AND8349-D Datasheet, PDF (3/6 Pages) ON Semiconductor – Automotive Applications The Use of Discrete

AND8349/D
D8
R1
MBRS140T3G
205
V_SUPPLY
13.5Vdc
HF3−R5570
D5
HF3−R5570
D6
HF3−R5570
D7
0
Figure 5.
Figure 5 shows a typical resistive bias for a single
CHMSL LED string. The resistor value is calculated to take
into account the V_fwd across the series connected LED
string. If a specific supply voltage, such as 13.5 V, is used,
a specific resistor can be chosen to supply a 30 mA drive
current.
Example for a 3 Red LED String:
V_supply –V_{sw_bat} – V_rpd – (I__led
V_{sw_bat} = 0 V
V_supply = 13.5 V Typical
V_rpd = 0.8 V
V_fwd = 2.20 V
I__led = 30 mA
R₁) – (3
V_fwd) = 0 V
13.5 V * 0.8 V * 3(2.20 V)
R₁ +
30 mA
+ 203 W or205 W (Standard 1% Value).
(eq. 1)
This method for setting the current with a specific resistor
is well known. By knowing the LEDs worst case V_fwd , and
the light intensity required, a specific range of resistor values
can be chosen. However, as the supply voltage varies from
9 V to 16 V, the current changes in the LED which affects the
intensity. With the same 205 W resistor and 9 V supply,
rearranging the equation and solving for I__led yields 7.8 mA.
Assuming all of the parameters remain constant and the
supply voltage is elevated to 16 V, an I__led value of 42 mA
is calculated. Again, the intensity of the LED is affected.
A CCR from ON Semiconductor would keep the current
and intensity constant over this supply voltage range
(Figure 2).
Here is how you can use ON Semiconductor’s CCR to
determine how many series LEDs it can drive.
D1
Anode
Cathode
+
− ^Vin
Q1
Q2
LED
HF3−R5570
LED
HF3−R5570
LED
HF3−R5570
LED
HF3−R5570
LED
HF3−R5570
LED
HF3−R5570
Qx
LED
HF3−R5570
LED
HF3−R5570
LED
HF3−R5570
Figure 6. Typical Application Circuit
(30 mA each LED String)
Number of LED’s that can be connected is determined by:
D1 is a reverse battery protection diode
LED’s = (V_in − (Q_X V_F + D1 V_F)) / LED V_F
Example: V_in = 12 Vdc, Q_X V_F = 3.5 Vdc, D1VF = 0.7 V
LED V_F = 2.2 Vdc @ 30 mA
(12 Vdc − 4.2 Vdc)/2.2 Vdc = 3 LEDs in series.
For application versatility, if more Current drive is
required, the CCRs can be connected in parallel to boost the
regulated current.
D1
Anode
Q1
Q2
Qx
Cathode
+
− ^Vin
LED
HF3−R5570
LED
HF3−R5570
LED
HF3−R5570
Figure 7. Typical Application Circuit
(90 mA each LED String)
Number of LED’s that can be connected is determined by:
D1 is a reverse battery protection diode
Example: V_in = 12 Vdc, Q_X V_F = 3.5 Vdc, D1VF = 0.7 V
LED V_F = 2.6 Vdc @ 90 mA
(12 Vdc − (3.5 + 0.7 Vdc))/2.6 Vdc = 3 LEDs in series.
Number of Drivers = LED current/30 mA
90 mA/30 mA = 3 Drivers (Q1, Q2, Q3)
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