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PDF LM27965 Data sheet ( Hoja de datos )
| Número de pieza | LM27965 | |
| Descripción | Dual Display White LED Driver with I2C Compatible Brightness Control | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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May 2006
LM27965
Dual Display White LED Driver with I2C Compatible
Brightness Control
General Description
Features
The LM27965 is a highly integrated charge-pump-based
dual-display LED driver. The device can drive up to 9 LEDs
in parallel with a total output current of 180mA. Regulated
internal current sinks deliver excellent current and brightness
matching in all LEDs.
The LED driver current sinks are split into three indepen-
dently controlled groups. The primary group can beconfig-
urabled with 4 or 5 LEDs, for backlighting a larger main
display and the second group can be configured with 2 or 3
LEDs, for backlighing a smaller secondary display. An addi-
tional, independently controlled led driver is provided for
driving an indicator or general purpose LED. The LM27965
has an I2C compatible interface that allows the user to
independently control the brightness on each bank of LEDs.
n 91% Peak LED Drive Efficiency
n No Inductor Required
n 0.3% Current Matching
n Drives LEDs with up to 30mA per LED
n 180mA of total drive current
n I2C Compatible Brightness Control Interface
n Adaptive 1x - 3/2x Charge Pump
n Resistor-Programmable Current Settings
n External Chip RESET Pin
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)
n 25mm2 total solution size
The device provides excellent efficiency without the use of
an inductor by operating the charge pump in a gain of 3/2, or Applications
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 LM27965 is available in National’s small 24-pin Lead-
n Mobile Phone Display Lighting
n PDA Backlighting
n General LED Lighting
less Leadframe Package (LLP-24).
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Typical Application Circuit
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© 2006 National Semiconductor Corporation
DS201550
20155001
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Electrical Characteristics (Notes 2, 8) (Continued)
Note 11: For the two groups of current sinks on a part (BankA and BankB), the following are determined: the maximum sink current in the group (MAX), the minimum
sink current in the group (MIN), and the average sink current of the group (AVG). For each group, 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 matching figure for a given part is considered to
be the highest matching figure of the two banks. The typical specification provided is the most likely norm of the matching figure for all parts.
Note 12: For each Dxx pin, headroom voltage is the voltage across the internal current sink connected to that pin. For Group A, B, and C current sinks, VHRx = 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.
Block Diagram
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Application Information (Continued)
PARALLEL CONNECTED AND UNUSED OUTPUTS
MAXIMUM OUTPUT CURRENT, MAXIMUM LED
VOLTAGE, MINIMUM INPUT VOLTAGE
Outputs D1A-5A or D1B-D3B may be connected together to
drive one or two LEDs at higher currents. In such a configu-
ration, all five parallel current sinks (BankA) of equal value
The LM27965 can drive 8 LEDs at 22.5mA each (BankA and
can drive a single LED. The LED current programmed for
BankB) from an input voltage as low as 3.2V, so long as the
BankA should be chosen so that the current through each of
LEDs have a forward voltage of 3.6V or less (room tempera-
the outputs is programmed to 20% of the total desired LED
ture).
current. For example, if 60mA is the desired drive current for
The statement above is a simple example of the LED drive
capabilities of the LM27965. The statement contains the key
a single LED, RSET should be selected such that the current
through each of the current sink inputs is 12mA.
application parameters that are required to validate an LED-
Connecting the outputs in parallel does not affect internal
drive design using the LM27965: 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 LM27965:
ILED_MAX = [(1.5 x VIN) - VLED - (IADDITIONAL x ROUT)] /
[(Nx x ROUT) + kHRx] (eq. 1)
ILED_MAX = [(1.5 x VIN ) - VLED - (IADDITIONAL x 2.75Ω)] /
[(Nx x 2.75Ω) + kHRx]
IADDITIONAL is the additional current that could be delivered
to the other LED banks.
operation of the LM27965 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.
Both BankA and BankB utilize LED forward voltage sensing
circuitry on each Dxx pin to optimize the charge-pump gain
for maximum efficiency. Due to the nature of the sensing
circuitry, it is not recommended to leave any of the DxA
(D1A-D4A) or DxB (D1B-D2B) pins open if either diode bank
is going to be used during normal operation. Leaving DxA
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
and/or DxB pins unconnected will force the charge-pump
into 3/2x mode over the entire VIN range negating any effi-
ciency gain that could have been achieved by switching to 1x
mode at higher input voltages.
pump, the loss parameter is modeled as a resistance. The
output resistance of the LM27965 is typically 2.75Ω (VIN =
3.6V, TA = 25˚C). In equation form:
If D5A is not used, it is recommended that the driver pin be
grounded and the general purpose register bit EN5A be set
to 0 to ensure proper gain transitions.
VPOUT = (1.5 x VIN) – [(NAx ILEDA + NB x ILEDB ) x
ROUT] (eq. 2)
The D3B driver can be completely turned on or off on the fly
using the general purpose register. The diode monitoring
kHR – Headroom constant. This parameter models the mini-
circuity is enabled and disabled with the driver. If D3B is not
mum voltage required to be present across the current sinkDsataSheueste4dU, i.tcisormecommended that the driver pin be grounded and
for them to regulate properly. This minimum voltage is pro-
the general purpose register bit EN3B be set to 0 to ensure
portional to the programmed LED current, so the constant
proper gain transitions.
has units of mV/mA. The typical kHR of the LM27965 is
8mV/mA. In equation form:
>(VPOUT – VLEDx) kHRx x ILEDx
(eq. 3)
Care must be taken when selecting the proper RSET value.
The current on any Dxx pin must not exceed the maximum
current rating for any given current sink pin.
Typical Headroom Constant Values
kHRA = 8mV/mA
kHRB = 8mV/mA
The "ILED-MAX" equation (eq. 1) is obtained from combining
the ROUT equation (eq. 2) with the kHRx 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
POWER EFFICIENCY
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 3/2x - 1x charge pump, the
input current is equal to the charge pump gain times the
output current (total LED current). The efficiency of the
LM27965 can be predicted as follows:
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.
PLEDTOTAL = (VLEDA x NA x ILEDA) +
(VLEDB x NB x ILEDB) + (VLEDC x ILEDC)
PIN = VIN x IIN
Total Output Current Capability
The maximum output current that can be drawn from the
LM27965 is 180mA. Each driver bank has a maximum allot-
ted current per Dxx sink that must not be exceeded.
PIN = VIN x (GAIN x ILEDTOTAL + IQ)
E = (PLEDTOTAL ÷ PIN)
The LED voltage is the main contributor to the charge-pump
gain selection process. Use of low forward-voltage LEDs
(3.0V- to 3.5V) will allow the LM27965 to stay in the gain of
DRIVER TYPE
MAXIMUM Dxx CURRENT
1x for a higher percentage of the lithium-ion battery voltage
DxA
30mA per DxA Pin
range when compared to the use of higher forward voltage
DxB
30mA per DxB Pin
LEDs (3.5V to 4.0V). See the LED Forward Voltage Monitor-
ing section of this datasheet for a more detailed description
DxC
30mA per DxB Pin
of the gain selection and transition process.
The 180mA load can be distributed in many different con-
figurations. Special care must be taken when running the
LM27965 at the maximum output current to ensure proper
DataSheet4fuUn.cctioomnality.
For an advanced analysis, it is recommended that power
consumed by the circuit (VIN x IIN) for a given load be
evaluated rather than power efficiency.
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| PDF Descargar | [ Datasheet LM27965.PDF ] | |
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