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marss
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DA14682 LED1/2/3 pins

- Is there anywhere more documentation for LED1/2/3 than datasheet table 685/843/844?
- DA14683_USB_KIT_VC shows 3 LEDs connected to LED1/2/3 and VBUS without any ballast resistor or any current-limiting or voltage-limiting external components. So LED1/2/3 are constant-current sinks controlled by the duty cycle of the PWM? For example, a constant current of 20mA (typ, max is 21mA) at duty cycle 100% and e.g. 2mA at duty cycle 10%?
- What means "after trimming" in Table 843
- Unrelated to LED1/2/3: is 14682 really not available in a WLCSP like DA14683?

Thanks for your help

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PS_Dialog
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Hi Marss,

Hi Marss,

Unfortunately that's all the documentation we have.

The DA1468x supports a LED current = 20 mA, this is the maximum possible current (current limiting driver).
The driver will try to draw 20 mA in the LED.
If the supply voltage is too low for the applied LED, or when a series resistor has been applied, the LED current will be less than 20 mA.
The LED_TRIM bits are meant for trimming/matching the LED driver currents.

Each LED driver uses it's own PWM (PWM2/3/4), so the brightness can be controlled individually for each LED port.

You are right, in the USB starter kit there's no limiting resistor, that's why we have choosen an LED with 20mA forward current LTST-C19HE1WT.

Regarding DA!4682: This is a SystemIn Package formed by a QSPI SFlash die + DA14683 die in the same package, that's why we do not have a WLCSP version.

BR, Paolo

 

marss
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Hi Paolo

Hi Paolo

Thanks for the helpful information. You are saying the current through LED1 can never exceed 20mA because the pin limits the current. The PWM is only to switch on/off the current of 20mA in the case of DA14683_USB_KIT_VC with LTST-C19HE1WT.

How efficient is this for wearables? Is the excessive voltage turned into heat? With DA14683_USB_KIT_VC, Iforward=20mA, Vforward=2.15V, VBUS_HUB=5V, loss=(5V-2.15V)*20mA?

LED_TRIM: those 16 states reduce the current from 20mA to 20mA*x/16 for all LED pins?

Cheers, Marc

LC_Dialog
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Joined: 2016-09-19 23:20
Hi marss,

Hi marss,

Yes, the 20mA is the max current and the PWM can be used to control the brightness.

There won't be any additional draw from the battery energy except the heat build up at the driver since this pulls out a constant 20mA current. Only the thermal will be affected and the 682 should be able to handle the heat dissipation internally for this block.

These trim bits are to get the current as close as possible to 20mA ideally for better efficiency and not for setting the current values.

Best regards,
LC_Dialog

marss
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Hi and thanks!

Hi and thanks!

Not sure I fully understand the efficiency part. In the case of DA14683_USB_KIT_VC:

a) are you saying, that the total power drawn from the battery is roughly 43mW (Iforward=20mA, Vforward=2.15V) plus some negligible loss (e.g. anywhere below 10%=4mW) inside 14682?

b) However, I suspect that the total power is 100mW (20mA * 5V VBUS_HUB).

a) and b) make a huge difference for battery operated devices... If b) was the case, we could reduce the power loss with a lower voltage provided to the LED.

Best,
Marc

LC_Dialog
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Hi marss,

Hi marss,

If you are supplying from a 5V battery directly, your math is right. You will be losing 100mW power in total. 
Having said that, we recommend you use a lower supply rail out of a switching converter which will transform the effective power from the battery (minus the efficiency losses) so that there is not so much loss at a bias resistor or at the driver. 
Of course, the input supply to the LED can vary depending on the color of LED you will use thereby you can minmize the drop across other elements in the network.

Best regards,
LC_Dialog

marss
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Thanks for the help, all

Thanks for the help, all answered!