LM2751SDX-A [NSC]

Regulated 2X, 1.5X Switched Capacitor White LED Driver; 稳压2X , 1.5X开关电容白光LED驱动器
LM2751SDX-A
型号: LM2751SDX-A
厂家: National Semiconductor    National Semiconductor
描述:

Regulated 2X, 1.5X Switched Capacitor White LED Driver
稳压2X , 1.5X开关电容白光LED驱动器

驱动器 开关
文件: 总11页 (文件大小:951K)
中文:  中文翻译
下载:  下载PDF数据表文档文件
April 2005  
LM2751  
Regulated 2X, 1.5X Switched Capacitor White LED Driver  
General Description  
Features  
n Regulated Output Options: 4.5V, 5.0V  
n Output Voltage Regulated within 3%  
n Peak Efficiency Over 90%  
The LM2751 is a constant frequency switched capacitor  
charge pump with regulated output voltage options of 4.5V,  
and 5.0V. Over the input voltage range of 2.8V to 5.5V the  
LM2751 provides up to 150mA of output current and requires  
only four low-cost ceramic capacitors.  
n 150mA (4.5V) or 80mA (5.0V) Output Current Capability  
n Input Voltage Range: 2.8V to 5.5V  
n Low Input and Output Voltage Ripple  
The LM2751 provides excellent efficiency without the use of  
an inductor by operating the charge pump in a gain of 3/2 or  
2. The proper gain for maintaining regulation is chosen so  
that efficiency is maximized over the input voltage range.  
<
n
1µA Typical Shutdown Current  
n Small Solution Size - NO INDUCTOR  
n Programmable 725kHz, 300kHz, 37kHz, or 9.5kHz  
Switching Frequencies  
LM2751 uses constant frequency pre-regulation to minimize  
conducted noise on the input and provide a predictable  
switching frequency. The switching frequency is program-  
mable to 725kHz, 300kHz, 37kHz, or 9.5kHz.  
n 10-pin LLP No-Pullback Package: 3mm x 3mm x 0.8mm  
Applications  
LM2751 is available in a 10-pin Leadless Leadframe No-  
Pullback Package: LLP-10.  
n White LED Display Backlights  
n White LED Keypad Backlights  
n General Purpose 2x, 1.5x Regulated Charge Pump  
Typical Application Circuit  
20112101  
LM2751 2x/1.5x Efficiency vs.  
2x Charge Pump Effciency  
20112128  
© 2005 National Semiconductor Corporation  
DS201121  
www.national.com  
Connection Diagram  
10-pin Leadless Leadframe Package (LLP-10) No Pullback  
3mm x 3mm x 0.8mm  
NS Package Number SDA10A  
20112102  
Pin Descriptions  
#
Pin  
1
Name  
VOUT  
C1+  
Description  
Pre-Regulated Output.  
2
Flying Capacitor C1 Connection.  
Input Supply Range: 2.8V to 5.5V.  
Frequency Select Input 0.  
Frequency Select Input 1.  
Enable Pin Logic Input.  
3
VIN  
4
CS0  
CS1  
EN  
5
6
7
C2−  
Flying Capacitor C2 Connection.  
Ground.  
8
GND  
C1−  
9
Flying Capacitor C1 Connection.  
Flying Capacitor C2 Connection.  
10  
C2+  
Ordering Information  
Version  
Voltage Option  
Order Number  
Package Marking  
Supplied As Tape and  
Reel  
A
A
B
B
5.0V  
5.0V  
4.5V  
4.5V  
LM2751SD-A  
LM2751SDX-A  
LM2751SD-B  
LM2751SDX-B  
XXXXX  
1000 Units  
YYYYY = L145B  
4500 Units  
XXXXX  
1000 Units  
YYYYY = L146B  
4500 Units  
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2
Absolute Maximum Ratings (Notes 1, 2)  
Operating Ratings (Notes 1, 2)  
Input Voltage Range  
If Military/Aerospace specified devices are required,  
please contact the National Semiconductor Sales Office/  
Distributors for availability and specifications.  
