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參數(shù)資料
| 型號(hào)： | LM27966SQX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 顯示驅(qū)動(dòng)器 |
| 英文描述： | White LED Driver with I2C Compatible Interface |
| 中文描述： | LED DISPLAY DRIVER, QCC24 |
| 封裝： | 4 X 4 MM, 0.80 MM HEIGHT, LLP-24 |
| 文件頁數(shù)： | 11/13頁 |
| 文件大?。?/td> | 673K |
| 代理商： | LM27966SQX |
Application Information
(Continued)
MAXIMUM OUTPUT CURRENT, MAXIMUM LED
VOLTAGE, MINIMUM INPUT VOLTAGE
The LM27966 can drive 6 LEDs at 30mA each (Main Display
and D
) from an input voltage as low as 3.2V, 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 LM27966. The statement contains the key
application parameters that are required to validate an LED-
drive design using the LM27966: LED current (I
LEDx
), num-
ber of active LEDs (N
x
), LED forward voltage (V
LED
), and
minimum input voltage (V
IN-MIN
).
The equation below can be used to estimate the maximum
output current capability of the LM27966:
I
LED_MAX
= [(1.5 x V
IN
) - V
- (I
x R
OUT
)] /
[(N
MAIN
x R
OUT
) + k
HR
] (eq. 1)
I
LED_MAX
= [(1.5 x V
IN
) - V
LED
- (I
DAUX
x 2.75
)] /
[(N
MAIN
x 2.75
) + k
HR
I
is the additional current that could be delivered to the
AUX LED.
R
– 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 LM27966 is typically 2.75
(V
IN
=
3.6V, T
A
= 25C). In equation form:
V
POUT
= (1.5 x V
IN
) – [N
MAIN
x I
LED-MAIN
x R
OUT
]
2)
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 LM27966 is
8mV/mA. In equation form:
(V
POUT
– V
LEDx
)
>
k
HR
x I
LEDx
Typical Headroom Constant Value
k
HR
= 8mV/mA
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.
(eq. 3)
Total Output Current Capability
The maximum output current that can be drawn from the
LM27966 is 180mA. Each driver bank has a maximum allot-
ted current per Dx sink that must not be exceeded.
MAXIMUM Dx CURRENT
30mA
The 180mA load can be distributed in many different con-
figurations. Special care must be taken when running the
LM27966 at the maximum output current to ensure proper
functionality.
PARALLEL CONNECTED AND UNUSED OUTPUTS
Outputs D1-5 may be connected together to drive one or two
LEDs at higher currents. In such a configuration, all five
parallel current sinks (Main Display) of equal value can drive
a single LED. The LED current programmed for Main Display
should be chosen so that the current through each of the
outputs is programmed to 20% of the total desired LED
current. For example, if 60mA is the desired drive current for
a single LED, R
should be selected such that the current
through each of the current sink inputs is 12mA.
Connecting the outputs in parallel does not affect internal
operation of the LM27966 and has no impact on the Electri-
cal Characteristics and limits previously presented. The
available diode output current, maximum diode voltage, and
all other specifications provided in the Electrical Character-
istics table apply to this parallel output configuration, just as
they do to the standard 5-LED application circuit.
Main Display utilizes LED forward voltage sensing circuitry
on each Dxx pin to optimize the charge-pump gain for maxi-
mum efficiency. Due to the nature of the sensing circuitry, it
is not recommended to leave any of the Dx (D1-D4) pins
unused if either diode bank is going to be used during normal
operation. Leaving Dx pins unconnected will force the
charge-pump into 1.5x mode over the entire V
range ne-
gating any efficiency gain that could be achieve by switching
to 1x mode at higher input voltages.
If D5 is not used, it is recommended that the driver pin be
grounded and the general purpose register bit EN-D5 be set
to 0 to ensure proper gain transitions.
Care must be taken when selecting the proper R
value.
The current on any Dx pin must not exceed the maximum
current rating for any given current sink pin.
POWER EFFICIENCY
The efficiency of LED drivers is commonly taken to be the
ratio of power consumed by the LEDs (P
) to the power
drawn at the input of the part (P
). With a 1.5x/1x charge
pump, the input current is equal to the charge pump gain
times the output current (total LED current). The efficiency of
the LM27966 can be predicted as follows:
P
LEDTOTAL
= (V
LED-MAIN
x N
MAIN
x I
LED-MAIN
) +
(V
LED-AUX
x I
LED-AUX
)
P
IN
= V
IN
x I
IN
P
IN
= V
IN
x (GAIN x I
LEDTOTAL
+ I
Q
)
E = (P
LEDTOTAL
÷ P
IN
)
It is also worth noting that efficiency as defined here is in part
dependent on LED voltage. Variation in LED voltage does
not affect power consumed by the circuit and typically does
not relate to the brightness of the LED. For an advanced
analysis, it is recommended that power consumed by the
circuit (V
IN
x I
IN
) be evaluated rather than power efficiency.
POWER DISSIPATION
The power dissipation (P
) and junction temperature (T
J
)
can be approximated with the equations below. P
IN
is the
power generated by the 1.5x/1x charge pump, P
is the
power consumed by the LEDs, T
is the ambient tempera-
ture, and
θ
is the junction-to-ambient thermal resistance
for the LLP-24 package. V
is the input voltage to the
LM27966, V
is the nominal LED forward voltage, N is the
number of LEDs and I
LED
is the programmed LED current.
P
DISS
= P
IN
- P
LEDA
P
DISS
= (GAIN x V
IN
x I
LEDA
) - (V
LEDA
x N
A
x I
LEDA
) -
(V
LED
x I
DAUX
)
L
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