NSC LM27961TL Dual-display white led driver with 3/2x switched capacitor boost Datasheet

LM27961
Dual-Display White LED Driver with 3/2x Switched
Capacitor Boost
General Description
Features
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
all LEDs.
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)
The LED driver current sources are split into two independently 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 commoncathode-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 independently 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 highefficiency 3/2x-gain charge pump.
The LM27961 is available in National’s chip-scale 18-bump
micro SMD package.
Applications
n
n
n
n
Mobile Phone Display Lighting
Mobile Phone Keypad Lighting
PDAs
General LED Lighting
Typical Application Circuit
20127901
© 2004 National Semiconductor Corporation
DS201279
www.national.com
LM27961 Dual-Display White LED Driver with 3/2x Switched Capacitor Boost
November 2004
LM27961
Connection Diagram
18-Bump Thin Micro SMD Package, Large Bump
NS Package Number TLA18
20127902
Pin Description
Pin #s
Pin Names
Pin Descriptions
C1
VIN
D2
GND
Ground
A3
POUT
Charge pump output. Approximately 1.5xVIN
A1, B2, A5, E1
C1+, C1-, C2+,
C2-
Flying capacitor connections.
D6, E5, D4, E3
D1A, D2A, D3A,
D4A
LED Outputs - Group A
C5, B4, C3
D1B, D2B, D3B
LED Outputs - Group B
B6
EN-A
Enable for Group-A LEDs (current outputs). Logic input.
High = Group-A LEDs ON. Low = Group A LEDs OFF.
Pulsing this pin with a PWM signal (100Hz-1kHz) can be used to dim LEDs.
A7
EN-B
Enable for Group-B LEDs (current outputs). Logic input.
High = Group-B LEDs ON. Low = Group B LEDs OFF.
Pulsing this pin with a PWM signal (100Hz-1kHz) can be used to dim LEDs.
E7
ISETA
Placing a resistor (RSETA) between this pin and GND sets the LED current for
Group A LEDs. LED Current = 100 x (1.25V ÷ RSETA).
C7
ISETB
Placing a resistor (RSETB) between this pin and GND sets the LED current for
Group B LEDs. LED Current = 100 x (1.25V ÷ RSETB).
Input voltage. Input range: 2.7V to 5.5V.
Operational States
ENA
ENB
L
L
Shutdown
Mode of Operation
H
L
Enabled. Group A LEDs ON. Group B LEDs OFF
L
H
Enabled. Group B LEDs ON. Group A LEDs OFF
H
H
Invalid for normal operation
Ordering Information
Order Information
Package
Supplied As
LM27961TL
TLA18 Micro SMD
250 Units, Tape & Reel
LM27961TLX
www.national.com
3000 Units, Tape & Reel
2
Operating Rating
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN pin voltage
Input Voltage Range
-0.3V to 7.1V
ENA, ENB pin voltages
-0.3V to (VIN+0.3V)
w/ 6.0V max
IDxx Pin Voltages
Internally Limited
Junction Temperature (TJ-MAX)
150oC
Storage Temperature Range
-65oC to +150o C
Maximum Lead Temperature
(Soldering, 10 sec.)
