STP24DP05 IC queries

[Meghla]

Hi Team,

We are using this STP24DP05 (24-bit constant current LED sink driver with output error detection) IC for a X*Y array of diodes to get high or low. The IC is interfaced with microcontroller using SDI pin of STP24DP05. We have some doubts kindly check-

1. How will the serial pin data transfers to  outputs R/G/B pin? does R1 , G1 & B1 gets data first then it goes to next R2,G2,B2? or it will first give data to R1-R8 then to next 8 bits goes to G1-G8 like that. 

2. Is LE/DM pin working as reset to all the pins? does the outputs of STP24DP05 gets refreshed when LE/DM is pulled down?

3. Can we keep OE pins as grounded ? 

[Peter BENSCH · Accepted solution]

1. See data sheet, section 7.6, table 14 (shifter register data flow control).
2. LE\DM is not a reset pin, but a Latch Enable pin enabling the latch circuit for a new set of data from the SDI chain.
3. To be checked, but you need to switch at least OE-R\DM to enter the error detection mode.

Hope that helps?

Regards
/Peter

[Meghla]

Hi Peter,

Thanks for the reply.

1. We donot want error detection mode. In that case can we keep all three output enables as low ?

2. At what interval SDO comes out? what I understood from the timing diagram after sending out all the 24 parallel outputs it will send the SDO.

[Peter BENSCH]

1. as described in section 7 of the data sheet: if you do not require error detection mode, do not set LE\DM and /OE-RE\DM to H at the same time and over some clock cycles

2. yes, in normal mode SDO can be seen as an output of a shift register, i.e. the input data will occur after 24 clock cycles

[Meghla]

Hi Peter,

Thanks for the reply. Could you please check the below schematic. Here STP is used for biasing several pin diodes. 

[Peter BENSCH]

No, the LEDs D1...D8 are connected completely incorrectly and also do not require any series resistors: STP24DP05 does not have high-side drivers without current limitation at its outputs, but current-controlled low-side drivers. So you only have to connect the LEDs with the anodes to the LED voltage and the cathodes without resistors to the RGB outputs of the STP24DP05.

What are the diodes CR1...CR11 for?

[Meghla]

Hi Peter,

CR1-CR11 are pin diodes.

As per our application, We need to drive the LEDs for on and off condition at 1.4V and 50mA current. STP24DP05 output pins voltage is 5V so to drop this to 1.4V we have connected series resistor.

When the microcontroller send bit 1 then LED(D1-D8) and Pin diodes (CR1-CR11) will be on and when when microcontroller send bit 0 then they will be off. 

Kindly check.

[Peter BENSCH]

No, your idea is still wrong: the STP24DP05 does not have an output voltage of 5V, but internally drives a current to GND. Please take another close look at the data sheet so that you understand: figure 2 already indicates this because it shows an example of a current source that drives the current to GND, figure 17 also shows the connection of the LEDs.

Pin diodes are not necessary in this application.

[Meghla]

Hi Peter,

As per our application we require 5V output Voltage and 50mA output current. According to Table 7 of STP24DP05 datasheet VOH(min)=(VDD-0.4)V. In our case VDD=5V so we can expect output voltage 5V on all serial output datas.

And in the same table maximum output current it can drive is 80mA. 

Could you please tell this will work as per our requirements or not ?

[Peter BENSCH]

Now I understand your confusion: the LED driver outputs are NOT serial, but parallel outputs, which can then also drive up to 80mA to GND. However, the specification VOL in table 7 refers exclusively to serial OUT, i.e. SDO (pin 35), not to R1...8, G1...8 and B1...8.

Please have another look at the AN3981, where the principle of the LED Driver is shown and discussed: all outputs of the LED array drivers work with the same principle, visible in fig 3, 7, 18 and 28.

Please also read sections 4 (Function of the STP24DP05) and 5 (Common Application Information) in the AN3981. You will then realise that you have to change the following points for a successful application:

• remove the unnecessary series resistors R5138, R18, etc), as the current is set with R5135, R5136, R5137, the STP24DP05 acts as current limiter for all LED outputs
• remove the 1nH coils L1... and the pin diodes (you have not yet explained their purpose)
• reverse the polarity of the LEDs (cathode to Rx/Bx/Gx) and finally
• connect the anodes of the LEDs directly to your LED voltage (= VLED+1V to 20V)

Hope that helps?

Regards
/Peter