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PDF LM27961 Data sheet ( Hoja de datos )
| Número de pieza | LM27961 | |
| Descripción | Dual-Display White | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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November 2004
LM27961
Dual-Display White LED Driver with 3/2x Switched
Capacitor Boost
General Description
The LM27961 is a charge-pump-based white-LED driver that
is ideal for mobile phone display backlighting. It is intended
to drive 4 LEDs for a main phone display backlight and 3
LEDs for a sub-display backlight. Regulated internal current
sources deliver excellent current and brightness matching in
www.DataSheet4U.caollmLEDs.
The LED driver current sources are split into two indepen-
dently controlled groups. The primary group (Group A) can
be used to backlight a main phone display with up to 4 LEDs.
The low-side current drivers of Group A accommodate
common-anode-type LEDs. The second group (Group B)
can backlight a secondary display with up to 3 LEDs. The
high-side current drivers of Group B accommodate common-
cathode-type LEDs. Both Group A and Group B can also
drive standard two-terminal LEDs, and provide other general
lighting functions (keypad lighting, fun lighting, etc). The
brightness of the two LED groups can be adjusted indepen-
dently with external resistors.
The LM27961 works off an extended Li-Ion input voltage
range (2.7V to 5.5V). Voltage boost is achieved with a high-
efficiency 3/2x-gain charge pump.
The LM27961 is available in National’s chip-scale 18-bump
micro SMD package.
Features
n Drives 4 Individual Common-Anode LEDs with up to
20mA each for a Main Display Backlight
n Drives 3 Individual Common-Cathode LEDs with up to
20mA each for a Sub-Display Backlight
n Independent Resistor-Programmable Current Setting
n Excellent Current and Brightness Matching
n High-Efficiency 3/2x Charge Pump
n Extended Li-Ion Input: 2.7V to 5.5V
n PWM Brightness Control: 100Hz - 1kHz
n 18-bump Thin Micro SMD Package:
(2.1mm x 2.4mm x 0.6mm)
Applications
n Mobile Phone Display Lighting
n Mobile Phone Keypad Lighting
n PDAs
n General LED Lighting
Typical Application Circuit
© 2004 National Semiconductor Corporation DS201279
20127901
www.national.com
1 page  
Typical Performance Characteristics Unless otherwise specified: VIN = 3.6V; VLEDxA = 3.6V; VLEDxB
= 3.6V; ENA = VIN and ENB = GND, or ENA = GND and ENB = VIN; RSETA = RSETB = 8.35kΩ; CIN, C1, C2 , and CPOUT =
1µF.
LED Current (D1A, D2A,D3A, D4A)
vs. Input Voltage
LED Current (DxA) vs. Input Voltage
www.DataSheet4U.com
20127904
Quiescent Current vs. Input Voltage,
20127905
Charge Pump Output Voltage
vs. Output Current
20127906
20127907
5 www.national.com
5 Page 
Circuit Description (Continued)
www.DataSheet4U.com
20127920
FIGURE 2. One Parallel Connected LED
Connecting outputs in parallel does not affect internal opera-
tion of the LM27961 and has no impact on the Electrical
Characteristics and limits previously presented. The avail-
able diode output current, maximum diode voltage, and all
other specifications provided in the Electrical Characteristics
table apply to parallel output configurations, just as they do
to the standard application circuit on pg1 of the datasheet.
SOFT START
The LM27961 contains internal soft-start circuitry to limit
input inrush currents when the part is enabled. Soft start is
implemented with a controlled turn-on of the internal voltage
reference. During soft start, the current through the LED
outputs rise at the rate of the reference voltage ramp. Due to
the soft-start circuitry, turn-on time of the LM27961 is ap-
proximately 350µs (typ.).
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM27961
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
power dissipation. The device will recover and operate nor-
mally when the junction temperature falls below 120˚C (typ.).
It is important that the board layout provides good thermal
conduction. This will help to keep the junction temperature
within specified operating ratings.
Applications Information
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 1.5x charge pump, the input
current is approximately 1.5x the output current (total LED
current). For a simple approximation, the current consumed
by internal circuitry can be neglected and the efficiency of
the LM27961 can be predicted as follows:
Neglecting IQ will result in a slightly higher efficiency predic-
tion, but this impact will be no more than a few percentage
points when several LEDs are driven at full power.
ADJUSTING LED BRIGHTNESS (PWM control)
Perceived LED brightness can be adjusted using a PWM
control signal to turn the LM27961 current sources ON and
OFF at a rate faster than perceptible by the eye. When this
is done, the total brightness perceived is proportional to the
duty cycle (D) of the PWM signal (D = the percentage of time
that the LED is on in every PWM cycle). A simple example: if
the LEDs are driven at 15mA each with a PWM signal that
has a 50% duty cycle, perceived LED brightness will be
about half as bright as compared to when the LEDs are
driven continuously with 15mA. A PWM signal thus provides
brightness (dimming) control for the solution.
The minimum recommended PWM frequency is 100Hz. Fre-
quencies below this may be visibly noticeable as flicker or
blinking. The maximum recommended PWM frequency is
1kHz. Frequencies above this may cause interference with
internal current driver circuitry.
In cases where a PWM signal must be connected to the ENx
pins, measures can be taken to reduce the magnitude of the
charge-pump turn-on voltage spikes. More input capaci-
tance, series resistors and/or ferrite beads may provide ben-
efits.
If the current and voltage spikes can be tolerated, connect-
ing the PWM signal to the EN pin does provide a benefit:
lower supply current when the PWM signal is active. When
the PWM signal is low, the LM27961 will be shutdown and
input current will only be a few micro-amps. This results in a
lower time-averaged input current.
CAPACITOR SELECTION
The LM27961 requires 4 external capacitors for proper op-
eration. Surface-mount multi-layer ceramic capacitors are
recommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR <20mW
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors are not recommended for use
with the LM27961 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
LM27961. These capacitors have tight capacitance toler-
ance (as good as ±10%) and hold their value over tempera-
ture (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 LM27961. Ca-
pacitors with these temperature characteristics typically
have wide capacitance tolerance (+80%, -20%) and vary
significantly 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 deviation is likely to cause Y5V and Z5U capaci-
tors to fail to meet the minimum capacitance requirements of
the LM27961.
11 www.national.com
11 Page | ||
| Páginas | Total 13 Páginas | |
| PDF Descargar | [ Datasheet LM27961.PDF ] | |
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