2.8V to 5.5V  
0V to VIN  
EN, CS0, CS1 Input Voltage Range  
Junction Temperature (TJ) Range  
Ambient Temperature (TA) Range  
(Note 5)  
-40˚C to 115˚C  
-40˚C to 85˚C  
VIN Pin  
−0.3V to 6.0V  
−0.3V to (VIN+0.3)  
w/ 6.0V max  
EN, CS0, CS1 Pins  
Recommended Maximum Load Current  
Continuous Power Dissipation  
(Note 3)  
Version  
B
Freq. = 725kHz  
Freq. = 300kHz  
Freq. = 37kHz  
Freq. = 9.5kHz  
Freq. = 725kHz  
Freq. = 300kHz  
Freq. = 37kHz  
Freq. = 9.5kHz  
150mA  
120mA  
40mA  
10mA  
80mA  
60mA  
16mA  
4mA  
Internally Limited  
Junction Temperature  
(TJ-MAX-ABS  
)
150˚C  
−65˚C to 150˚C  
265˚C  
Storage Temperature Range  
Maximum Lead Temperature  
(Soldering, 10sec.)  
Version  
A
ESD Rating (Note 4)  
Human-body model:  
Machine model:  
2kV  
200V  
Thermal Properties  
Junction-to-Ambient Thermal  
Resistance, LLP-10  
55˚C/W  
Package (θJA) (Note 6)  
Electrical Characteristics (Notes 2, 7)  
Limits in standard typeface are for TA = 25oC. Limits in boldface type apply over the full operating ambient temperature range  
(-40˚C TA +85˚C) . Unless otherwise noted, specifications apply to the LM2751 Typical Application Circuit (pg. 1) with: VIN  
= 3.6V, V(EN) = VIN, CS0 = CS1 = VIN, C1 = C2 = 1.0µF, CIN = COUT = 2.2µF (Note 8).  
Symbol  
Parameter  
Output Voltage  
Conditions  
Version A, 2.8V VIN 5.5V,  
Freq. = 300kHz, 725kHz, TA = 25˚C  
IOUT = 0 to 60mA  
Min  
Typ  
Max  
5.150  
(+3%)  
Units  
VOUT  
4.850  
(-3%)  
5.0  
V
Version A, 2.8V VIN 5.5V,  
Freq. = 300kHz, IOUT = 0 to 60mA  
Freq. = 725kHz, IOUT = 0 to 80mA  
Version B, 2.8V VIN 5.5V,  
Freq. = 300kHz, 725kHz, TA = 25˚C  
IOUT = 0 to 120mA  
4.775  
(-4.5%)  
5.225  
(+4.5%)  
4.343  
4.5  
4.658  
(-3.5%)  
(+3.5%)  
Version B, 2.8V VIN 5.5V,  
Freq. = 300kHz, IOUT = 0 to 120mA  
Freq. = 725kHz, IOUT = 0 to 150mA  
2.8V VIN 5.5V  
4.275  
(-5%)  
4.725  
(+5%)  
VR  
IQ  
Output Ripple  
8
mV  
µA  
IOUT = 60mA  
Quiescent Current  
Freq. = 9.5kHz, IOUT = 0mA, VIN = 3.7V  
Freq. = 37kHz, IOUT = 0mA, VIN = 3.7V  
Freq. = 300kHz, IOUT = 0mA, VIN = 3.7V  
Freq. = 725kHz, IOUT = 0mA, VIN = 3.7V  
425  
450  
700  
1000  
0.77  
1.0  
600  
640  
900  
1500  
1.3  
ISD  
E
Shutdown Supply Current V(EN) = 0V  
V(EN) = 0V, TA = 85˚C  
µA  
%
Efficiency  
IOUT = 80mA (Version A, 5.0V)  
Freq. = 300kHz, 725kHz  
92  
IOUT = 150mA (Version B, 4.5V)  
Freq. = 300kHz, 725kHz  
83  
3
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Electrical Characteristics (Notes 2, 7) (Continued)  
Limits in standard typeface are for TA = 25oC. Limits in boldface type apply over the full operating ambient temperature range  
(-40˚C TA +85˚C) . Unless otherwise noted, specifications apply to the LM2751 Typical Application Circuit (pg. 1) with: VIN  
= 3.6V, V(EN) = VIN, CS0 = CS1 = VIN, C1 = C2 = 1.0µF, CIN = COUT = 2.2µF (Note 8).  
