ST AN3090 APPLICATION NOTE

AN3090

Application note

Power saving and LED status detection using the STP16DPPS05 device

Introduction

Today, the use of LEDs is growing in many market segments, including portable applications, due to their improving performance and lower cost.

This new scenario requires that LED drivers be capable of providing good performance at low current, and include power saving capabilities.To address these requirements, STMicroelectronics has introduced the STP16xPPx05 family of LED drivers.

This application note describes the STP16DPPS05 low voltage 16-bit constant current LED sink driver, with particular focus on two important features: output error detection and auto power-saving.

December 2009

Doc ID 16496 Rev 1

1/19

www.st.com

Contents	AN3090

Contents

1	Device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
2	Normal mode functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
3	Error detection feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		8
4	Error detection output test circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13
5	Auto power-saving feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14
	5.1	Value added by the auto power-saving feature . . . . . . . . . . . . . . . . . . . .	15
6	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17
7	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		18

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AN3090	List of tables

List of tables

Table 1. Output error detection sequence after OE/DM2 acquisition . . . . . . . . . . . . . . . . . . . . . . . . 10 Table 2. Error threshold test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Table 3. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

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List of figures	AN3090

List of figures

Figure 1. STP16DPPS05 block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 2. Typical data transfer from SDI to SDO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 3. Typical output_n functionality in normal mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 4. Typical application of the STP16DPPS05 device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5. Error detection sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 6. Typical error detection results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 7. Entering output error detection timing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 8. Resuming normal mode timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 9. IDD consumption in normal mode and auto power-saving condition. . . . . . . . . . . . . . . . . . 14 Figure 10. First out “ON” after resuming normal mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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AN3090	Device description

1 Device description

The STP16DPPS05 device is a monolithic, low voltage, low current power shift register. It contains a 16-bit serial IN, parallel OUT shift register that feeds a 16-bit D-type storage register. Sixteen regulated currents are present in the output stage, which is capable of providing 3-40 mA of constant current to drive the LEDs. Figure 1 shows the application block diagram of the STP16DPPS05 device.

Figure 1. STP16DPPS05 block diagram

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Two important features of STP16DPPS05 are output status checking to detect LED errors during driving (error detection), and power saving when the device is not managing data (auto power safe).

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Normal mode functionality	AN3090

2 Normal mode functionality

During normal operation, the serial data present on the SDI pin is transferred to the shift register in synchronization with the CLK rising edge. After 16 CLK pulses, the data loaded on the SDI pin is shifted to the SDO pin with a typical delay of 22 ns, as shown on the plot in

Figure 2.

This delay guarantees the correct synchronization of the CLK and SDI signals if two or more STP16DPPS05 devices are cascaded.

Figure 2. Typical data transfer from SDI to SDO

The data present in any register is transferred to the respective latch when the latch-enable (LE/DM1) signal is high (serial-to-parallel conversion).

After this step, the data is transferred to the outputs via the output-enable pin (OE/DM2), which turns on the LEDs at the current set by the external resistor.

By changing the OE/DM2 duty cycle, it is possible to set the output dimming if the application requires slow or fast frame visualization.

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