LM27951
LM27951 White LED Adaptive 1.5X/1X Switched Capacitor Current Driver
Literature Number: SNVS416A
LM27951
White LED Adaptive 1.5X/1X Switched Capacitor Current
Driver
General Description
The LM27951 is a switched capacitor white-LED driver ca-
pable of driving up to 4 LEDs with 30mA through each LED.
Its 4 tightly regulated current sources ensure excellent LED
current and brightness matching. LED drive current is pro-
grammed by an external sense resistor. The LM27951 oper-
ates over an input voltage range from 2.8V to 5.5V and
requires only four low-cost ceramic capacitors.
The LM27951 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 LM27951 uses constant frequency pre-regulation to
minimize conducted noise on the input. It has a fixed 750kHz
switching frequency optimized for portable applications. The
LM27951 consumes less than 1µA of supply current when
shut down.
The LM27951 is available in a 14-pin No-Pullback Leadless
Leadframe Package: LLP-14.
Features
nDrives up to 4 LEDs with up to 30mA each
nRegulated current sources with 0.2%(typ.) matching
n3/2x, 1x Gain transition based on LED V
F
nPeak Efficiency Over 85%
nInput Voltage Range: 2.8V to 5.5V
nPWM Brightness Control
nVery Small Solution Size - NO INDUCTOR
nFixed 750kHz Switching Frequency
n<1µA Shutdown Current
n14-pin LLP Package: 4.0mm X 3.0mm X 0.8mm
Applications
nWhite LED Display Backlights
nWhite LED Keypad Backlights
nGeneral Purpose LED Lighting
Typical Application Circuit
20171701
November 2005
LM27951 White LED Adaptive 1.5X/1X Switched Capacitor Current Driver
© 2006 National Semiconductor Corporation DS201717 www.national.com
Connection Diagram
LM27951
14-pin No-Pullback Leadless Leadframe Package (LLP-14)
4mm x 3mm x 0.8mm
NS Package Number SDA14A
20171702
Pin Descriptions
Pin Name Description
1 C2+ Flying Capacitor C2 Connection
2V
OUT
Pre-Regulated Charge Pump Output
3 C1+ Flying Capacitor C1 Connection
4 D4 Regulated Current Source Output.
5 D3 Regulated Current Source Output.
6 D2 Regulated Current Source Output.
7 D1 Regulated Current Source Output.
8I
SET
Current Set Input. Placing a resistor (R
SET
) between this pin and GND sets
the LED current for all the LEDs. LED Current = 200 x (1.25V ÷ R
SET
).
9 EN Enable Logic Input Pin. Logic Low = Shut Down, Logic High = Enabled. There
is a 150kΩ(typ.) resistor connected internally between the EN pin and GND.
10 PWM Current Source Modulation Logic Input Pin. Logic Low = Off, Logic High = On.
Applying a Pulse Width Modulated (PWM) signal to this pin allows the
regulated current sources to be modulated without shutting down the internal
Charge Pump and the V
OUT
node.
11 V
IN
Input Supply Range: 2.8V to 5.5V.
