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LT3003
APPLICATIONS INFORMATION
? I Q =
+ 3 mA = 29 . 25 mA
Inaddition,LT3003hasaninternal150°Covertemperature
protection circuitry that resets the chip to zero LED current
mode. This prevents the chip from continuous operation
at high temperature.
Thermal Calculations
To maximize output power capability in an application
without exceeding the LT3003 125°C maximum operational
junction temperature, it is useful to be able to calculate
power dissipation within the IC. The power dissipation
within the LT3003 comes from four main sources: switch
DC loss, switch loss due to LED V LED mismatch and input
quiescent current.
1. Switch DC Loss:
P SW(DC) = I LED ? V LED ? 3
(See (V LED1,2,3 – V EE ) vs I LED Typical Performance
Characteristics graph.)
2. Switch Loss due to V LED Mismatch:
P SW( Δ VLED) = Total V LED mismatch ? I LED
3. Input Quiescent Loss:
P Q = (V IN – V EE ) ? (I Q – 1mA) + 1mA ? V IN
Example
V IN = 3V; V EE = 0V; I LED = 350mA/string;
3 ? 350mA
40
Total V LED mismatch = 1V:
? e.g., LED string 1 voltage drop = 6V;
LED string 2 voltage drop = 5.7V;
LED string 3 voltage drop = 5.3V
? Total V LED mismatch = (6V – 5.7V) + (6V – 5.3V) = 1V
V LED = 1.1V at I LED = 350mA (see (V LED – V EE ) vs I LED
Typical Performance Characteristics graph).
1. P SW(DC) = 3 ? 350mA ? (1.1V) = 1.16W
2. P SW(LED) = 1000mV ? 350mA = 350mW
3. P Q = (3 – 0) ? (29.25 – 1)mA + 3 ? 1mA = 88mW
4. P TOT = 1.1W + 350mW + 88mW ≈1.6W
The LT3003 uses a thermally enhanced 10-lead MSE
package. With proper soldering of the Exposed Pad to
the underside of the package, combined with a full copper
plane underneath the device, the thermal resistance ( θ JA )
is about 35°C/W. For an ambient temperature of T A = 25°C,
I Q =
3 ? I LED
40
+ 3 mA
the junction temperature of the LT3003, for the example
application described above, can be calculated as:
4. Total Power Dissipation:
P TOT = P SW(DC) + P SW( Δ VLED) + P Q
5. LT3003 Junction Temperature:
T J (LT3003) = T A + θ JA (P TOT );
θ JA (P TOT ) = P TOT ? 35°C/W
5. T J = T A + θ JA ? P TOT = 25°C + 35°C/W ? 1.6W
= 81°C
Minimizing LT3003 Internal Power Dissipation
The LT3003 requires at least 3V headroom between V IN
and V EE . Hence, for systems with high system input volt-
age and low V EE (such as running multiple series LEDs
in a Buck Mode), it is bene?cial to lower the level of V IN
pin voltage (LT3003 upper rail) with an external zener to
reduce power dissipation in the chip. Therefore, it is recom-
mended to limit (V IN – V EE ) to less than 10V. To achieve
best performance, (V IN – V EE ) should equal 3V.
3003fa
9
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