Guess who is back to crunching simulations in LTspice, this time trying to
figure out how well the
RGB LED driver
works ;^)
RGB LED, Configuration, & Driver Overview
The RGB LED I have decided to use is the
BROADCOM ASMT-YTD7-0AA02, which also comes with a diffused silicone cap to mix the emitted
light
The LEDs will be divided into 4 zones/strings inside the AR2 Barrel:
Finally, after comparing LED drivers like
HV9980,
LT3597,
LP5009... I decided to settle on the
LT3496
to drive the above configuration. The thing that makes this driver special
is that it has a Buck-Boost mode, which is a must when the nominal battery
voltage hovers close to the total forward voltage of the LED sting
(particular for the GRN & BLU channels). Below is the LTspice simulation
for the RED string:
A few key points about this LT3496
configuration:
-
Reducing the current sense voltage (CTRL pin) from 100mV to 50mV lowers
the peak LED current during ON-OFF transition from 110mA to 80mA (think
reducing swing range of error amplifier). LEDs are rated for 100mA
peak 100ns pulse and though the 110mA pulse was only for 25ns, I really
wanted to be sure I am not hitting the 100mA maximum limit
-
Reducing the switching frequency from 2.1MHz to 1.25MHz improves converter
efficiency, as ON losses are lowered from 96mW to 76mW. Reducing the
frequency beyond this brings diminishing returns, as losses are more or
less ~70mW, while LED ripple current is increased (for same size
inductor, 10μH)
-
Chosen VC filter (22K & 470P) helps minimize LED current overshot and
improves settling time (during OFF-ON transition)
-
OVP resistor divider sets the LED string over-voltage protection to 35V.
So if in the unlikely scenario that say a single LED fails in the RED
string, the string will be safely shutdown to make sure it does not impact
the GRN/BLU strings
RGB LED in Detail (Or Why We Need a Buck-Boost Mode)
Remember when I said having a
Buck-Boost mode is super useful when the nominal battery voltage hovers
close to the total forward voltage of the LED sting? Well lets look at this
in more detail... First off I will be using a 4S LiFePO₄ battery, so I expect the
voltage to be:
- 14.4V maximum
- 13.6V nominal
- 10.0V minimum
I plan to drive each LED channel at 80% maximum DC current rating, so I
expect the individual forward voltage to be:
- RED 2.3Vf nominal @ 40mA, & a maximum of 3.0Vf
- GRN 3.1Vf nominal @ 40mA, & a maximum of 3.6Vf
- BLU 3.1Vf nominal @ 40mA, & a maximum of 3.6Vf
So with a 3 series LED string the forward voltage will be:
- RED 6.9Vf nominal & 9.0Vf maximum
-
GRN
9.3Vf nominal & 10.8Vf maximum
-
BLU
9.3Vf nominal & 10.8Vf maximum
And with a 4 series LED string the forward voltage will be:
-
RED
9.2Vf nominal & 12.0Vf maximum
-
GRN
12.4Vf nominal & 14.4Vf maximum
-
BLU
12.4Vf nominal & 14.4Vf maximum
Note how the LED string forward voltage spans the range of the battery
voltage, meaning we can't just use a Buck or Boost regulator, as we will run
into cases where battery voltage is too high/low for regulator to function.
This is exactly where the Buck-Boost mode saves the day, as it can happily
regulate the string voltage to required value :D
Linear vs Switching (Or When Things Get Hot)
So first of all we can't use a linear regulator to drive 4 series LEDs,
unless we increase the
battery voltage (as it would need to be ~2V higher than maximum forward
voltage of the string). A linear regulator can just about drive 3 series
LEDs, as in this scenario the battery voltage mostly gives enough
headroom. With that said, lets compare how a linear regulator compares
to a switching one
NOTE: I am setting the LED string brightness with a 1kHz PWM
waveform that is at 50% duty
So using a switching regulator reduces the average dissipated power by
~70% (from 145mW to 40mW), and the ON dissipated power by ~80% (from
290mW to 56mW)... and that's just for the RED LED string/segment (as in
not including the GRN/BLU string)!
Next lets expand the simulation to include the GRN/BLU LED driver losses
and see what the expected IC (just the one, not the 4 I need to drive
all barrel zones) temperature rise above ambient is:
| IC |
Type |
RθJA, [°C/W] |
IC losses average, [mW] |
IC losses maximum, [mW] |
Temp above ambient, [°C] |
|
LP5009
|
Linear |
54°C/W |
RED 145mW
GRN 120mW
BLU 120mW |
RED 290mW
GRN 239mW
BLU 239mW |
21°C to 42°C |
|
LT3496
|
Switching |
34°C/W |
RED 40mW
GRN 46mW
BLU 46mW |
RED 56mW
GRN 69mW
BLU 69mW |
5°C to 7°C |
And with all that waffle out of the way... we can see that using a
switching regulator is the way to go, as:
-
It is more energy efficient, resulting in a cooler AR2 Barrel ;^)
-
It can actually drive 4 or more RGB LEDs in series, while being
powered from same 4S LiFePO₄ battery