12 C2- Flying Capacitor C2 Connection.
13 GND Power Supply Ground Connection.
14 C1- Flying Capacitor C1 Connection.
Ordering Information
Order Number Package Description Package Marking Supplied as Tape and Reel
(Units)
LM27951SD No-Pullback
LLP-14
XXXXX = ¢Z¢2¢X
YYYYY = D006B
1000
LM27951SDX 4500
LM27951
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Absolute Maximum Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
IN
-0.3V to 6.0V
EN, PWM -0.3V to (V
IN
+ 0.3V)
w/ 6.0V max
Continuous Power Dissipation
(Note 3) Internally Limited
Junction Temperature
(T
J-MAX-ABS
) 150˚C
Storage Temperature Range -65˚C to 150˚C
Lead Temp. (Soldering, 5 sec.) 260˚C
ESD Rating (Note 4)
Human Body Model 2kV
Operating Ratings (Notes 2, 7)
Input Voltage V
IN
2.8V to 5.5V
LED Voltage Range 2.5V to 3.9V
Junction Temperature Range (T
J
) -40˚C to +115˚C
Ambient Temperature Range (T
A
)
(Note 5) -40˚C to +85 ˚C
Thermal Information
Junction-to-Ambient Thermal Resistance,
LLP-14 Package (θ
JA
) (Note 6) 45˚C/W
Electrical Characteristics (Notes 2, 7)
Limits in standard typeface are for T
A
= 25˚C, and limits in boldface type apply over the full operating junction temperature
range (-40˚C to +85 ˚C). Unless otherwise noted, specifications apply to the LM27951 Typical Application Circuit (pg.1) with V
IN
= 3.6V, V(EN) = 1.8V, V(PWM) = 1.8V, 4 LEDs, V
DX
= 3.6V, C
IN
=C
OUT
= 3.3µF, C
1
=C
2
= 1µF, R
SET
= 12.5kΩ(Note 8)
Symbol Parameter Conditions Min Typ Max Units
I
DX
LED Current Regulation 3.0V ≤V
IN
≤5.5V
R
SET
= 12.5kΩ
I
VOUT
= 0mA
18.4
(−8%)
20 21.6
(+8%)
mA
3.0V ≤V
IN
≤5.5V
R
SET
= 8.32kΩ
I
VOUT
= 0mA
30
3.0V ≤V
IN
≤5.5V
R
SET
= 24.9kΩ
I
VOUT
= 0mA
10
I
D-MATCH
LED Current Matching
(Note 9)
R
SET
= 8.32kΩ0.2 1.5 %
I
Q
Quiescent Supply Current D
(1-4)
= OPEN
R
SET
= OPEN
1.5 1.9 mA
I
SD
Shutdown Supply Current 3.0V ≤V
IN
≤5.5V
V(EN) = 0V
0.1 1µA
V
SET
I
SET
Pin Voltage 3.0V ≤V
IN
≤5.5V 1.25 V
I
DX
/I
SET
Output Current to Current
Set Ratio
200
V
HR
Current Source Voltage
Headroom Requirement
(Note 10)
I
DX
= 95% I
DX
(nom.)
R
SET
= 8.32kΩ
(I
DX
nom. = 30mA)
360 mV
I
DX
= 95% I
DX
(nom.)
R
SET
= 12.5kΩ
(I
DX
nom. = 20mA)
240
f
SW
Switching Frequency 525
(-30%)
750 975
(+30%)
kHz
V
IH
Logic Input High Input Pins: EN, PWM
3.0V ≤V
IN
≤5.5V
1.0 V
IN
V
V
IL
Logic Input Low Input Pins: EN, PWM
3.0V ≤V
IN
≤5.5V
0 0.4
LM27951
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Electrical Characteristics (Notes 2, 7) (Continued)
Limits in standard typeface are for T
A
= 25˚C, and limits in boldface type apply over the full operating junction temperature
range (-40˚C to +85 ˚C). Unless otherwise noted, specifications apply to the LM27951 Typical Application Circuit (pg.1) with V
IN
= 3.6V, V(EN) = 1.8V, V(PWM) = 1.8V, 4 LEDs, V
DX
= 3.6V, C
IN
=C
OUT
= 3.3µF, C
1
=C
2
= 1µF, R
SET
= 12.5kΩ(Note 8)
Symbol Parameter Conditions Min Typ Max Units
I
IH
Logic Input High Current Input Pin: PWM
V(PWM) = 1.8V
10 nA
Input Pin: EN
V(EN) = 1.8V (Note 11)
12 µA
I
IL
Logic Input Low Current Input Pins: EN, PWM
V(EN, PWM) = 0V
10 nA
R
OUT
Charge Pump Output
Resistance (Note 12)
3.3 Ω
V
GDX
1x to 3/2x Gain Transition
Voltage Threshold on V
DX
(V
OUT
−V
DX
) Falling 500 mV
t
ON
Startup Time I
DX
= 90% steady state 330 µs
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=150˚C (typ.) and disengages at TJ=
140˚C (typ.).
