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PDF LM2755 Data sheet ( Hoja de datos )
| Número de pieza | LM2755 | |
| Descripción | Charge Pump LED Controller | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LM2755 (archivo pdf) en la parte inferior de esta página. Total 14 Páginas | ||
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October 2007
LM2755
Charge Pump LED Controller with I2C Compatible Interface
in Micro SMDwww.datasheet4u.com
General Description
The LM2755 is a charge-pump-based, constant current LED
driver capable of driving 3 LEDs with a total output current up
to 90mA. The diode current waveforms of each LED can be
trapezoidal with timing and level parameters (rise time, fall
time, high level, low level, delay, high time, low time) pro-
grammed via an I2C compatible interface. The 32 brightness
levels found on the LM2755 are exponentially spaced (as op-
posed to linearly spaced) to better match the response of the
human eye to changing brightness levels.
The device requires only four small and low-cost ceramic ca-
pacitors. The LM2755 provides excellent efficiency without
the use of an inductor by operating the charge pump in a gain
of 3/2 or in a gain of 1. Maximum efficiency is achieved over
the input voltage range by actively selecting the proper gain
based on the LED forward voltage requirements.
The pre-regulation scheme used by the LM2755 is optimized
to ensure low conducted noise on the input. An internal soft-
start circuitry eliminates high inrush current at start-up. The
LM2755 consumes 3µA (typ.) of supply current in shut-down.
The LM2755 is available in National’s tiny 18-bump thin micro
SMD package.
Features
■ 90% Peak Efficiency
■ Total solution size < 13mm2
■ No Inductor Required: Only 4 Inexpensive Ceramic Caps
■ 3 Independently Controlled Constant Current Outputs
■ Programmable Trapezoidal Dimming Waveform on Each
Output
■ Programmable Timing Control Via Internal Registers and
External Clock Synchronization Input
■ 32 Exponential Dimming Steps with 800:1 Dimming Ratio
■ Programmable brightness control via I2C compatible
interface
■ Hardware Enable Pin
■ Wide input voltage range: 2.7V to 5.5V
■ Tiny 18-bump thin micro SMD: 1.8mm x 1.6mm x 0.6mm
Applications
■ Indicator LEDs
■ Keypad LED Backlight
■ Display LED Backlight
■ Fun-light LEDs
Typical Application Circuit
20180901
20180904
Minimum Solution Size
© 2007 National Semiconductor Corporation 201809
www.national.com
1 page  
Typical Performance Characteristics Unless otherwise specified: TA = 25°C; VIN = 3.6V; VHWEN = VIN;
VD1 = VD2 = VD3 = 3.6V; RSET = 12.5kΩ; C1=C2= 0.47µF, CIN = CVOUT = 1µF; ENA = ENB = ENC = '1'.
LED Drive Efficiency vs Input Voltage
Diode Current vs Input Voltage
www.datasheet4u.com
20180910
Current Matching vs Input Voltage
3 LEDs
20180909
Diode Current vs Brightness Code
20180917
Quiescent Current vs Input Voltage
20180916
Shutdown Current vs Input Voltage
20180919
5
20180918
www.national.com
5 Page 
Total Output Current Capability
The maximum output current that can be drawn from the
LM2755 is 90mA. Each driver Group has a maximum allotted
current per Dx sink that must not be exceeded.
DRIVER TYPE
MAXIMUM Dx CURRENT
Dx 30mA per Dx Pin
The 90mA load can be distributed in many different configu-
rations. Special care must be taken when running the LM2755
wwawt t.hdeatmasahxeimetu4mu.cooumtput current to ensure proper functionality.
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 3/2× - 1× charge pump, the
input current is equal to the charge pump gain times the output
current (total LED current). The efficiency of the LM2755 can
be predicted as follow:
PLEDTOTAL = (VLEDA × NA × ILEDA) +
(VLEDB × NB × ILEDB) + (VLEDC × ILEDC)
PIN = VIN × IIN
PIN = VIN × (GAIN × 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 LM2755 to stay in the gain of 1×
for a higher percentage of the lithium-ion battery voltage
range when compared to the use of higher forward voltage
LEDs (3.5V to 4.0V). See the LED Forward Voltage Monitor-
ing section of this datasheet for a more detailed description
of the gain selection and transition process.
For an advanced analysis, it is recommended that power con-
sumed by the circuit (VIN x IIN) for a given load 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 3/2× - 1× charge pump, PLED is the
power consumed by the LEDs, TA is the ambient temperature,
and θJA is the junction-to-ambient thermal resistance for the
µSMD 18-bump package. VIN is the input voltage to the
LM2755, VLED is the nominal LED forward voltage, N is the
number of LEDs and ILED is the programmed LED current.
PDISS = PIN - PLED1 - PLED2 - PLED3
PDISS= (GAIN × VIN × ID1 + D2+ D3 ) - (VLED1 × ILED1) - (VLED2 ×
ILED2) - (VLED3 × ILED3)
TJ = TA + (PDISS x θJA)
The junction temperature rating takes precedence over the
ambient temperature rating. The LM2755 may be operated
outside the ambient temperature rating, so long as the junc-
tion temperature of the device does not exceed the maximum
operating rating of 105°C. The maximum ambient tempera-
ture rating must be derated in applications where high power
dissipation and/or poor thermal resistance causes the junc-
tion temperature to exceed 105°C.
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM2755
when the junction temperature exceeds 160°C (typ.). This
feature protects the device from being damaged by high die
temperatures that might otherwise result from excessive pow-
er dissipation. The device will recover and operate normally
when the junction temperature falls below 155°C (typ.). It is
important that the board layout provide good thermal conduc-
tion to keep the junction temperature within the specified
operating ratings.
CAPACITOR SELECTION
The LM2755 requires 4 external capacitors for proper opera-
tion (CIN = COUT = 1µF, C1 = C2 = 0.47µF). Surface-mount
multi-layer ceramic capacitors are recommended. These ca-
pacitors are small, inexpensive and have very low equivalent
series resistance (ESR <20mΩ typ.). Tantalum capacitors,
OS-CON capacitors, and aluminum electrolytic capacitors are
not recommended for use with the LM2755 due to their high
ESR, as compared to ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM2755. These capacitors have tight capacitance tolerance
(as good as ±10%) and hold their value over temperature
(X7R: ±15% over -55°C to 125°C; X5R: ±15% over -55°C to
85°C).
Capacitors with Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2755. Ca-
pacitors with these temperature characteristics typically have
wide capacitance tolerance (+80%, -20%) and vary signifi-
cantly over temperature (Y5V: +22%, -82% over -30°C to
+85°C range; Z5U: +22%, -56% over +10°C to +85°C range).
Under some conditions, a nominal 1µF Y5V or Z5U capacitor
could have a capacitance of only 0.1µF. Such detrimental de-
viation is likely to cause Y5V and Z5U capacitors to fail to
meet the minimum capacitance requirements of the LM2755.
The recommended voltage rating for the capacitors is
10V to account for DC bias capacitance losses.
11 www.national.com
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| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet LM2755.PDF ] | |
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