LM2759SDEV datasheet(15/22 Pages) TI1

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LM2759

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SNVS577D – JUNE 2008 – REVISED MAY 2013

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 junction temperature of the device does not

exceed the maximum operating rating of 105°C. The maximum ambient temperature rating must be derated in

applications where high power dissipation and/or poor thermal resistance causes the junction 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 minimum input voltage, and the charge pump gain:

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

(9)

VHR or the voltage required across the current sink to remain in regulation can be approximated by (ILED x KHR),

where KHR 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 (VF + VHR) portion of the equation can not be

greater than the nominal output regulation voltage for a particular gain. In other words, when making calculations

for an application where the term (VF + VHR) is higher than a particular gain’s regulation voltage, the next higher

gain level must be used for the calculation.

Example: VF = 4V @ 1A, Charge Pump in the Gain of 2x with a ROUT of 2.25Ω (typ.)

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

VIN_MIN > 3.53V (typ.)

The maximum power dissipation in the LM2759 must also be taken into account when selecting the conditions

for an application, 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 application 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 design. 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, resulting in poor regulation or

instability. Poor layout can also result in re-flow problems leading to poor solder joints between the WSON

package and board pads. Poor solder joints can result in erratic or degraded performance.

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