Hello i am trying to understand the application report slva280, but since my knowlegde about electronics are limited i need some kind of help mostly on my calculations.
I am unable to calculate VOUTx correctly,
Lets say i use LEDs with Vf = 3 Volt @20mA,
Question 1: When LED is on the equivalent circuit is a voltage source (Vf) 3V with a series resistor(Rled),right;
Question 2: When LED is off the equivalnet circuit is a resistor (Rled), right;
With that in mind lets do some calculations
Figure 1
Case when LED is on: Vcc = Vf + VOUTx (we ignore Rled to keep things simple), so VOUTx = 5 - 3 = 2 V, -0,3 <VOUTx < 18 we are in the safe area.
Case when LED is off: I assume Id = 0 A, so VOUTx = 5 Volts.
Figure 2
Three LEDS in series, lets calculate the same thing.
Case when leds are off: Vout = 5 Volt
Case when LEDS on: VOUTx = Vcc - 3*Vf = 5 - 9 = -4 Volts, so VOUTx is now negative exceeds the limits.
In order to correct this problem we have to use a bigger value for Vcc lets say 20Volts.
Lets do the calculation oncemore
Case Leds on: VOUTx = 20 - 9 = 11Volts. OKEY
Case Leds off: VOUTx = 20Volts. PROBLEM!
Figure 3
Figure 3 shows a typical solution if you want to connect many Leds in series, using a Mosfet
.I assume we use the mosfet in two regions, cutoff region, and triode region (Ron).
Lets do some calculations again
Case 1 when led is ON:
We must ensure Vgs > Vth and Vds < Vgs - Vth, if those conditions are true VOUTx must be
VOUTx = Vcc - id*Ron - Vf = 5 - 0,02*10 - 3 = 1,8 Volt.
Case 2 when led is OFF :
we must ensure Vgs < Vth , since MOSFET is an open circuit
VOUTx = high Impedance ???
Okey guys thats what i have done so far,please if you find some errors in my logic correct me.
lets get back to the application report
Page 3, says
Even though the driver is off,the LEDs have a forward voltage drop due to leakage currents that flow through them.
When the driver is off and no current is flowing, the voltage across the LEDs is 29V.
I cant understand why the voltage across the LEDs is 29V.
The voltage on the OUTx pin is 2V,which is greater than VCC– VGS(th),or greater than 3.3V–1.5V = 1.8V.
I cant understand why VOUTx is 2V
THANKS