Symbol  
fsw  
Parameter  
Conditions  
CS0 = High, CS1 = Low  
Min  
6.7  
Typ  
Max  
12.3  
Units  
Switching Frequency  
9.5  
kHz  
2.8V VIN 5.5V  
CS0 = Low, CS1 = Low  
2.8V VIN 5.5V  
CS0 = Low, CS1 = High  
2.8V VIN 5.5V  
CS0 = High, CS1 = High  
2.8V VIN 5.5V  
Input Pins: EN, CS0, CS1  
2.8V VIN 5.5V  
Input Pins: EN, CS0, CS1  
2.8V VIN 5.5V  
Input Pins: CS0, CS1  
V(CSx) = 1.8V  
(−30%)  
26  
(−30%)  
210  
(−30%)  
508  
(−30%)  
1.00  
(+30%)  
48  
(+30%)  
390  
(+30%)  
942  
(+30%)  
VIN  
37  
300  
725  
VIH  
VIL  
IIH  
Logic Input High  
V
V
Logic Input Low  
0
.30  
Logic Input High Current  
10  
2
nA  
µA  
nA  
V
Input Pin: EN  
V(EN) = 1.8V(Note 9)  
Input Pins: EN, CS0, CS1  
V(EN, CSx) = 0V  
1.5X to 2X  
IIL  
Logic Input Low Current  
10  
VG  
Gain Transition Voltage  
(Version A, B)  
3.50  
3.58  
80  
2X to 1.5X  
Hysteresis  
40  
150  
mV  
mA  
ISC  
tON  
Short Circuit Output  
Current  
VOUT = 0V  
250  
VOUT Turn-On Time  
(Note 10)  
300  
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 T =150˚C (typ.) and disengages at  
J
T =140˚C (typ.).  
J
Note 4: The Human body model is a 100 pF capacitor discharged through a 1.5kresistor 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 (T  
) is dependent on the maximum operation junction temperature (T  
A-MAX  
= 115oC), the maximum power  
J-MAX-OP  
dissipation of the device in the application (P  
), and the junction-to ambient thermal resistance of the part/package in the application (θ ), as given by the  
D-MAX  
JA  
following equation: T  
= T  
- (θ x P  
).  
A-MAX  
J-MAX-OP  
JA  
D-MAX  
Note 6: Junction-to-ambient thermal resistance (θ ) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the JEDEC  
JA  
standard JESD51-7. The test board is a 4 layer FR-4 board measuring 102mm x 76mm x 1.6mm with a 2 x 1 array 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 22oC, still air. Power  
dissipation is 1W.  
The value of θ of the LM2751 in LLP-10 could fall in a range as wide as 50oC/W to 150oC/W (if not wider), depending on PWB material, layout, and environmental  
JA  
conditions. In applications where high maximum power dissipation exists (high V , high I  
), special care must be paid to thermal dissipation issues. For more  
IN  
OUT  
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 represent the most likely norm.  
Note 8: C , C  
, C , and C : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics.  
1 2  
IN  
OUT  
Note 9: EN Logic Input High Current (I ) is due to a 1M(typ.) pull-down resistor connected internally between the EN pin and GND.  
IH  
Note 10: Turn-on time is measured from when the EN signal is pulled high until the output voltage on V  
crosses 90% of its final value.  
OUT  
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4
Block Diagram  
20112103  
5
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Typical Performance Characteristics Unless otherwise specified: TA = 25˚C, VIN = 3.6V, CS0 =  
CS1 = VIN, V(EN) = VIN, CIN = COUT = 2.2µF, C1 = C2 = 1µF.  