Note 4: The Human-body model is a 100 pF capacitor discharged through a 1.5kΩresistor into each pin.
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 operation junction temperature (TJ-MAX-OP =115
oC), 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 =T
J-MAX-OP -(θJA xP
D-MAX).
Note 6: Junction-to-ambient thermal resistance (θJA) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the JEDEC
standard JESD51-7. The test board is a 4 layer FR-4 board measuring 102mm x 76mm x 1.6mm witha2x1array of thermal vias. The ground plane on the board
is 50mm x 50mm. Thickness of copper layers are 36µm/18µm /18µm/36µm (1.5oz/1oz/1oz/1.5oz). Ambient temperature in simulation is 22˚C, still air. Power
dissipation is 1W.
The value of θJA of the LM27951 in LLP-14 could fall in a range as wide as 45oC/W to 150oC/W (if not wider), depending on PWB material, layout, and environmental
conditions. In applications where high maximum power dissipation exists (high VIN, high IOUT), special care must be paid to thermal dissipation issues. For more
information on these topics, please refer to Application Note 1187: Leadless Leadframe Package (LLP) and the Power Efficiency and Power Dissipation
section of this datasheet..
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,C
OUT,C
1,C
2: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics
Note 9: LED Current Matching is based on two calculations: [(IMAX -I
AVG)÷I
AVG] and [(IAVG -I
MIN)÷I
AVG]. IMAX and IMIN are the highest and lowest respective
Dx currents, and IAVG is the average Dx current of all four current sources. The largest number of the two calculations (worst case) is considered the matching figure
for the part. The typical specification provided is the most likely norm of the matching figure for all parts.
Note 10: Headroom Voltage (VHR)=V
OUT −V
DX. If headroom voltage requirement is not met, LED current regulation will be compromised.
Note 11: EN Logic Input High Current (IIH) is due to a 150kΩ(typ.) pull-down resistor connected internally between the EN and GND pins.
Note 12: The open loop output resistance (ROUT) models all voltage losses in the charge pump. ROUT can be used to estimate the voltage at the charge pump
output VOUT and the maximum current capability of the device under low VIN and high IOUT conditions, beyond what is specified in the electrical specifications table:
VOUT =(GxV
IN)-(R
OUT xI
OUT). In the equation, G is the charge pump gain mode, and IOUT is the total output current (sum of all active Dx current sources and
all current drawn from VOUT).
Note 13: Turn-on time is measured from when the EN signal is pulled high until the output voltage on VOUT crosses 90% of its final value.
LM27951
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Block Diagram
20171703
LM27951
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Typical Performance Characteristics Unless otherwise specified: T
A
= 25˚C, 4 LEDs, V
DX
= 3.6V,
V
IN
= 3.6V, V
EN
=V
IN
,V
PWM
=V
IN
,C
1
=C
2
= 1µF, C
IN
=C
OUT
= 3.3µF. Capacitors are low-ESR multi-layer ceramic capaci-
tors (MLCC’s).
LED Current Regulation vs. Input Voltage Average LED Current Regulation vs. Input Voltage
20171704 20171706
Converter Efficiency vs. Input Voltage LED Current vs. R
SET
20171708 20171709
Input and Output Voltage Ripple Startup Response
20171712
VIN = 3.6V, Load = 15mA/LED, 4 LEDs
CH1 (TOP): VIN; Scale: 20mV/Div, AC Coupled
CH2 (BOTTOM): VOUT; Scale: 20mV/Div, AC Coupled
Time scale: 400ns/Div
20171713
VIN = 3.6V, Load = 20mA/LED, 4 LEDs
CH1 (TOP): VEN; Scale: 1V/Div
CH2 (BOTTOM): VOUT; Scale: 1V/Div
Time scale: 100µs/Div
LM27951
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Application Information
CIRCUIT DESCRIPTION
The LM27951 is an adaptive 1.5x/1x CMOS charge pump,
optimized for driving white LEDs used in small-format dis-
play backlighting. It provides four constant current outputs
capable of sourcing up to 30mA through each LED. The
well-matched current sources ensure the current through all
the LEDs are virtually identical, providing a uniform bright-
ness across the entire display.
