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PDF LM27966 Data sheet ( Hoja de datos )
| Número de pieza | LM27966 | |
| Descripción | White LED Driver | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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August 2006
LM27966
White LED Driver with I2C Compatible Interface
General Description
The LM27966 is a highly integrated charge-pump-based
display LED driver. The device can drive up to 6 LEDs in
parallel with a total output current of 180mA. Regulated
internal current sources deliver excellent current and bright-
ness matching in all LEDs.
The LED driver current sources are split into two indepen-
dently controlled groups. The primary group, which can be
www.DataSheet4U.ccoomnfigured with 4 or 5 LEDs, can be used to backlight the
main phone display. An additional, independently controlled
led driver is provided for driving an indicator or other general
purpose LED function. The LM27966 has an I2C compatible
interface that allows the user to independently control the
brightness on each bank of LEDs.
The device provides excellent efficiency without the use of
an inductor by operating the charge pump in a gain of 3/2, or
in Pass-Mode. The proper gain for maintaining current regu-
lation is chosen, based on LED forward voltage, so that
efficiency is maximized over the input voltage range.
The LM27966 is available in National’s small 24-pin Lead-
less Leadframe Package (LLP-24).
Features
n 91% Peak LED Drive Efficiency
n No Inductor Required
n 0.3% Current Matching
n Drives 6 LEDs with up to 30mA per LED
n 180mA of total driver current
n I2C Compatible Brightness Control Interface
n Adaptive 1x- 3/2x Charge Pump
n Resistor-Programmable Current Settings
n External Chip RESET Pin (RESET)
n Extended Li-Ion Input: 2.7V to 5.5V
n Small low profile industry standard leadless package,
LLP 24 : (4mm x 4mm x 0.8mm)
Applications
n Mobile Phone Display Lighting
n PDAs Backlighting
n General LED Lighting
Typical Application Circuit
© 2006 National Semiconductor Corporation DS201901
20190101
www.national.com
1 page  
Electrical Characteristics (Notes 2, 8) (Continued)
Note 10: The maximum total output current for the LM27966 should be limited to 180mA. The total output current can be split among any of the three banks (IDxA
= IDxC = 30mA Max.). Under maximum output current conditions, special attention must be given to input voltage and LED forward voltage to ensure proper current
regulation. See the Maximum Output Current section of the datasheet for more information.
Note 11: For the Main Display group of outputs on a part, the following are determined: the maximum output current in the group (MAX), the minimum output current
in the group (MIN), and the average output current of the group (AVG). Two matching numbers are calculated: (MAX-AVG)/AVG and (AVG-MIN)/AVG. The largest
number of the two (worst case) is considered the matching figure for the bank. The typical specification provided is the most likely norm of the matching figure for
all parts.
Note 12: For each IDxx output pin, headroom voltage is the voltage across the internal current sink connected to that pin. For Main and Aux outputs, VHR = VOUT
-VLED. If headroom voltage requirement is not met, LED current regulation will be compromised.
Note 13: SCL and SDIO signals are referenced to VIO and GND for minimum VIO voltage testing.
Note 14: SCL and SDIO should be glitch-free in order for proper brightness control to be realized.
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5 www.national.com
5 Page 
Application Information (Continued)
MAXIMUM OUTPUT CURRENT, MAXIMUM LED
VOLTAGE, MINIMUM INPUT VOLTAGE
The LM27966 can drive 6 LEDs at 30mA each (Main Display
and DAUX) 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 (ILEDx), num-
ber of active LEDs (Nx), LED forward voltage (VLED), and
minimum input voltage (VIN-MIN).
The equation below can be used to estimate the maximum
output current capability of the LM27966:
ILED_MAX = [(1.5 x VIN) - VLED - (IDAUX x ROUT)] /
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[(NMAIN x ROUT) + kHR] (eq. 1)
ILED_MAX = [(1.5 x VIN ) - VLED - (IDAUX x 2.75Ω)] /
[(NMAIN x 2.75Ω) + kHR]
IDAUX is the additional current that could be delivered to the
AUX LED.
ROUT – Output resistance. This parameter models the inter-
nal losses of the charge pump that result in voltage droop at
the pump output POUT. 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 2.75Ω (VIN =
3.6V, TA = 25˚C). In equation form:
VPOUT = (1.5 x VIN) – [NMAINx ILED-MAIN x ROUT]
2)
(eq.
kHR – 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 kHR of the LM27966 is
8mV/mA. In equation form:
>(VPOUT – VLEDx) kHR x ILEDx
(eq. 3)
Typical Headroom Constant Value
kHR = 8mV/mA
The "ILED-MAX" equation (eq. 1) is obtained from combining
the ROUT equation (eq. 2) with the kHR equation (eq. 3) and
solving for ILEDx. 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.
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, RSET 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 VIN 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 RSET 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 (PLED) to the power
drawn at the input of the part (PIN). 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:
PLEDTOTAL = (VLED-MAIN x NMAIN x ILED-MAIN) +
(VLED-AUX x ILED-AUX)
PIN = VIN x IIN
PIN = VIN x (GAIN x ILEDTOTAL + IQ)
E = (PLEDTOTAL ÷ PIN)
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 (VIN x IIN) be evaluated rather than power efficiency.
POWER DISSIPATION
The power dissipation (PDISS) and junction temperature (TJ)
can be approximated with the equations below. PIN is the
power generated by the 1.5x/1x charge pump, PLED is the
power consumed by the LEDs, TA is the ambient tempera-
ture, and θJA is the junction-to-ambient thermal resistance
for the LLP-24 package. VIN is the input voltage to the
LM27966, VLED is the nominal LED forward voltage, N is the
number of LEDs and ILED is the programmed LED current.
PDISS = PIN - PLEDA
PDISS= (GAIN x VIN x ILEDA ) - (VLEDA x NA x ILEDA) -
(VLED x IDAUX)
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| Páginas | Total 13 Páginas | |
| PDF Descargar | [ Datasheet LM27966.PDF ] | |
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