TABLE 2. White LED Selection

Manufacturer

Contact

Osram

www.osram-os.com

Nichia

www.nichia.com

PARALLEL DX OUTPUTS FOR INCREASED CURRENT

DRIVE

Outputs D

single LED. In such a configuration, two parallel current

sources of equal value drive the single LED. The voltage on

SD-BRGT should be chosen so that the current through

each of the outputs is programmed to 50% of the total

desired LED current. For example, if 30mA is the desired

drive current for the single LED, SD-BRGT should be se-

lected so that the current through each of the outputs is

15mA. Connecting the outputs in parallel does not affect

internal operation of the LM2793 and has no impact on the

Electrical Characteristics and limits previously presented.

The available diode output current, maximum diode voltage,

and all other specifications provided in the Electrical Char-

acteristics table apply to this parallel output configuration,

just as they do to the standard 2-LED application circuit.

P

OUT

P

the LM2793 1.5x charge pump. Pre-regulation uses the

voltage present at P

switched capacitor charge pump. Pre-regulation helps to

reduce input current noise and large input current spikes

normally associated with switched capacitor charge pumps.

At voltages below 3.3V (typ.), the LM2793 acts as an open

loop charge pump. When the device is in open loop, the

voltage at P

voltages higher than 3.3V (typ.) P

4.9V (typ.).

POWER EFFICIENCY

Figure 2 shows the efficiency of the LM2793.

Efficiency (E) of the LM2793 is defined here as the ratio of

the power consumed by LEDs (P

from the input source (P

quiescent current of the LM2793, I

through one LED, and V

LED current. In the input power calculation, the 1.5 multiplier

reflects the 3/2 switched capacitor gain of the LM2793.

P

P

P

E=(P

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 (V

Power consumption of the LM2793 Typical Application Cir-

cuit is shown in Figure 3.

THERMAL PROTECTION

When the junction temperature exceeds 150˚C, the LM2793

internal thermal protection circuitry disables the part. This

feature protects the device from damage due to excessive

power dissipation. The device will recover and operate nor-

mally when the junction temperature falls below 125˚C. It is

important to have good thermal conduction with a proper

layout to reduce thermal resistance.

POWER DISSIPATION

When operating within specified operating ratings, the peak

power dissipation (P

input voltage of 5.5V. Assuming a typical junction-to-ambient

thermal resistance (

θJA) for the LLP-10 package of 55˚C/W,

a LED forward voltage (V

of 32mA for two White LEDs connected to D

power dissipation and junction temperature (T

lated below for a part operating at the maximum rated am-

bient temperature (T

the input voltage to the LM2793, P

by the 1.5x charge pump, and P

by the LEDs.

P

= (1.5V

= ((1.5 x 5.5V) - 3.6V) x 0.032A

= 149mW

20063612

FIGURE 2. Efficiency vs. V

IN

2 LEDs, V

20063613

FIGURE 3. I

2 LEDs, 2.5

≤ V

LED

≤ 3.9V, I

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