265oC
ESD Rating (Note 4)
Human Body Model - IDxx Pins:
Human Body Model - All other Pins:
Machine Model - IDxx Pins:
Machine Model - All Other Pins:
Electrical Characteristics
2.7V to 5.5V
Junction Temperature (TJ) Range
-30˚C to +125˚C
Ambient Temperature (TA) Range
(Note 5)
-30˚C to +85˚C
Thermal Properties
-0.3V to
(VPOUT+0.3V)
w/ 6.0V max
Continuous Power Dissipation
(Note 3)
(Notes 1, 2)
100˚C/W
Juntion-to-Ambient Thermal
Resistance (θJA), (Note 6)
1.0kV
2.0kV
100V
200V
(Notes 2, 7)
Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating temperature range. Unless otherwise specified: VIN = 3.6V; VDxA = 0.6V; VDxB = 3.6V; ENA = 1.5V and ENB = GND, or ENA = GND and ENB =
1.5V; RSETA = RSETB = 8.35kΩ; CIN, C1, C2 , and CPOUT = 1µF. Specifications related to output current(s) and current setting
pins (IDxx and ISETx) apply to both Group A and Group B. (Note 8)
Symbol
Parameter
Condition
3.0V ≤ VIN ≤ 4.2V, and VIN = 5.5V
0.45V ≤ VDxA ≤ 3.8V or
2.5V ≤ VDxB ≤ 3.8V;
RSET = 8.35kΩ
IDxx
Output Current Regulation
IDxx-MATCH Current Matching Between Any
Two Group A Outputs or Group
B Outputs
Min
Typ
Max
Units
13.5
(-10%)
15
16.5
(+10%)
mA
(%)
3.0V ≤ VIN ≤ 5.5V;
0.6V ≤ VDxA ≤ 3.8V or
2.5V ≤ VDxB ≤ 3.8V;
RSET = 6.25kΩ
20
mA
3.0V ≤ VIN ≤ 5.5V;
0.3V ≤ VDxA ≤ 3.8V or
2.5V ≤ VDxB ≤ 3.8V;
RSET = 12.5kΩ
10
mA
2.7V ≤ VIN ≤ 3.0V;
0.45V ≤ VDxA ≤ 3.8V or
2.5V ≤ VDxB ≤ 3.8V;
RSET = 8.35kΩ
15
mA
VIN = 3.0V (Note 9)
0.6
%
IQ
Quiescent Supply Current
2.7V ≤ VIN ≤ 4.2V;
No Load Current,
ENA or ENB = ON
4.4
6.75
mA
ISD
Shutdown Supply Current
2.7V ≤ VIN ≤ 5.5V,
ENA and ENB = OFF
2.3
5
µA
VSET
ISET Pin Voltage
2.7V ≤ VIN ≤ 5.5V
1.25
IDxx/ISET
Output Current to Current Set
Ratio
V
100
3
www.national.com
LM27961
Absolute Maximum Ratings (Notes 1, 2)
LM27961
Electrical Characteristics (Notes 2, 7)
(Continued)
Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating temperature range. Unless otherwise specified: VIN = 3.6V; VDxA = 0.6V; VDxB = 3.6V; ENA = 1.5V and ENB = GND, or ENA = GND and ENB =
1.5V; RSETA = RSETB = 8.35kΩ; CIN, C1, C2 , and CPOUT = 1µF. Specifications related to output current(s) and current setting
pins (IDxx and ISETx) apply to both Group A and Group B. (Note 8)
Symbol
Parameter
Condition
Min
Typ
Max
Units
ROUT
Charge Pump Output Resistance VIN = 3.0V
(Note 10)
2.7
Ω
VHR
Current Source Headroom
Voltage Requirement (Note 11)
IDxx = 95% X IDxx (nom)
RSET = 8.35kΩ
(IDxx (nom) ≈ 15mA)
320
mV
fSW
Switching Frequency
3.0V ≤ VIN ≤ 4.2V
tSTART
Start-up Time
IDx = 90% steady state
350
µs
1.5x/1x
Charge pump gain cross-over:
Gain = 1.5 when VIN is below
threshold. Gain = 1 when VIN is
above threshold.
1.5x to 1x Threshold
4.75
V
1x to 1.5x Threshold
4.55
V
375
500
625
kHz
Logic Pin Specifications: EN, ENA, ENB
VIL
Input Logic Low
2.7V ≤ VIN ≤ 5.5V
0
0.5
V
VIH
Input Logic High
2.7V ≤ VIN ≤ 5.5V
1.1
VIN
V
ILEAK
Input Leakage Current
VENx = 0V
0.1
VENx = 3V (Note 12)
10
µA
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of
the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the
Electrical Characteristics tables.
Note 2: All voltages are with respect to the potential at the GND pin.
Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ = 160˚C (typ.) and disengages at TJ =
120˚C (typ.). The thermal shutdown function is guaranteed by design.
