LM2759SDEV datasheet(14/22 Pages) TI1

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LM2759

SNVS577D – JUNE 2008 – REVISED MAY 2013

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temperature (Y5V: +22%, -82% over -30°C to +85°C range; Z5U: +22%, -56% over +10°C to +85°C range).

Under some conditions, a nominal 1

μF Y5V or Z5U capacitor could have a capacitance of only 0.1 μF. Such

detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance

requirements of the LM2759. The voltage rating of the output capacitor should be 6.3V or more. For example, a

6.3V 0603 4.7

μF output capacitor (TDK C1608X5R0J475) is acceptable for use with the LM2759, as long as the

capacitance on the output does not fall below a minimum of 3

μF in the intended application. All other capacitors

should have a voltage rating at or above the maximum input voltage of the application and should have a

minimum capacitance of 1

μF.

Table 5. Suggested Capacitors and Suppliers

MFG Part No.

Type

MFG

Voltage Rating

Case Size

Inch (mm)

4.7 µF for COUT

C1608X5R0J475

Ceramic X5R

TDK

6.3V

0603 (1608)

JMK107BJ475

Ceramic X5R

Taiyo-Yuden

6.3V

0603 (1608)

2.2 µF for C1, C2, CIN

C1608X5R0J225

Ceramic X5R

TDK

6.3V

0603 (1608)

JMK107BJ225

Ceramic X5R

Taiyo-Yuden

6.3V

0603 (1608)

POWER EFFICIENCY

Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LED (PLED) to the power

drawn at the input of the part (PIN). 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:

PLED = VLED × ILED

(1)

PIN = VIN × IIN

(2)

PIN = VIN × (Gain × ILED + IQ)

(3)

E = (PLED ÷ PIN)

(4)

For a simple approximation, the current consumed by internal circuitry (IQ) can be neglected, and the resulting

efficiency will become:

E = VLED ÷ (VIN × Gain)

(5)

Neglecting IQ will result in a slightly higher efficiency prediction, but this impact will be negligible due to the value

of IQ being very low compared to the typical Torch and Flash current 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 consumed by the circuit (VIN 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 power 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 conduction to keep the junction

temperature within the specified operating ratings.

POWER DISSIPATION

The power dissipation (PDISSIPATION) and junction temperature (TJ) can be approximated with the equations

below. PIN is the power generated by the 1x, 1.5x, 2x charge pump, PLED is the power consumed by the LED, TA

is the ambient temperature, and

θJA is the junction-to-ambient thermal resistance for the 12 pin WSON package.

VIN is the input voltage to the LM2759, VLED is the nominal LED forward voltage, and ILED is the programmed

LED current.

PDISSIPATION = PIN - PLED

(6)

= (Gain × VIN × ILED) − (VLED × ILED)

(7)

TJ = TA + (PDISSIPATION × θJA)

(8)

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