LT3003 datasheet(9/16 Pages) LINER

9 / 16 page

LT3003

3003fa

In addition, LT3003 has an internal 150°C overtemperature

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 VLED mismatch and input

quiescent current.

1. Switch DC Loss:

PSW(DC) = ILED • VLED • 3

(See (VLED1,2,3 – VEE) vs ILED Typical Performance

Characteristics graph.)

2. Switch Loss due to VLED Mismatch:

PSW(ΔVLED) = Total VLED mismatch • ILED

3. Input Quiescent Loss:

PQ = (VIN – VEE) • (IQ – 1mA) + 1mA • VIN

LED

•

4. Total Power Dissipation:

PTOT = PSW(DC) + PSW(ΔVLED) + PQ

5. LT3003 Junction Temperature:

TJ (LT3003) = TA + θJA(PTOT);

θJA(PTOT) = PTOT • 35°C/W

Example

VIN = 3V; VEE = 0V; ILED = 350mA/string;

• I

Q =+

3 350

329 25

•

Total VLED 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 VLED mismatch = (6V – 5.7V) + (6V – 5.3V) = 1V

VLED = 1.1V at ILED = 350mA (see (VLED – VEE) vs ILED

Typical Performance Characteristics graph).

1. PSW(DC) = 3 • 350mA • (1.1V) = 1.16W

2. PSW(LED) = 1000mV • 350mA = 350mW

3. PQ = (3 – 0) • (29.25 – 1)mA + 3 • 1mA = 88mW

4. PTOT = 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 (θJA)

is about 35°C/W. For an ambient temperature of TA = 25°C,

the junction temperature of the LT3003, for the example

application described above, can be calculated as:

5. TJ = TA + θJA • PTOT = 25°C + 35°C/W • 1.6W

= 81°C

Minimizing LT3003 Internal Power Dissipation

The LT3003 requires at least 3V headroom between VIN

and VEE. Hence, for systems with high system input volt-

age and low VEE (such as running multiple series LEDs

in a Buck Mode), it is beneﬁcial to lower the level of VIN

pin voltage (LT3003 upper rail) with an external zener to

reduce power dissipation in the chip. Therefore, it is recom-

mended to limit (VIN – VEE) to less than 10V. To achieve

best performance, (VIN – VEE) should equal 3V.

APPLICATIONS INFORMATION

LT3003 Arkusz danych(PDF) 9 Page - Linear Technology

LT3003 Arkusz danych(HTML) 9 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Arkusz danych Pobierz

Link URL