LM2794EVAL National Semiconductor, LM2794EVAL Datasheet - Page 12

• Manufacturer Part Number: LM2794EVAL
• Description: BOARD EVALUATION LM2794
• Manufacturer: National Semiconductor

Specifications of LM2794EVAL

• Current - Output / Channel: 80mA
• Outputs And Type: 4, Non-Isolated
• Voltage - Output: 4 V
• Features: Charge Pump
• Voltage - Input: 2.7 ~ 5.5V
• Utilized Ic / Part: LM2794
• Lead Free Status / RoHS Status: Not applicable / Not applicable

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Application Information
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
LED HEADROOM VOLTAGE (V
Four current sources are connected internally between P
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:
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.
OUT
DX
can be predicted with the following equations:
1
OUT
), is referred to as headroom voltage (V
-D
4
. The voltage across each current source, (V
voltage is an important factor in determining the
I
TOTAL
V
POUT
HR
= I
, typically 20mV/mA in the LM2794/5, is a
= 1.5 x V
V
D1
HR-MIN
+ I
HR
D2
of 25mV/mA is recommended.
= k
IN
+ I
− I
HR
D3
HR
TOTAL
x I
+ I
)
OUT
X
DX
D4
output currents plus
FIGURE 2. Keypad LEDs Connected to P
x R
pin, the voltage at
+ I
(Continued)
OUT
POUT
OUT
HR
voltage pro-
). The cur-
POUT
OUT
12
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.
OUT
FIGURE 3. I
4 LEDs, V
LED
IN
= 3.0V
vs V
HR
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