MC10SX1130DR2G 데이터 시트보기 (PDF) - ON Semiconductor

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MC10SX1130DR2G

LED Driver

ON Semiconductor 

MC10SX1130

First, the RSET resistor must be chosen to set the desired

nominal modulation current based on the following

equation:

RSET = VSET/IMOD (Equation 1)

The voltage at VSET is a function of the RTCO tracking

resistor, so the desired tracking rate (VTR) must also be

chosen. To determine this, the equation must be normalized

to correspond to how the LED has been specified.

Temp Co = VTR/VSET (Equation 2)

The data sheet has three temperature tracking rates for

different values of the RTCO resistor. By using the VSET

values at 25°C and substituting those numbers into

Equation 2, normalized tracking rates can be calculated.

Table 5. Normalized Tracking at 25°C

RTCO

Tracking %/°C

Short

+0.20

1 KW

+0.52

2 KW

+0.89

To match the LED chosen, a 1 kW resistor can be used.

Now that this is known, the value of the voltage at the VSET

can be substituted into Equation 1 to determine the value of

RSET resistor which, for this example is 10 W.

The Stretch circuit can be used to compensate for the

turn-on/turn-off delay of the LED. The circuit has been

designed for ease of use so the pin is designed to be strapped

to one of the two power plane levels to select the

pre-distortion value. If no pre-distortion is desired, the pin

can be left open. In this +5 V example, the maximum amount

of pre-distortion is desired, so the STRETCH pin is

connected to ground.

In addition a resistor must be placed between IOUT and

VCC. In selecting this resistor, just as in the case of the RSET,

the resistor type should be chosen to dissipate the worst case

power and derated for the worst case temperature. As a rule

of thumb, the voltage drop across the resistor should match

the forward voltage across the diode. The voltage can be

larger to minimize the power dissipated on chip when the

LED is not ’ON’. Although, the voltage drop across this

resistor should not be greater than 2 V. For this example:

R @ IOUT = VF/IMOD

IMOD(max)

+

VSET@85°C

RSET

+

855mV

10W

+

86mA

R @ IOUT = 1.5V/86mA = 17W

Because of the positive tracking circuitry in the LED

driver, the modulation current will increase over

temperature. It is important to now go back and re-calculate

the numbers under the worst case environmental conditions

to ensure that operating conditions have not been exceeded.

Thermal Management

LED devices tend to require large amounts of current for

most efficient operation. This requirement is then translated

into the design of the LED Driver. When large modulation

currents are required, power dissipation becomes a critical

issue and the user must be concerned about the junction

temperature of the device. The following equation can be

used to estimate the junction temperature of a device in a

given environment:

TJ = TA + PD * qJA (Equation 3)

TJ

Junction Temperature

TA

Ambient Temperature

PD

Power Dissipation

qJA

Average Thermal Resistance

(Junction-Ambient)

A specially designed thermally enhanced leadframe has

been used to house the LED Driver. Below is a graph of the

average qJA plotted against air flow.

110

100

90

80

70

0

100

200

300

400

500

AIRFLOW (LFPM)

Figure 4. Typical qJA versus Airflow

The power dissipation of the device has two components;

the quiescent power drain related to the pre-drive circuitry,

and the power dissipated in the current switch when driving

the LED.

Pd = Pstatic + Pswitching

(Equation 4)

The power dissipated in the current switch is a function of

the IMOD current, the LED forward voltage, and the value

of RSET. For example in a +5 V application, the following

equations can be used:

Pstatic = VCC * ICC (Equation 5)

Pswitching = (VCC-VF-VSET)* IMOD (Equation 6)

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