Output Voltage vs. Output Current,  
Version A (5V), 300kHz  
Output Voltage vs. Output Current,  
Version B (4.5V), 300kHz  
20112116  
20112110  
Output Voltage vs. Output Current,  
Version A (5V), 725kHz  
Output Voltage vs. Output Current,  
Version B (4.5V), 725kHz  
20112111  
20112117  
Input Current vs. Input Voltage,  
Version A (5V)  
Input Current vs. Input Voltage,  
Version B (4.5V)  
20112115  
20112121  
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6
Typical Performance Characteristics Unless otherwise specified: TA = 25˚C, VIN = 3.6V, CS0 = CS1  
= VIN, V(EN) = VIN, CIN = COUT = 2.2µF, C1 = C2 = 1µF. (Continued)  
Output Voltage vs. Input Voltage,  
Version A (5V), 300kHz  
Output Voltage vs. Input Voltage,  
Version B (4.5V), 300kHz  
20112112  
20112118  
Output Voltage vs. Input Voltage,  
Version A (5V), 725kHz  
Output Voltage vs. Input Voltage,  
Version B (4.5V), 725kHz  
20112113  
20112119  
Efficiency vs. Input Voltage,  
Version A (5V)  
Efficiency vs. Input Voltage,  
Version B (4.5V)  
20112114  
20112120  
7
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Typical Performance Characteristics Unless otherwise specified: TA = 25˚C, VIN = 3.6V, CS0 = CS1  
= VIN, V(EN) = VIN, CIN = COUT = 2.2µF, C1 = C2 = 1µF. (Continued)  
Output Voltage Ripple vs. Input Voltage  
Version B (4.5V), Load = 120mA  
Output Voltage Ripple,  
Version B (4.5V)  
20112126  
V
IN  
= 3.6V, Load = 150mA  
CH1: V  
; Scale: 10mV/Div, AC Coupled  
OUT  
Time scale: 400ns/Div  
20112129  
Line Step Response,  
Version B (4.5V)  
Load Step Response,  
Version B (4.5V)  
20112124  
20112127  
V
= 3.2V - 4.2V Step, Load = 150mA  
V
= 3.6V, Load = 20mA - 150mA Step  
IN  
IN  
CH1 (top): V ; Scale: 1V/Div, DC Coupled  
CH1 (top): V  
; Scale: 50mV/Div, AC Coupled  
OUT  
IN  
CH2: V  
; Scale: 50mV/Div, AC Coupled  
CH2: Output Current; Scale: 50mA/Div  
Time scale: 200µs/Div  
OUT  
Time scale: 200µs/Div  
Start-up Behavior,  
Version A (5V), Load = 80mA  
Start-up Behavior,  
Version B (4.5V), Load = 150mA  
20112122  
20112123  
CH1: EN pin; Scale: 2V/Div  
CH2: V ; Scale: 2V/Div  
CH1: EN pin; Scale: 2V/Div  
CH2: V ; Scale: 2V/Div  
OUT  
OUT  
Time scale: 100µs/Div  
Time scale: 100µs/Div  
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8
Application Information  
CIRCUIT DESCRIPTION  
drive of the switched capacitor charge pump. This regulation  
is done before the voltage is gained up by the charge pump,  
giving rise to the term "pre-regulation". Pre-regulation helps  
to reduce input current noise and large input current spikes  
normally associated with switched capacitor charge pumps.  
The LM2751 is a Switched Capacitor Convertor with gains of  
2x and 1.5x. It is capable of continuously supplying up to  
150mA at 4.5V or up to 80mA at 5V depending on the output  
voltage option. The LM2751’s fixed frequency pre-regulation  
maintains the output voltage to within 3% (typ.), making it  
well suited for driving White LEDs. There are also four user  
programmable switching frequencies to reduce the quies-  
cent current consumption at light loads.  
The LM2751 switched capacitor charge pump has gains of  
2x and 1.5x. When the input voltage to the device is greater  
than 3.58V (typ.), the LM2751 operates in a gain of 1.5x.  
When the input voltage falls below 3.5V (typ.), the device  
switches to a gain of 2x.  