Note 4: The Human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. The machine model is a 200pF capacitor discharged
directly into each pin. MIL-STD-883 3015.7
Note 5: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be
derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = 125˚C), the maximum power
dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the
following equation: TA-MAX = TJ-MAX-OP – (θJA x PD-MAX).
Note 6: Junction-to-ambient thermal resistance is highly dependent on application and board layout. In applications where high maximum power dissipation exists,
special care must be paid to thermal dissipation issues in board design.
Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm.
Note 8: CIN, CPOUT, C1, and C2 : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics
Note 9: For the two groups of outputs on a part (Group A and Group B), 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). 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 group. The matching figure for a given part is considered to
be the highest matching figure of the two groups. The typical specification provided is the most likely norm of the matching figure for all parts.
Note 10: Output resistance (ROUT) models all voltage losses in the charge pump. ROUT can be used to estimate the voltage at the charge pump output (POUT):
VPout = (1.5 x VIN) – (ROUT x IOUT). In the equation, IOUT is the total output current: the sum of all active Dxx output currents and all current drawn from POUT. The
equation applies when the charge pump is operating with a gain of 3/2 (VIN ≤ 4.75V typ.).
Note 11: Headroom voltage: VHR = VPout – VLEDx . If headroom voltage requirement is not met, LED current regulation will be compromised.
Note 12: There is a 300kΩ(typ.) pull-down resistor connected internally between each enable pin (ENA, ENB) and GND.
www.national.com
4
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
20127904
20127905
Charge Pump Output Voltage
vs. Output Current
Quiescent Current vs. Input Voltage,
20127906
20127907
5
www.national.com
LM27961
Typical Performance Characteristics
LM27961
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. (Continued)
Charge Pump Output Voltage
vs. Output Current
Charge Pump Output Voltage
vs. Input Voltage (No Load Current)
20127910
20127908
Charge Pump Output Resistance
vs Output Current
Input Current vs. Input Voltage
20127911
20127909
www.national.com
6
= 3.6V; ENA = VIN and ENB = GND, or ENA = GND and ENB = VIN; RSETA = RSETB = 8.35kΩ; CIN, C1, C2 , and CPOUT =
1µF. (Continued)
Charge Pump Switching Frequency
vs. Input Voltage
Diode Current (DxA)
vs. Headroom Voltage (DxA)
20127913
20127912
Diode Current (DxB)
vs. Headroom Voltage (DxB)
Diode Current (DxA or DxB)
vs. PWM Duty Cycle (ENA or ENB)
20127915
20127914
7
www.national.com
LM27961
Typical Performance Characteristics Unless otherwise specified: VIN = 3.6V; VLEDxA = 3.6V; VLEDxB
LM27961
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. (Continued)
Diode Current (DxA)
vs. RSETx
Input Voltage (Top)
and Output Voltage (Bottom) Waveforms
20127917
Vertical Scale = (100mV/div),
Horizontal Scale = 1µs/div)
20127916
ENx Signal (Top)
and Charge Pump Start-Up (Bottom) Waveforms
20127918
Vertical Scale = (2V/div),
Horizontal Scale = 100µs/div)
www.national.com
8
LM27961
Block Diagram
20127903
output voltage gain of the charge pump is 1.5, and the
voltage at the output of the charge pump will be approximately 1.5x the input voltage (V(POUT) ≈ 1.5 * VIN ). When in
the 1.5x mode, the charge pump provides the voltage boost
that is required to drive white LEDs from a Li-Ion battery.
(White LEDs typically have a forward voltage in the range of
3.3V to 4.0V. A Li-Ion battery typically is not considered to be
fully discharged until the battery voltage falls to 3.0V (approx.) )
The charge pump operates in the 1x mode when the input
voltage is above 4.75V (typ.). In these conditions, voltage
boost is not required to drive the LEDs, so the charge pump
merely passes the input voltage to POUT (V(POUT) ≈ VIN).
This reduces the input current and the power dissipation of
the LM27961 when the input voltage is high.