Each current source is internally connected to the charge
pump output, V
OUT
. LED drive current is programmed by
connecting a resistor, R
SET
, to the current set pin, I
SET
. LED
brightness is adjusted by applying a Pulse Width Modulated
(PWM) signal to the dedicated PWM input pin.
CHARGE PUMP
The input to the 1.5x/1x charge pump is connected to the V
IN
pin, and the loosely regulated output of the charge pump is
connected to the V
OUT
pin. The recommended input voltage
range of the LM27951 is 3.0V to 5.5V. The device’s loosely-
regulated charge pump has both open loop and closed loop
modes of operation. When the device is in open loop, the
voltage at V
OUT
is equal to the gain times the voltage at the
input. When the device is in closed loop, the voltage at V
OUT
is loosely regulated to 4.5V (typ.). The charge pump gain
transitions are actively selected to maintain regulation based
on LED forward voltage and load requirements. This allows
the charge pump to stay in the most efficient gain (1x) over
as much of the input voltage range as possible, reducing the
power consumed from the battery.
SOFT START
The LM27951 contains internal soft-start circuitry to limit
input inrush currents when the part is enabled. Soft start is
implemented internally with a controlled turn-on of the inter-
nal voltage reference. Due to the soft-start circuitry, startup
time of the LM27951 is approximately 330µs (typ.).
ENABLE AND PWM PINS
The LM27951 has 2 logic control pins. Both pins are active-
high logic (HIGH = ON). There is an internal pull-down
resistor (150kΩtyp.) connected between the enable pin (EN)
and GND. There is no pull-up or pull-down connected to the
Pulse Width Modulated (PWM) pin.
The EN pin is the master enable pin for the part. When the
voltage on this pin is low (<0.4V), the part is in shutdown
mode. In this mode, all internal circuitry is OFF and the part
consumes very little supply current (<1µA typ.). When the
voltage on the EN pin is high (>1.0V), the part will activate
the charge pump and regulate the output voltage to its
nominal value.
The PWM pin serves as a dedicated logic input for LED
brightness control. When the voltage on this pin is low
(<0.4V), the current sources will be turned off and no current
will flow through the LEDs. When the voltage on this pin is
high (>1.0V), the currents sources will turn on and regulate
to the current level set by the resistor connected to the I
SET
pin.
SETTING LED CURRENTS
The current through the four LEDs connected to D
1-4
can be
set to a desired level simply by connecting an appropriately
sized resistor (R
SET
) between the I
SET
pin of the LM27951
and GND. The LED currents are proportional to the current
that flows out of the I
SET
pin and are a factor of 200 times
greater than the I
SET
current. The feedback loop of an inter-
nal amplifier sets the voltage of the I
SET
pin to 1.25V (typ.).
The statements above are simplified in the equations below:
I
Dx
= 200 x(V
SET
/R
SET
)
R
SET
= 200 x (1.25V / I
Dx
)
ADJUSTING LED BRIGHTNESS (PWM control)
Perceived LED brightness can be adjusted using a PWM
control signal on the LM27951 PWM logic input pin, turning
the current sources ON and OFF at a rate faster than per-
ceptible by the human 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.
