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參數(shù)資料
| 型號： | LM2792LDX-H |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 顯示驅(qū)動器 |
| 英文描述： | Silver Mica Capacitor; Capacitance:6800pF; Capacitance Tolerance:+/- 5%; Series:CDV30; Voltage Rating:1500VDC; Capacitor Dielectric Material:Mica; Termination:Radial Leaded; Lead Pitch:11.1mm; Leaded Process Compatible:No RoHS Compliant: No |
| 中文描述： | LED DISPLAY DRIVER, DSO10 |
| 封裝： | 3 X 3 MM, 0.80 MM HEIGHT, LLP-10 |
| 文件頁數(shù)： | 8/10頁 |
| 文件大?。?/td> | 204K |
| 代理商： | LM2792LDX-H |
Application Information
(Continued)
be used for most applications or a surface mount diode such
as BAT54-series and MA2J704 can be used to reduce the
circuit size.
Table 2
below lists suggested schottky diode
manufactures.
TABLE 2. Diode Manufactures
Manufacturer
ON
Semiconductor
Phillips
Semiconductors
Panasonic
Semiconductors
Contact
(800) 344
3860
(800) 234
7381
(408) 945
5622
website
www.onsemi.com
www.philipssemi-
conductor.com
www.panasonic.com
LED SELECTION
The LM2792 is designed to drive LEDs with a forward volt-
age of about 3.0V to 4.0V or higher. The typical and maxi-
mum V
depends highly on the manufacturer and their tech-
nology. Table 3 lists two suggested manufactures and
example part numbers. Each supplier makes many LEDs
that work well with the LM2792. The LEDs suggested below
are in a surface mount package and TOPLED or SIDEVIEW
configuration with a maximum forward current of 20mA.
These diodes also come in SIDELED or SIDEVIEW configu-
ration and various chromaticity groups. For applications that
demand color and brigthness matching, care must be taken
to select LEDs from the same chromaticity group. Forward
current matching is assured over the LED process variations
due to the constant current output of the LM2792. For best fit
selection for an application, consult the manufacturer for
detailed information.
TABLE 3. White LED Selection:
Component
LWT673/LWT67C
NSCW100/NSCW215 Nichia
Manufacture Contact
Osram
www.osram-os.com
www.nichia.com
I
SET
PIN
An external resistor, R
, sets the mirror current that is
required to provide a constant current through the LEDs. The
current through R
SET
and the LED is set by the internal
current mirror circuitry with a ratio of 25:1 The currents
through each LED are matched within 0.3%. R
SET
should be
chosen not to exceed the maximum current delivery capa-
bility of the device. Table 3 shows a list of R
SET
values when
maximun BRGT=3V is applied. For other BRGT voltages,
R
SET
can be calculated using this formula:
R
SET
= ((0.42*BRGT) / I
SET
)*25
TABLE 4. R
SET
Selections ( when BRGT pin = 3V
maximum)
I
LED
per LED
15mA
10mA
5mA
R
SET
2.1K
3.15K
6.3K
BRGT PIN
The BRGT pin can be used to smoothly vary the brightness
of the LEDs. In the LM2792, current on BRGT is connected
to an internal resistor divider which gives a factor of 0.42
(see
Figure 1
) . This voltage is fed to the operational ampli-
fier that controls the current through the mirror resistor R
SET
.
The nominal range on BRGT is 0V to 3V.
This means some current must be provided on the BRGT pin
or no current will flow through the LEDs. The LM2792 can
provide an infinite ratio, from fully off (essentially zero cur-
rent) to the maximum current set by the R
SET
resistor. Care
must be taken to prevent voltages on BRGT that cause LED
current to exceed a total of 34mA. Although this will not
cause damage to the IC, it will not meet the guaranteed
specifications listed in the Electrical Characteristics.
Calculation of LED Current When Using BRGT :
V
IN
= 3.6V
R
SET
= 1800
I
SET
= ((V
BRGT
*
(0.42) / R
SET
)
*
25
I
SET
= ((2.75
*
(0.42)) / 1800 )
*
25 = 16mA
Note that making V
BRGT
= 0V results in I
SET
～
= 0mA
BRIGHTNESS CONTROL USING PWM
Brigthness control can be implemented by pulsing a signal at
the SD pin. The recommended signal should be between
100Hz to 1kHz. If the operating PWM frequency is much less
than 100Hz, flicker may be seen in the LEDs. Likewise, if
frequency is much higher, brightness in the LEDs will not be
linear. When a PWM signal is used to drive the SD pin of the
LM2792, connect BRGT pin to a maximun of 3V to ensure
the widest range. Similarly, the voltage at the BRGT pin can
be set higher than 3V without damage to the IC, it will not
increase the brigthness of the LED significantly. R
SET
value
is selected using the above I
SET
equation as if BRGT pin is
used. The brigthness is controlled by increasing and de-
creasing the duty cycle of the PWM. Zero duty cycle will turn
off the brigthness and a 50% duty cycle waveform produces
an average current of 7.5mA if R
SET
is set to produce a
maximum LED current of 15mA. So the LED current varies
linearly with the duty cycle.
PARALLEL DX OUTPUTS FOR INCREASED CURRENT
DRIVE
Outputs D1 and D2 may be connected together to drive a
single LED. In such a configuration, two parallel current
sources of equal value drive the single LED. R
and
V
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, R
and V
should be selected so that
the current through each of the outputs is 15mA. Connecting
the outputs in parallel does not affect internal operation of
the LM2792 and has no impact on the Electrical Character-
istics and limits previously presented. The available Dx out-
put current, maximum Dx voltage, and all other specifica-
tions provided in the Electrical Characteristics table apply to
this parallel output configuration, just as they do to the
standard 2-LED application circuit.
THERMAL PROTECTION
The LM2792 has internal thermal protection circuitry to dis-
able the charge pump if the junction temperature exceeds
150C. This feature will protect the device from damage due
to excessive power dissipation. The device will recover and
operate normally when the junction temperature falls below
the maximum operating junction temperature of 100C. It is
L
www.national.com
8
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