Circuit Description
OVERVIEW
The LM27961 is primarily intended for Lithium-Ion battery
driven white-LED drive applications, and is well suited to
drive white LEDs that are used for backlighting small-format
displays. The part has seven matched constant-current outputs, each capable of driving up to 20mA (or more) through
white LEDs. The well-matched current sources ensure the
current through all the LEDs is virtually identical. This keeps
brightness of all LEDs matched to near perfection so that
they can provide a consistent backlight over the entire display.
CHARGE PUMP
The core of the LM27961 is a 1.5x/1x dual-mode charge
pump. The input of the charge pump is connected to the VIN
pin. The recommended input voltage range of the LM27961
is 2.7V to 5.5V. The output of the charge pump is the POUT
pin (“Pump OUTput”). The output voltage of the charge
pump is unregulated and varies with input voltage and load
current.
The charge pump operates in the 1.5x mode when the input
voltage is below 4.75V (typ.). In this mode, the input-to-
REGULATED CURRENT OUTPUTS
The matched current outputs are generated with a precision
current mirror that is biased off the charge pump output.
Matched currents are ensured with the use of tightly
matched internal devices and internal mismatch cancellation
circuitry.
There are seven regulated current outputs. These seven
outputs are split into two groups, a group of 4 common
9
www.national.com
LM27961
Circuit Description
VPOUT = 1.5xVIN – NxILEDxROUT
(eq. 2)
kHR – Headroom constant. This parameter models the minimum 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 constant has units of mV/mA. The typical kHR of the LM27961 is
22mV/mA. In equation form:
(eq. 3)
(VPOUT – VLED) > kHRxILED
The "ILED-MAX" equation (eq. 1) is obtained from combining
the ROUT equation (eq. 2) with the kHR equation (eq. 3) and
solving for ILED. 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.
(Continued)
anode outputs and a group of 3 common cathode outputs.
There is an ON/OFF control pin for each group (ENA and
ENB).
The DC current through the LEDs is programmed with an
external resistor. Changing currents on-the-fly can be
achieved with the use of digital pulse (PWM) signals.
ENABLE PINS: ENA, ENB
The LM27961 has 2 enable pins. Both are active-high logic
(HIGH = ON). There are internal pull-down resistors (300kΩ
typ.) that are connected internally between each of the enable pins and GND.
ENA and ENB can both enable and disable the part. When
the voltage on both pins are low ( < 0.5V), the part is in
shutdown mode. All internal circuitry is OFF and the part
consumes very little supply current when the LM27961 is
shutdown. When the voltage on either ENx pin is high
( > 1.1V), the part is active. The charge pump is ON, and the
corresponding output current drivers are active.
PARALLEL Dx OUTPUTS FOR INCREASED CURRENT
CAPABILITY
Outputs D1A through D4A, or D1B through D3B may be
connected together in any combination to drive higher currents through fewer LEDs. For example in Figure 1, outputs
D1A and D2A are connected together to drive one LED. D3A
and D4A are connected to drive a second LED.
ENA and ENB are used to turn the output currents ON and
OFF. ENA activates/deactivates the four GroupA outputs
(D1A-D4A). ENB activates/deactivates the three GroupB
outputs (D1B-D3B).
SETTING LED CURRENTS
The output currents of the LM27961 can be set to a desired
value simply by connecting an appropriately sized resistor
(RSETx) between the ISETx pins of the LM27961 and GND.
RSETA sets the current for the GroupA outputs and RSETB
sets the current for the GroupB outputs. The output currents
(LED currents) are proportional to the current that flows out
of the ISETx pins. The output currents are a factor of 100
greater than the ISETx current. The feedback loop of an
internal amplifier sets the voltage of the ISETx pin to 1.25V
(typ.). Placing a resistor between ISETx and GND programs
the ISETx current, and thus the LED currents. The statements
above are simplified in the equations below:
IDxx = 100 x (VSETx / RSETx)
RSETx = 100 x (1.25V / IDxx)
20127919
FIGURE 1. Two Parallel Connected LEDs
MAXIMUM OUTPUT CURRENT, MAXIMUM LED
VOLTAGE, MINIMUM INPUT VOLTAGE
The LM27961 can drive 4 LEDs at 15mA each from an input
voltage as low as 2.7V, so long as the LEDs have a forward
voltage of 3.5V or less (room temperature).