The minimum recommended PWM frequency is 100Hz. Fre-
quencies below this may be visible as flicker or blinking. The
maximum recommended PWM frequency is 1kHz. Frequen-
cies above this may cause interference with internal current
driver circuitry and/or noise in the audible range. Due to the
regulation control loop, the maximum frequency and mini-
mum duty cycle applied to the PWM pin should be chosen
such that the minimum ON time is no less than 30µs in
duration. If a PWM signal is applied to the EN pin instead,
the maximum frequency and minimum duty cycle should be
chosen to accommodate both the LM27951 startup time
(330µs typ.) and the 30µs control loop delay.
The preferred method to adjust brightness is to keep the
master EN voltage ON continuously and apply a PWM signal
to the dedicated PWM input pin. The benefit of this type of
connection can be best understood with a contrary example.
When a PWM signal is connected to the master enable (EN)
pin, the charge pump repeatedly turns on and off. Every time
the charge pump turns on, there is an inrush of current as the
capacitors, both internal and external, are recharged. This
inrush current results in a current spike and a voltage dip at
the input of the part. By applying the PWM signal to PWM
logic input pin, the charge pump remains active, resulting in
much lower input noise.
In cases where a PWM signal must be connected to the EN
pin, measures can be taken to reduce the magnitude of the
charge-pump turn-on transient response. More input capaci-
tance, series resistors and/or ferrite beads may provide ben-
efits. If the current spikes and voltage dips can be tolerated,
connecting the PWM signal to the EN pin does provide a
benefit of lower supply current consumption. When the PWM
signal to the EN pin is low, the LM27951 will be shutdown
and input current will only be a few micro-amps. This results
in a lower time-averaged input current than the prior sugges-
tion, where EN is kept on continuously.
MAXIMUM OUTPUT CURRENT, MAXIMUM LED
VOLTAGE, MINIMUM INPUT VOLTAGE
The LM27951 can drive 4 LEDs at 30mA each from an input
voltage as low as 3.0V, 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 LM27951. The statement contains key
application parameters required to validate an LED-drive
LM27951
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Application Information (Continued)
design using the LM27951: LED current (I
LED
), number of
active LEDs (N), LED forward voltage (V
LED
), and minimum
input voltage (V
IN-MIN
).
The equation below can be used to estimate the total output
current capability of the LM27951:
I
LED_MAX
= ((1.5 x V
IN
)-V
LED
)/((NxR
OUT
)+k
HR
) (eq. 1)
I
LED_MAX
= ((1.5 x V
IN
)-V
LED
) / ((N x 3.3Ω) + 12mV/mA)
R
OUT
– Output resistance. This parameter models the inter-
nal losses of the charge pump that result in voltage droop at
the pump output V
OUT
. 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 LM27951 is typically 3.3Ω(V
IN
=
3.0V, T
A
= 25˚C). In equation form:
V
VOUT
=1.5xV
IN
–NxI
LED
xR
OUT
(eq. 2)
k
HR
– Headroom constant. This parameter models the mini-
mum voltage required across the current sources for proper
regulation. This minimum voltage is proportional to the pro-
grammed LED current, so the constant has units of mV/mA.
The typical k
HR
of the LM27951 is 12mV/mA. In equation
form:
(V
VOUT
–V
LED
)>k
HR
xI
LED
(eq. 3)
The "I
LED-MAX
" equation (eq. 1) is obtained from combining
the R
OUT
equation (eq. 2) with the k
HR
equation (eq. 3) and
solving for I
LED
. 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 LEDs with a
lower forward voltage. Excessive power dissipation may also
limit output current capability of an application.
CAPACITOR SELECTION
The LM27951 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 <20mΩ
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors are not recommended for use
with the LM27951 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
LM27951. 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 LM27951. 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 LM27951.
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.
PARALLEL DX OUTPUTS FOR INCREASED CURRENT
DRIVE
Outputs D
1-4
may be connected together to drive a one or
two LEDs at higher currents. In a one LED configuration, all
four parallel current sources of equal value are connected
together to drive a single LED. The LED current pro-
grammed should be chosen such that the current provided
from each of the outputs is programmed to 25% of the total
desired LED current. For example, if 60mA is the desired
drive current for the single LED, R
SET
should be selected
such that the current out of each current source is 15mA.
