LM2794EVAL National Semiconductor, LM2794EVAL Datasheet - Page 2

• 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

www.national.com
R
R
It is left intentionally for the end users to set the desired
current range. Equations below are used for determining
R
For example, if 15mA is the desired current per LED and
BRGT = 3V, using the above equation 1 yields R
ohms. Table 1 below shows a typical LED current when
BRGT is at 3V(LM2792) maximum and Table 2 shows a
typical LED current when BRGT is at 0V(LM2791). If BRGT
voltage is other than 3V, Table 5 shows R
combination per LED current.
R
Similar to the LM2791/2, R
users to set the desired current range. Below is the equation
for determining R
For example, if 15mA is the desired current per LED and
BRGT = 0V (or ground), using the above equation 3 yields
R
when BRGT is tied to ground and Table 4 shows typical LED
current when BRGT is at 3V maximum. If BRGT is used in
the application, Table 5 shows R
per LED current.
SET
SET
SET
SET
SET
is the resistor that sets the current through both LEDs.
value:
= 124 ohms. Table 3 below shows typical LED current
R
R
SET
Selection (LM2791/2)
SET
Selection (LM2794/5)
R
TABLE 1. R
TABLE 2. R
= ((0.188 + (0.385 • BRGT)) / ILED) • 10
TABLE 3. R
TABLE 4. R
*Rset value is rounded off to the nearest 1% value
= (((0.42 • BRGT) + 0.200)/ ILED) • 25
SET
* Use 1% resistor for Rset
*Use 1% resistor for Rset
(example given for LM2792)
(example given for LM2791)
SET
= ((0.42 • BRGT) / ILED) • 25
15mA
10mA
15mA
10mA
15mA
10mA
5mA
I
I
5mA
5mA
I
I
Eq. 3 for LM2794/5:
LED
LED
LED
LED
Eq. 1 for LM2792:
Eq. 2 for LM2791:
value:
SET
SET
SET
SET
SET
when BRGT = 3V
when BRGT = 0V
when BRGT = 0V
when BRGT = 3V
is left intentionally for the end
SET
3.15kΩ
* R
*R
2.1kΩ
6.3kΩ
330Ω
500Ω
*R
1KΩ
124Ω
196Ω
383Ω
R
and BRGT combination
SET
SET
SET
SET
SET
SET
and BRGT
= 2.1K
2
BRGT (LM2791/2)
A voltage from 0 to 3V may be applied to the BRGT pin to
control the brightness of the LEDs by varying the current
through the LEDs. Note that some voltage must be provided
at BRGT pin or no current will flow through the LEDs for the
LM2792. BRGT pin is connected to an internal resistor di-
vider that gives a factor of 0.42 (LM2792). The product of this
factor and the voltage at BRGT is fed to the input of an
internal amplifier that sets the current mirror resistor R
Table 5 shows the relationship between LED current with
various R
ensure that the voltage at BRGT does not cause LEDs
current to exceed a total of 34mA (LM2792). Note that
calculations are based on when both D1 and D2 are in use.
(Values Highlighted in Boldface exceed maximum current
range of the device if both D1 & D2 are in use)
By rearranging equation 1, the following equation can be
used to determine ILED:
BRGT (LM2794/5)
A voltage from 0 to 3V may be applied to the BRGT pin to
control the brightness of the LEDs by varying the current
through the LEDs. BRGT pin is connected to an internal
resistor divider and summed with an offset voltage from the
internal bandgap (188mV). This voltage is fed to the input of
an internal amplifier that sets the current mirror resistor
R
current with various R
taken to ensure that the voltage at BRGT does not cause
LEDs current to exceed total of 80mA. By rearranging equa-
tion 3, the following equation can be used to determine the
ILED:
Voltage on
SET
BRGT (V)
(Example for LM2792 & both D1 & D2 are in use)
TABLE 5. LED Current When Using BRGT Input
. Table 6 below shows the relationship between LED
0.5
1.0
1.5
2.0
2.5
3.0
ILED = ((0.188 + (0.385 • BRGT)) / R
0
ILED = (((0.42) + 0.200) • BRGT) / R
SET
ILED = ((0.42 • BRGT) / R
*Rset value is rounded off to the nearest 1% value
and BRGT values. Care must be taken to
1000Ω
10.05
15.75
26.25
5.25
31.5
15mA
10mA
5mA
21
0
Eq. 4 for LM2792:
Eq. 5 for LM2791:
SET
Current through LED (mA)
and BRGT values. Care must be
Eq. 6:
1500Ω
10.5
17.5
3.5
14
21
0
7
1.4KΩ
2.67Ω
909Ω
R
SET
SET
2000Ω
) • 25
15.75
5.25
7.88
10.5
13.1
2.6
SET
SET
0
) • 25
) • 10
2500Ω
10.5
12.6
SET
2.1
4.2
6.3
8.4
0
.