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參數資料
| 型號： | LM27961 |
| 廠商： | National Semiconductor Corporation |
| 元件分類： | LED驅動器 |
| 英文描述： | Dual-Display White LED Driver with 3/2x Switched Capacitor Boost |
| 中文描述： | 雙顯示器白光LED驅動3/2x開關電容升壓 |
| 文件頁數： | 10/13頁 |
| 文件大?。?/td> | 711K |
| 代理商： | LM27961 |
Circuit Description
(Continued)
anode outputs and a group of 3 common cathode outputs.
There is an ON/OFF control pin for each group (ENA and
ENB).
The DC current through the LEDs is programmed with an
external resistor. Changing currents on-the-fly can be
achieved with the use of digital pulse (PWM) signals.
ENABLE PINS: ENA, ENB
The LM27961 has 2 enable pins. Both are active-high logic
(HIGH = ON). There are internal pull-down resistors (300k
typ.) that are connected internally between each of the en-
able pins and GND.
ENA and ENB can both enable and disable the part. When
the voltage on both pins are low (
<
0.5V), the part is in
shutdown mode. All internal circuitry is OFF and the part
consumes very little supply current when the LM27961 is
shutdown. When the voltage on either ENx pin is high
(
>
1.1V), the part is active. The charge pump is ON, and the
corresponding output current drivers are active.
ENA and ENB are used to turn the output currents ON and
OFF. ENA activates/deactivates the four GroupA outputs
(D1A-D4A). ENB activates/deactivates the three GroupB
outputs (D1B-D3B).
SETTING LED CURRENTS
The output currents of the LM27961 can be set to a desired
value simply by connecting an appropriately sized resistor
(R
SETx
) between the I
SETx
pins of the LM27961 and GND.
R
SETA
sets the current for the GroupA outputs and R
SETB
sets the current for the GroupB outputs. The output currents
(LED currents) are proportional to the current that flows out
of the I
pins. The output currents are a factor of 100
greater than the I
current. The feedback loop of an
internal amplifier sets the voltage of the I
pin to 1.25V
(typ.). Placing a resistor between I
and GND programs
the I
current, and thus the LED currents. The statements
above are simplified in the equations below:
I
Dxx
= 100 x (V
SETx
/ R
SETx
)
R
SETx
= 100 x (1.25V / I
Dxx
)
MAXIMUM OUTPUT CURRENT, MAXIMUM LED
VOLTAGE, MINIMUM INPUT VOLTAGE
The LM27961 can drive 4 LEDs at 15mA each from an input
voltage as low as 2.7V, so long as the LEDs have a forward
voltage of 3.5V or less (room temperature).
The statement above is a simple example of the LED drive
capabilities of the LM27961. The statement contains the key
application parameters that are required to validate an LED-
drive design using the LM27961: LED current (I
LEDx
), num-
ber of active LEDs (N), LED forward voltage (V
LED
), and
minimum input voltage (V
IN-MIN
).
The equation below can be used to estimate the total output
current capability of the LM27961:
I
LED_MAX
= ((1.5 x V
IN
) - V
LED
) / ((N x R
OUT
) + k
HR
) (eq. 1)
I
LED_MAX
= ((1.5 x V
IN
) - V
LED
) / ((N x 2.7
) + 22mV/mA)
R
OUT
– Output resistance. This parameter models the inter-
nal losses of the charge pump that result in voltage droop at
the pump output P
. Since the magnitude of the voltage
droop is proportional to the total output current of the charge
pump, the loss parameter is modeled as a resistance. The
output resistance of the LM27961 is typically 2.7
(V
IN
=
3.0V, T
A
= 25C). In equation form:
V
POUT
= 1.5xV
IN
– NxI
LED
xR
OUT
k
HR
– Headroom constant. This parameter models the mini-
mum voltage required to be present across the current
sources for them to regulate properly. This minimum voltage
is proportional to the programmed LED current, so the con-
stant has units of mV/mA. The typical k
HR
of the LM27961 is
22mV/mA. In equation form:
(V
POUT
– V
LED
)
>
k
HR
xI
LED
The "I
LED-MAX
" equation (eq. 1) is obtained from combining
the R
OUT
equation (eq. 2) with the k
HR
equation (eq. 3) and
solving for I
LED
. Maximum LED current is highly dependent
on minimum input voltage and LED forward voltage. Output
current capability can be increased by raising the minimum
input voltage of the application, or by selecting an LED with
a lower forward voltage. Excessive power dissipation may
also limit output current capability of an application.
(eq. 2)
(eq. 3)
PARALLEL Dx OUTPUTS FOR INCREASED CURRENT
CAPABILITY
Outputs D1A through D4A, or D1B through D3B may be
connected together in any combination to drive higher cur-
rents through fewer LEDs. For example in
Figure 1
, outputs
D1A and D2A are connected together to drive one LED. D3A
and D4A are connected to drive a second LED.
With this configuration, two parallel current sources of equal
value provide current to one of the LEDs. R
should
therefore be chosen so that the current through each output
is programmed to 50% of the desired current through the
parallel connected LED. For example, if 40mA is the desired
drive current for the parallel connected LED, R
should
be selected so that the current through each of the outputs is
20mA. Other combinations of parallel outputs may be imple-
mented in similar fashions, such as in
Figure 2
.
20127919
FIGURE 1. Two Parallel Connected LEDs
L
www.national.com
10
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