Output resistance results in droop in the P

portional to the amount of current delivered by the pump.

The P

total output current capability of an application. Taking total

output current to be the sum of all D

the current delivered through the P

P

I

V

LED HEADROOM VOLTAGE (V

Four current sources are connected internally between P

OUT

and D

−V

rent sources require a sufficient amount of headroom voltage

to be present across them in order to regulate properly.

Minimum required headroom voltage is proportional to the

current flowing through the current source, as dictated by the

equation:

V

The parameter k

proportionality constant that represents the ON-resistance of

the internal current mirror transistors. For worst-case design

calculations, using a k

(Worst-case recommendation accounts for parameter shifts

from part-to-part variation and applies over the full operating

temperature range). Figure 3 shows how output current of

the LM2794/5 varies with respect to headroom voltage.

On the flat part of the graph, the currents regulate properly

as there is sufficient headroom voltage for regulation. On the

sloping part of the graph the headroom voltage is too small,

the current sources are squeezed, and their current drive

capability is limited. Changes in headroom voltage from one

output to the next, possible with LED forward voltage mis-

match, will result in different output currents and LED bright-

ness mismatch. Thus, operating the LM2794/5 with insuffi-

cient headroom voltage across the current sources should

be avoided.

20028535

FIGURE 2. Keypad LEDs Connected to P

OUT

20028539

FIGURE 3. I

4 LEDs, V

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