Aside from powering LEDs, the LM2751 is suitable for driv-  
ing other devices with power requirements up to 150mA. The  
LM2751 operates over the extended Li-Ion battery range  
from 2.8V to 5.5V. The LM2751 limits output current to  
250mA (typ.) during an output short circuit condition. LED  
brightness is controlled by applying a PWM (Pulse Width  
Modulation) signal to the Enable pin (EN). (see PWM  
BRIGHTNESS CONTROL section).  
OUTPUT VOLTAGE RIPPLE  
The primary contributor in keeping the output voltage ripple  
of the LM2751 low is its switching topology. The output  
capacitance, input voltage, switching frequency and output  
current also play a significant part in determining the output  
voltage ripple. Due to the complexity of the LM2751 opera-  
tion, providing equations or models to approximate the mag-  
nitude of the ripple cannot be easily accomplished. However,  
the following general statements can be made.  
SOFT START  
Soft Start is engaged when the device is taken out of Shut-  
down mode (EN = logic HIGH) or when voltage is supplied  
simultaneously to the VIN and EN pins. During Soft Start, the  
voltage on VOUT will ramp up in proportion to the rate that the  
reference voltage is being ramped up. The output voltage is  
programmed to rise from 0V to the regulated output voltage  
level (4.5V or 5V) in 300µs (typ.).  
The LM2751 has very low output ripple when compared to  
typical boost regulators due to its double-pump topology,  
where charge is continually supplied to the output during  
both 2x and 1.5x modes. Combined with fixed frequency  
operation modes, double-pumping allows for the use of a  
very small, low value ceramic capacitor on the output node  
while still achieving minimal output ripple. Increasing the  
capacitance by adding a higher value capacitor or placing  
multiple capacitors in parallel can further reduce the ripple  
magnitude.  
ENABLE MODE  
The Enable logic pin (EN) disables the part and reduces the  
quiescent current to 0.77µA (typ.). The LM2751 has an  
active-high enable pin (LOW = shut down, HIGH = operat-  
ing) which can be driven with a low-voltage CMOS logic  
signal (1.5V logic, 1.8V logic, etc). There is an internal 1MΩ  
pull-down resistor between the EN and GND pins of the  
LM2751.  
CAPACITOR SELECTION  
The LM2751 requires 4 external capacitors for proper opera-  
tion. Surface-mount multi-layer ceramic capacitors are rec-  
ommended. These capacitors are small, inexpensive and  
have very low equivalent series resistance (ESR, 15mΩ  
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-  
num electrolytic capacitors are generally not recommended  
for use with the LM2751 due to their high ESR, as compared  
to ceramic capacitors.  
FREQUENCY MODE SELECT  
The LM2751 switching frequency is user programmable via  
two logic input pins, CS0 and CS1. Both logic input pins have  
active-high logic (LOW = un-selected, HIGH = selected) and  
can be driven with a low-voltage CMOS logic signal (1.5V  
logic, 1.8V logic, etc). There are no internal pull-down or  
pull-up resistors between the CSx and GND pins of the  
LM2751. The CSO and CS1 can be controlled independently  
or with the same logic signal.  
For most applications, ceramic capacitors with X7R or X5R  
temperature characteristic are preferred for use with the  
LM2751. These capacitors have tight capacitance tolerance  
(as good as 10%), hold their value over temperature (X7R:  
15% over −55˚C to 125˚C; X5R: 15% over −55˚C to  
85˚C), and typically have little voltage coefficient when com-  
pared to other types of capacitors. However selecting a  
capacitor with a voltage rating much higher than the voltage  
it will be subjected to, will ensure that the capacitance will  
stay closer to the capacitor’s nominal value. Capacitors with  
Y5V or Z5U temperature characteristic are generally not  
recommended for use with the LM2751. Capacitors with  
these temperature characteristics typically have wide ca-  
pacitance tolerance (+80%, −20%), vary significantly over  
temperature (Y5V: +22%, −82% over −30˚C to +85˚C range;  
Z5U: +22%, −56% over +10˚C to +85˚C range), and have  
poor voltage coefficients. 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 LM2751.  