Similarly, if two LEDs are to be driven by pairing up the D
1-4
outputs (i.e D
1-2
,D
3-4
), R
SET
should be selected such that
the current out of each current source output is 50% of the
desired LED current.
Connecting the outputs in parallel does not affect the internal
operation of the LM27951 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 4-LED application circuit.
POWER EFFICIENCY
Efficiency of LED drivers is commonly taken to be the ratio of
power consumed by the LEDs (P
LED
) to the power drawn at
the input of the part (P
IN
). With a 1.5x/1x charge pump, the
input current is equal to the charge pump gain times the
output current (total LED current). For a simple approxima-
tion, the current consumed by internal circuitry can be ne-
glected and the efficiency of the LM27951 can be predicted
as follows:
P
LED
=NxV
LED
xI
LED
P
IN
=V
IN
xI
IN
P
IN
=V
IN
x (GainxNxI
LED
+I
Q
)
E=(P
LED
÷P
IN
)
Neglecting I
Q
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. It is also
worth noting that efficiency as defined here is in part depen-
dent 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 analy-
sis, it is recommended that power consumed by the circuit
(V
IN
xI
IN
) be evaluated rather than power efficiency.
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM27951
when the junction temperature exceeds 150˚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 140˚C (typ.).
It is important that the board layout provide good thermal
conduction to keep the junction temperature within the speci-
fied operating ratings.
POWER DISSIPATION
The power dissipation (P
DISSIPATION
) and junction tempera-
ture (T
J
) can be approximated with the equations below. P
IN
is the power generated by the 1.5x/1x charge pump, P
LED
is
the power consumed by the LEDs, T
A
is the ambient tem-
perature, and θ
JA
is the junction-to-ambient thermal resis-
LM27951
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Application Information (Continued)
tance for the LLP-14 package. V
IN
is the input voltage to the
LM27951, V
LED
is the nominal LED forward voltage, and
I
LED
is the programmed LED current.
P
DISSIPATION
=P
IN
-P
LED
= [Gain x V
IN
x(4xI
LED
)]−(V
LED
x4xI
LED
)
T
J
=T
A
+(P
DISSIPATION
xθ
JA
)
The junction temperature rating takes precedence over the
ambient temperature rating. The LM27951 may be operated
outside the ambient temperature rating, so long as the junc-
tion temperature of the device does not exceed the maxi-
mum operating rating of 115˚C. The maximum ambient tem-
perature rating must be derated in applications where high
power dissipation and/or poor thermal resistance causes the
junction temperature to exceed 115˚C.
PCB Layout Considerations
The LLP is a leadframe based Chip Scale Package (CSP)
with very good thermal properties. This package has an
exposed DAP (die attach pad) at the center of the package
measuring 3.0mm x 1.6mm. The main advantage of this
exposed DAP is to offer lower thermal resistance when it is
soldered to the thermal land on the PCB. For PCB layout,
National highly recommends a 1:1 ratio between the pack-
age and the PCB thermal land. To further enhance thermal
conductivity, the PCB thermal land may include vias to a
ground plane. For more detailed instructions on mounting
LLP packages, please refer to National Semiconductor Ap-
plication Note AN-1187.
LM27951
www.national.com9
Physical Dimensions inches (millimeters) unless otherwise noted
14-Pin LLP
NS Package Number SDA14A
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 manufactures products and uses packing materials that 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.
Leadfree products are RoHS compliant.
National Semiconductor
Americas Customer
Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
National Semiconductor
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Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
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Support Center
Email: ap.support@nsc.com
National Semiconductor
Japan Customer Support Center
Fax: 81-3-5639-7507
Email: jpn.feedback@nsc.com
Tel: 81-3-5639-7560
www.national.com
LM27951 White LED Adaptive 1.5X/1X Switched Capacitor Current Driver
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