LM2759 datasheet(12/14 Pages) NSC

12 / 14 page

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POWER EFFICIENCY

Efficiency of LED drivers is commonly taken to be the ratio of

power consumed by the LED (P

LED) to the power drawn at the

input of the part (P

IN). With a 1x, 1.5x, 2x charge pump, the

input current is equal to the charge pump gain times the output

current (total LED current). The efficiency of the LM2759 can

be predicted as follows:

LED = VLED × ILED

IN = VIN × IIN

IN = VIN × (Gain × ILED + IQ)

E = (P

LED ÷ PIN)

For a simple approximation, the current consumed by internal

circuitry (I

Q) can be neglected, and the resulting efficiency will

become:

E = V

LED ÷ (VIN × Gain)

Neglecting I

Q will result in a slightly higher efficiency predic-

tion, but this impact will be negligible due to the value of I

being very low compared to the typical Torch and Flash cur-

rent levels (100mA - 1A). It is also worth noting that efficiency

as defined here is in part dependent on LED voltage. Variation

in LED voltage does not affect power consumed by the circuit

and typically does not relate to the brightness of the LED. For

an advanced analysis, it is recommended that power con-

sumed by the circuit (V

IN x IIN) be evaluated rather than power

efficiency.

THERMAL PROTECTION

Internal thermal protection circuitry disables the LM2759

when the junction temperature exceeds 150°C (typ.). This

feature protects the device from being damaged by high die

temperatures that might otherwise result from excessive pow-

er dissipation. The device will recover and operate normally

when the junction temperature falls below 120°C (typ.). It is

important that the board layout provide good thermal conduc-

tion to keep the junction temperature within the specified

operating ratings.

POWER DISSIPATION

The power dissipation (P

DISSIPATION) and junction temperature

J) can be approximated with the equations below. PIN is the

power generated by the 1x, 1.5x, 2x charge pump, P

LED is the

power consumed by the LED, T

A is the ambient temperature,

and

JA is the junction-to-ambient thermal resistance for the

12 pin LLP package. V

IN is the input voltage to the LM2759,

LED is the nominal LED forward voltage, and ILED is the pro-

grammed LED current.

DISSIPATION = PIN - PLED

= (Gain × V

IN × ILED) − (VLED × ILED)

J = TA + (PDISSIPATION × θJA)

The junction temperature rating takes precedence over the

ambient temperature rating. The LM2759 may be operated

outside the ambient temperature rating, so long as the junc-

tion temperature of the device does not exceed the maximum

operating rating of 105°C. The maximum ambient tempera-

ture rating must be derated in applications where high power

dissipation and/or poor thermal resistance causes the junc-

tion temperature to exceed 105°C.

MAXIMUM OUTPUT CURRENT

The maximum LED current that can be used for a particular

application depends on the rated forward voltage of the LED

used, the input voltage range of the application, and the Gain

mode of the LM2759’s charge pump. The following equation

can be used to approximate the relationship between the

maximum LED current, the LED forward voltage, the mini-

mum input voltage, and the charge pump gain:

IN_MIN x Gain) > (VF + VHR) + (ILED x ROUT_GAIN)

HR or the voltage required across the current sink to remain

in regulation can be approximated by (I

LED x KHR), where

HR is 0.8 mV/mA (typ). ROUT_GAIN is the output impedance of

the charge pump according to its gain mode. When using the

equation above, keep in mind that the (V

F + VHR) portion of

the equation can not be greater than the nominal output reg-

ulation voltage for a particular gain. In other words, when

making calculations for an application where the term (V

F +

HR) is higher than a particular gain’s regulation voltage, the

next higher gain level must be used for the calculation.

Example: V

F = 4V @ 1A, Charge Pump in the Gain of 2x with

a R

OUT of 2.25Ω (typ.)

IN_MIN > [(4V + 0.8V) + (1A x 2.25Ω) ] ÷ 2

IN_MIN > 3.53V (typ.)

The maximum power dissipation in the LM2759 must also be

taken into account when selecting the conditions for an ap-

plication, such that the junction temperature of the device

never exceeds its rated maximum. The input voltage range,

operating temperature range, and/or current level of the ap-

plication may have to be adjusted to keep the LM2759 within

normal operating ratings.

BOARD LAYOUT CONSIDERATIONS

PC board layout is an important part of DC-DC converter de-

sign. Poor board layout can disrupt the performance of a DC-

DC converter and surrounding circuitry by contributing to EMI,

ground bounce, and resistive voltage loss in the traces. These

can send erroneous signals to the DC-DC converter IC, re-

sulting in poor regulation or instability. Poor layout can also

result in re-flow problems leading to poor solder joints be-

tween the LLP package and board pads. Poor solder joints

can result in erratic or degraded performance.

LM2759 Datasheet(PDF) 12 Page - National Semiconductor (TI)

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