The statement above is a simple example of the LED drive
capabilities of the LM27961. The statement contains the key
application parameters that are required to validate an LEDdrive design using the LM27961: LED current (ILEDx), number of active LEDs (N), LED forward voltage (VLED), and
minimum input voltage (VIN-MIN).
The equation below can be used to estimate the total output
current capability of the LM27961:
ILED_MAX = ((1.5 x VIN) - VLED) / ((N x ROUT) + kHR) (eq. 1)
ILED_MAX = ((1.5 x VIN ) - VLED) / ((N x 2.7Ω) + 22mV/mA)
ROUT – Output resistance. This parameter models the internal 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 LM27961 is typically 2.7Ω (VIN =
3.0V, TA = 25˚C). In equation form:
www.national.com
With this configuration, two parallel current sources of equal
value provide current to one of the LEDs. RSET should
therefore be chosen so that the current through each output
is programmed to 50% of the desired current through the
parallel connected LED. For example, if 40mA is the desired
drive current for the parallel connected LED, RSETx should
be selected so that the current through each of the outputs is
20mA. Other combinations of parallel outputs may be implemented in similar fashions, such as in Figure 2.
10
LM27961
Circuit Description
(Continued)
Neglecting IQ will result in a slightly higher efficiency prediction, 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.
20127920
The minimum recommended PWM frequency is 100Hz. Frequencies 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 capacitance, series resistors and/or ferrite beads may provide benefits.
If the current and voltage spikes can be tolerated, connecting 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.
FIGURE 2. One Parallel Connected LED
Connecting outputs in parallel does not affect internal operation of the LM27961 and has no impact on the Electrical
Characteristics and limits previously presented. The available 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 approximately 350µs (typ.).
CAPACITOR SELECTION
The LM27961 requires 4 external capacitors for proper operation. 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 aluminum 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 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 LM27961. Capacitors 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 capacitors to fail to meet the minimum capacitance requirements of
the LM27961.
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 normally 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:
11
www.national.com
LM27961
Applications Information
MICRO SMD MOUNTING
The LM27961 is an 18-bump micro SMD with a bump size of
approximately 300 micron diameter. The micro SMD package requires specific mounting techniques detailed in National Semiconductor Application Note 1112 (AN-1112).
(Continued)
The voltage rating of the output capacitor should be 10V or
more. All other capacitors should have a voltage rating at or
above the maximum input voltage of the application.
CIRCUIT BOARD LAYOUT
For optimal, low-noise performance, all capacitors (CIN,
CPOUT, C1, C2) should be placed very close to the LM27961.
A solid ground plane should be used for IC and component
GND connections. Refer to the LM27961 Evaluation Board
for an example layout.
www.national.com
12
inches (millimeters) unless otherwise noted
TLA18EHA: 18-Bump Thin Micro SMD, Large Bump
X1 = 2.098 ± 0.030mm
X2 = 2.403mm ± 0.030
X3 = 0.600mm ± 0.075mm
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship
Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned
Substances’’ as defined in CSP-9-111S2.
National Semiconductor
Americas Customer
Support Center
Email: [email protected]
Tel: 1-800-272-9959
www.national.com
National Semiconductor
Europe Customer Support Center
Fax: +49 (0) 180-530 85 86
Email: [email protected]
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
National Semiconductor
Asia Pacific Customer
Support Center
Email: [email protected]
National Semiconductor
Japan Customer Support Center
Fax: 81-3-5639-7507
Email: [email protected]
Tel: 81-3-5639-7560
LM27961 Dual-Display White LED Driver with 3/2x Switched Capacitor Boost
Physical Dimensions
Similar pages