The selectable switching frequencies are 9.5kHz, 37kHz,  
300kHz, 725kHz. The switching frequency is programmed  
according to Table 1  
TABLE 1. Frequency Modes  
CS0  
CS1  
Frequency  
37kHz  
0
0
1
1
0
1
0
1
300kHz  
9.5kHz  
725kHz  
VOUT REGULATION  
The LM2751 uses pre-regulation to regulate the output volt-  
age to 4.5V or 5.0V depending on the voltage option. Pre-  
regulation uses the voltage present at VOUT to limit the gate  
9
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THERMAL PROTECTION  
Application Information (Continued)  
When the junction temperature exceeds 150˚C (typ.), inter-  
nal thermal protection circuitry disables the device. This  
feature protects the LM2751 from damage due to excessive  
power dissipation. The device will recover and operate nor-  
mally when the junction temperature falls below 140˚C (typ.).  
It is important to have good thermal conduction with a proper  
layout to reduce thermal resistance.  
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.  
DRIVING WHITE LEDS  
The desired LED current is set by placing a resistor (R) in  
series with each LED, and is determined by the equation:  
POWER EFFICIENCY  
ILED = (VOUT - VLED) ÷R  
Charge-Pump efficiency is derived in the following two ideal  
equations (supply current and other losses are neglected for  
simplicity):  
In the equation above, ILED is the current that flows through  
a particular LED, and VLED is the forward voltage of the LED  
at the given current. The output voltage (VOUT) of the  
LM2751 is tightly regulated to 4.5V or 5V depending on the  
output voltage option. However, LED forward voltage varies  
from LED to LED, and LED current will vary accordingly.  
Mismatch of LED currents will result in brightness mismatch  
from one LED to the next. Therefore it is suggested that LED  
groups with tightly controlled I-V characteristics ("Binned"  
LEDs) be used. LEDs with looser tolerance can be used in  
applications where brightness matching is not critical, such  
as in keypad or general backlighting. The typical and maxi-  
mum diode forward voltage depends highly on the manufac-  
turer and their technology.  
IIN = G x IOUT  
E = (VOUT x IOUT) ÷ (VIN x IIN) = VOUT ÷ (G x VIN  
)
In the equations, G represents the charge pump gain. Effi-  
ciency is at its highest as G x VIN approaches VOUT. Refer to  
the efficiency graph in the Typical Performance Character-  
istics section for the detailed efficiency data.  
POWER DISSIPATION  
The power dissipation (PDISSIPATION) and junction tempera-  
ture (TJ) can be approximated with the equations below. PIN  
is the product of the input current and input voltage, POUT is  
the power consumed by the load connected to the output,  
TAis the ambient temperature, and θJA is the junction-to-  
ambient thermal resistance for the LLP-10 package. VIN is  
the input voltage to the LM2751, VVOUT is the voltage at the  
output of the device, and IOUT is the total current supplied to  
PWM BRIGHTNESS CONTROL  
Perceived LED brightness can be adjusted using a PWM  
control signal on the Enable pin of the LM2751, to turn the  
voltage output 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, per-  
ceived LED brightness will be about half as bright as com-  
pared to when the LEDs are driven continuously with 15mA.  
the load connected to VOUT  
.
PDISSIPATION = PIN - POUT  
= (VIN x IIN) − (VVOUT x IOUT  
)
TJ = TA + (PDISSIPATION x θJA  
)
The junction temperature rating takes precedence over the  
ambient temperature rating. The LM2751 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.  
For linear brightness control over the full duty cycle adjust-  
ment range, the PWM frequency (f) should be limited to  
accommodate the turn-on time (typ. TON = 300µs) of the  
device.  
>
D x (1/f) TON  
fMAX = DMIN ÷ TON  
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 noise in the au-  
dible range.  
www.national.com  
10  
Physical Dimensions inches (millimeters) unless otherwise noted  
10-Pin LLP  
NS Package Number SDA10A  
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.  
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WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR  
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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  
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