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參數(shù)資料
| 型號： | LM2796 |
| 廠商： | National Semiconductor Corporation |
| 元件分類： | LED驅(qū)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(qū)動3/2x開關(guān)電容升壓 |
| 文件頁數(shù)： | 6/8頁 |
| 文件大小： | 172K |
| 代理商： | LM2796 |
Circuit Description
OVERVIEW
The LM2796 is primarily intended for Lithium-Ion battery
driven white-LED drive applications, and is well suited to
drive white LEDs that are used for backlighting small-format
displays. The part has seven matched constant-current out-
puts, each capable of driving up to 20mA (or more) through
white LEDs. The well-matched current sources ensure the
current through all the LEDs is virtually identical. This keeps
brightness of all LEDs matched to near perfection so that
they can provide a consistent backlight over the entire dis-
play.
The core of the LM2796 is a 1.5x/1x dual-mode charge
pump. The input of the charge pump is connected to the V
IN
pin. The recommended input voltage range of the LM2796 is
2.7V to 5.5V. The output of the charge pump is the P
pin
( “Pump OUTput”). The output voltage of the charge pump is
unregulated and varies with input voltage and load current.
The charge pump operates in the 1.5x mode when the input
voltage is below 4.75V (typ.). In this mode, the input-to-
output voltage gain of the charge pump is 1.5, and the
voltage at the output of the charge pump will be approxi-
mately 1.5x the input voltage (V(P
)
≈
1.5 * V
). When in
the 1.5x mode, the charge pump provides the voltage boost
that is required to drive white LEDs from a Li-Ion battery.
(White LEDs typically have a forward voltage in the range of
3.3V to 4.0V. A Li-Ion battery typically is not considered to be
fully discharged until the battery voltage falls to 3.0V (ap-
prox.) )
The charge pump operates in the 1x mode when the input
voltage is above 4.75V (typ.). In these conditions, voltage
boost is not required to drive the LEDs, so the charge pump
merely passes the input voltage to P
OUT
(V(P
OUT
)
≈
V
IN
).
This reduces the input current and the power dissipation of
the LM2796 when the input voltage is high.
The matched current outputs are generated with a precision
current mirror that is biased off the charge pump output.
Matched currents are ensured with the use of tightly
matched internal devices and internal mismatch cancellation
circuitry. Top-side current drive allows LEDs to be connected
between each current output and GND, simplifying PWB
routing and connectivity.
There are seven regulated current outputs. These seven
outputs are split into two groups, a group of 4 outputs and a
group of 3 outputs. There is an ON/OFF control pin for each
group.
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: EN, ENA, ENB
The LM2796 has 3 enable pins. All three are active-high
logic (HIGH = ON). There are internal pull-down resistors
(300k
typ.) that are connected internally between each of
the enable pins and GND.
The EN pin is the master enable pin for the part. When
voltage on this pin is low (
<
0.5V), the part is in shutdown
mode. All internal circuitry is OFF and the part consumes
very little supply current when the LM2796 is shutdown.
When the voltage on the EN pin is high (
>
1.1V), the part is
active. The charge pump is ON, and it is possible to turn on
the output currents to drive the LEDs.
ENA and ENB are used to turn the output currents ON and
OFF. ENA activates/deactivates the four group-A outputs
(D1A-D4A). ENB activates/deactivates the three group-B
outputs (D1B-D3B).
SETTING LED CURRENTS
The output currents of the LM2796 can be set to a desired
value simply by connecting an appropriately sized resistor
(R
SET
) between the I
SET
pin of the LM2796 and GND. The
output currents (LED currents) are proportional to the current
that flows out of the I
SET
pin. The output currents are a factor
of 100 greater than the I
SET
current. The feedback loop of an
internal amplifier sets the voltage of the I
SET
pin to 1.25V
(typ.). Placing a resistor between I
SET
and GND programs
the I
SET
current, and thus the LED currents. The statements
above are simplified in the equations below:
I
Dxx
= 100 x(V
SET
/ R
SET
)
R
SET
= 100 x (1.25V / I
Dxx
)
Maximum Output Current, Maximum LED Voltage,
Minimum Input Voltage
The LM2796 can drive 7 LEDs at 15mA each from an input
voltage as low as 3.0V, so long as the LEDs have a forward
voltage of 3.6V or less (room temperature).
The statement above is a simple example of the LED drive
capabilities of the LM2796. The statement contains the key
application parameters that are required to validate an LED-
drive design using the LM2796: LED current (I
LED
), number
of active LEDs (N), LED forward voltage (V
LED
mum input voltage (V
IN-MIN
).
The equation below can be used to estimate the total output
current capability of the LM2796:
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
OUT
. 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 LM2796 is typically 2.7
(V
IN
= 3.0V,
T
A
= 25C). In equation form:
V
POUT
= 1.5xV
IN
– NxI
LED
xR
OUT
k
– 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 LM2796 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
equation (eq. 3) and
solving for I
. 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)
Soft Start
The LM2796 contains internal soft-start circuitry to limit input
inrush currents when the part is enabled. Soft start is imple-
mented internally with a controlled turn-on of the internal
voltage reference. During soft start, the current through the
L
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6
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