OUTPUT CURRENT CAPABILITY

The primary constraint on the total current capability is the

headroom voltage requirement of the internal current

sources. Combining the V

previous two sections yields the basic inequality for deter-

mining the validity of an LM2794/5 LED-drive application:

V

V

V

≥ V

HR-MIN

1.5xV

≥ (k

Rearranging this inequality shows the estimated total output

current capability of an application:

I

TOTAL

≤ [(1.5 x V

Examining the equation above, the primary limiting factors

on total output current capability are input and LED forward

voltage. A low input voltage combined with a high LED

voltage may result in insufficient headroom voltage across

the current sources, causing them to fall out of regulation.

When the current sources are not regulated, LED currents

will be below desired levels and brightness matching will be

highly dependent on LED forward voltage matching.

Typical LM2794/5 output resistance is 3.0

Ω. For worst-case

design calculations, using an output resistance of 3.5

Ω is

recommended. LM2794/5 has a typical k

HR

constant of

20mV/mA. For worst-case design calculations, use k

25mV/mA. (Worst-case recommendations account for pa-

rameter shifts from part-to-part variation and apply over the

full operating temperature range). R

slightly with temperature, but losses are typically offset by

the negative temperature coefficient properties of LED for-

ward voltages. Power dissipation and internal self-heating

may also limit output current capability but is discussed in a

later section.

PARALLEL Dx OUTPUTS FOR INCREASED CURRENT

DRIVE

Outputs D

combination to drive higher currents through fewer LEDs.

For example in Figure 4, outputs D

together to drive one LED while D

together to drive a second LED.

With this configuration, two parallel current sources of equal

value provide current to each LED. R

therefore be chosen so that the current through each output

is programmed to 50% of the desired current through the

parallel connected LEDs. For example, if 30mA is the de-

sired drive current for 2 parallel connected LEDs , R

V

the outputs is 15mA. Other combinations of parallel outputs

may be implemented in similar fashions, such as in Figure 5.

Connecting outputs in parallel does not affect internal opera-

tion of the LM2794/95 and has no impact on the Electrical

Characteristics and limits previously presented. The avail-

able diode output current, maximum diode voltage, and all

other specifications provided in the Electrical Characteristics

table apply to parallel output configurations, just as they do

to the standard 4-LED application circuit.

THERMAL PROTECTION

When the junction temperature exceeds 150˚C (typ.), the

LM2794/5 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 normally when the junction temperature falls below

140˚C (typ.). It is important to have good thermal conduction

with a proper layout to reduce thermal resistance.

POWER EFFICIENCY

Figure 6 shows the efficiency of the LM2794/5. The change

in efficiency shown by the graph comes from the transition

from Pass Mode to a gain of 1.5.

Efficiency (E) of the LM2794/5 is defined here as the ratio of

the power consumed by LEDs (P

from the input source (P

quiescent current of the LM2794/5, I

through one LED, V

current, and N is the number of LEDs connected to the

regulated current outputs. In the input power calculation, the

1.5 represents the switched capacitor gain configuration of

the LM2794/5.

P

P

P

20028533

FIGURE 4. Two Parallel Connected LEDs

20028534

FIGURE 5. One Parallel Connected LED

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