G-
T
W
-000
5534.2.2
LED Forward Voltage, V
F
Pad Voltage, V
PAD
; R
ON
= 20Ω
R
LED
LED[2, 1 or 0]
Resistor Voltage Drop, V
R
LED Supply
I
LE
D
Figure 8.1: LED Equivalent Circuit
From Figure 8.1 it is possible to derive Equation 8.1 to calculate I
LED
. If a known value of current is required through
the LED to give a specific luminous intensity, then the value of R
LED
is calculated.
I
LED
=
VDD − VF
RLED + RON
Equation 8.1: LED Current
For the LED pads to act as resistance, the external series resistor, R
LED
, needs to be such that the voltage drop
across it, V
R
, keeps V
PAD
below 0.5V. Equation 8.2 also applies.
VDD = V
F
+ V
R
+ V
PAD
Equation 8.2: LED PAD Voltage
Note:
The LED current adds to the overall current. Conservative LED selection extends battery life.
Advance Information
This material is subject to CSR's non-disclosure agreement
© Cambridge Silicon Radio Limited 2011
Page 35 of 110
CS-209182-DSP1
CSR8640 BGA
Data Sheet
