ElectronicsStarter
- Mar 10, 2016
- 3
- Joined
- Mar 10, 2016
- Messages
- 3
Hi All,
I am working on a low intensity LED array for medical applications. Specifically the application is transcranial photobiomodulation. I would like the device to have these specifications
1) 100 LED of the Vishay VSMF4720-GS08 type. Ideally I would like to have a 10 x 10 array of LED.
2) From the datasheet for that LED I will need 1.45V across each LED with a current of 100mA.
3) Each string will have 14.5V and .1A going through it.
4) Ability to switch on/off each string of LED to get a power output that can step up.
5) I will need a constant current through the LED string.
To meet these specifications I have been searching for a suitable LED driver IC. I came across the CAT4106 (I understand that this can only drive 4 channels but I can use multiple of these if need be). From the specifications in the datasheet and the design notes given in the back I think I have a fair idea of how to design the structure but I have some questions.
*I have attached the relevant datasheets*
Question 1) - From the datasheet in the back on the section about LED1 - LED4 it says the following
"These provide the regulated current source for driving each of the LED strings with a tightly matched constant current. To ensure optimal performance, the bias voltage on the LED channels should be set at a nominal 0.6 V or higher. Each channel is capable of driving a current up to 175 mA. All channels immediately enter a high impedance mode whenever the EN/PWM is taken LOW. "
- How do I set the bias voltage to .6V
Question 2) - From the datasheet this is what is said about CTRL Pin.
CTRL pin is a multiplexer output which selects the lowest operating voltage appearing on any of the four LED output drive channels. This control signal represents the cathode terminal voltage of the LED string with greatest forward voltage (VF). An external resistor network from CTRL to FB can be used to set the lowest operating voltage of each channel. External current loading of the CTRL pin is recommended to be less than 25uA.
- How I can select the external resistor loading to force the current to be less than 25uA?
Question 3) - From the datasheet this is what is said about the VFMIN pin.
VFMIN pin uses a pair of external resistors (R6 & R7) to program the worst case, minimum LED string forward voltage (VFMIN) expected in the specific application. If any LED string enters full regulation before the output voltage reaches VFMIN, the output will continue to rise to VFMIN where it will signal an LED short−circuit and a fault condition will be flagged. The detection only works during during power−up and works best when VFMIN is set at least twice the value of VL.
- So R6 & R7 together set the forward voltage drop of a single LED string am I right? Similar can be said about R4 & R5.
- How do I know the maximum voltage drop allowable along each string? 10 LED will require a typical voltage drop of 14.5V and consequently how do I know what the maximum voltage drop can be allowed for the combination of strings? I mean if each string uses 14.5V and I have 4 strings in parallel will I need a more powerful power supply?
I am working on a low intensity LED array for medical applications. Specifically the application is transcranial photobiomodulation. I would like the device to have these specifications
1) 100 LED of the Vishay VSMF4720-GS08 type. Ideally I would like to have a 10 x 10 array of LED.
2) From the datasheet for that LED I will need 1.45V across each LED with a current of 100mA.
3) Each string will have 14.5V and .1A going through it.
4) Ability to switch on/off each string of LED to get a power output that can step up.
5) I will need a constant current through the LED string.
To meet these specifications I have been searching for a suitable LED driver IC. I came across the CAT4106 (I understand that this can only drive 4 channels but I can use multiple of these if need be). From the specifications in the datasheet and the design notes given in the back I think I have a fair idea of how to design the structure but I have some questions.
*I have attached the relevant datasheets*
Question 1) - From the datasheet in the back on the section about LED1 - LED4 it says the following
"These provide the regulated current source for driving each of the LED strings with a tightly matched constant current. To ensure optimal performance, the bias voltage on the LED channels should be set at a nominal 0.6 V or higher. Each channel is capable of driving a current up to 175 mA. All channels immediately enter a high impedance mode whenever the EN/PWM is taken LOW. "
- How do I set the bias voltage to .6V
Question 2) - From the datasheet this is what is said about CTRL Pin.
CTRL pin is a multiplexer output which selects the lowest operating voltage appearing on any of the four LED output drive channels. This control signal represents the cathode terminal voltage of the LED string with greatest forward voltage (VF). An external resistor network from CTRL to FB can be used to set the lowest operating voltage of each channel. External current loading of the CTRL pin is recommended to be less than 25uA.
- How I can select the external resistor loading to force the current to be less than 25uA?
Question 3) - From the datasheet this is what is said about the VFMIN pin.
VFMIN pin uses a pair of external resistors (R6 & R7) to program the worst case, minimum LED string forward voltage (VFMIN) expected in the specific application. If any LED string enters full regulation before the output voltage reaches VFMIN, the output will continue to rise to VFMIN where it will signal an LED short−circuit and a fault condition will be flagged. The detection only works during during power−up and works best when VFMIN is set at least twice the value of VL.
- So R6 & R7 together set the forward voltage drop of a single LED string am I right? Similar can be said about R4 & R5.
- How do I know the maximum voltage drop allowable along each string? 10 LED will require a typical voltage drop of 14.5V and consequently how do I know what the maximum voltage drop can be allowed for the combination of strings? I mean if each string uses 14.5V and I have 4 strings in parallel will I need a more powerful power supply?