it (due to an incorrect OE timing or LE timing), no error detection is performed, so data coming out of the SDO pin does not represent the error status code, as expected.

Figure 16. Failed error detection due to wrongly entered digital key

Error Detection Problem due to OE signal

No detection was

Observed

Error Detection Problem due to LE signal

No detection was

Observed

AM10548v1

Another incorrect result of the error detection can occur if the choice of the LED supply voltage is not appropriate. The rules to suitably define the LED supply voltage are discussed in Section 5.1.

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AN3981	16-bit LED array drivers

3 16-bit LED array drivers

3.1Main features

The 16-bit LED array driver family contains several products with the following common features:

●Sixteen constant current output channels

●Adjustable output current through an external resistor (current ranges indicated in

Table 2 below)

●Error detection (only some devices, see Table 2)

●Auto shutdown and auto wake-up functions (only some devices, see Table 2)

●20 V current generators rated voltage

●Supply voltage from 3.3 V to 5.5 V.

Figure 17. Typical application circuit for STP16xPyy05 devices

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Table 2 provides a general overview of the products belonging to this family.

Table 2.	16 bit LED array driver overview
P/N		ILED [mA]	Error detection	Auto shutdown	Balanced TON/TOFF
STP16CP05		5÷100
STP16CPC05		5÷100			•
STP16CPP05		3÷40
STP16CPS05		5÷100		•
STP16CPPS05		3÷40		•
STP16DP05		5÷100	•
STP16DPP05		3÷40	•
STP16DPS05		5÷100	•	•
STP16DPPS05		3÷40	•	•
STP16CPC26		3÷90			•(1)

1. almost balanced, see dedicated section

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