Datasheet LM2795EVAL, LM2795BLX, LM2795BL, LM2795TL Datasheet (NSC)

LM2794/LM2795
Current Regulated Switched Capacitor LED Supply with
Analog and PWM Brightness Control

General Description

The LM2794/95 is a fractional CMOS charge-pump that
provides four regulated current sources. It accepts an input
voltage range from 2.7V to 5.5V and maintains a constant
current determined by an external sense resistor.

The LM2794/5 delivers up to 80mA of load current to accom­modate four White LEDs. The switching frequency is
325kHz. (min.) to keep the conducted noise spectrum away
from sensitive frequencies within portable RF devices.

Brightness can be controlled by both linear and PWM tech­niques. A voltage between 0V and 3.0V may be applied to
the BRGT pin to linearly vary the LED current. Alternatively,
a PWM signal can be applied to the SD pin to vary the
perceived brightness of the LED. The SD pin reduces the
operating current to 2.3µA (typ.) The LM2794 is shut down
when the SD pin is low, and the LM2795 is shut down when
the SD pin is high.

The LM2794/95 is available in a micro SMD-14 CSP pack­age.

Features

n Regulated current sources with±0.5% matching

between any two outputs

n High efficiency 3/2 boost function
n Drives one, two, three or four white LEDs
n 2.7V to 5.5V Input Voltage
n Up to 80mA output current
n Analog brightness control
n Active-low or high shutdown input (’94/95)
n Very small solution size and no inductor
n 2.3µA (typ.) shutdown current
n 325kHz switching frequency (min.)
n Constant Frequency generates predictable noise

spectrum

n Standard Micro SMD-14 package: 2.08mm X 2.403mm

X 0.845mm High

n Thin Micro SMD-14 package: 2.08mm X 2.403mm X

0.600mm High

Applications

n White LED Display Backlights
n White LED Keypad Backlights
n 1-Cell Li-Ion battery-operated equipment including

PDAs, hand-held PCs, cellular phones

Basic Application Circuit

20028503

May 2003

LM2794/LM2795 Current Regulated Switched Capacitor LED Supply with Analog and PWM

Brightness Control

© 2003 National Semiconductor Corporation DS200285 www.national.com

Connection Diagram

20028523

Bottom View

Ordering Information

Standard Micro SMD Package:

Order Number Shutdown Polarity Package Number Package

Marking

Supplied As

LM2794BL Active Low BLP14EHB I LOG 250 Units, Tape and Reel

LM2794BLX Active Low BLP14EHB I LOG 3000 Units, Tape and Reel

LM2795BL Active High BLP14EHB I LOJ 250 Units, Tape and Reel

LM2795BLX Active High BLP14EHB I LOJ 3000 Units, Tape and Reel

Thin Micro SMD Package:

Order Number Shutdown Polarity Package Number Package

Marking

Supplied As

LM2794TL Active Low TLP14EHA I LOG 250 Units, Tape and Reel

LM2794TLX Active Low TLP14EHA I LOG 3000 Units, Tape and Reel

LM2795TL Active High TLP14EHA I LOJ 250 Units, Tape and Reel

LM2795TLX Active High TLP14EHA I LOJ 3000 Units, Tape and Reel

Pin Description

Pin(*) Name Function

A1 C1+ Positive terminal of C1

B2 C1− Negative terminal of C1

C1 V

IN

Power supply voltage input

D2 GND Power supply ground input

E1 C2− Negative terminal of C2

E3,E5,E7,D6 D1−4 Current source outputs. Connect directly to LED

C7 I

SET

Current Sense Input. Connect 1% resistor to ground to set constant current through LED

B6 BRGT Variable voltage input controls output current

A7 SD The LM2794 has an active-low shutdown pin (LOW = shutdown, HIGH = operating). The

LM2795 has an active-high shutdown pin (HIGH = shutdown, LOW = operating) that has a
pull-up to V

IN

.

A5 C2+ Positive terminal of C2

A3 P

OUT

Charge pump output

(*) Note that the pin numbering scheme for the Micro SMD package was revised in April, 2002 to conform to JEDEC standard. Only the pin numbers were revised.
No changes to the physical location of the inputs/outputs were made. For reference purpose, the obsolete numbering had C1+ as pin 1, C1- as pin 2, VIN as pin
3, GND as pin 4, C2- as pin 5, D1-D4 as pin 6,7,8 & 9, Iset as pin 10, BRGT as pin 11, SD as pin 12, C2+ as pin 13, Pout as pin 14

LM2794/LM2795

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Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

V

IN

−0.5 to 6.2V max

SD −0.5 to (V

IN

+0.3V) w/

6.2V max

BRGT −0.5 to (V

IN

+0.3V) w/

6.2V max

Continuous Power Dissipation
(Note 2) Internally Limited

T

JMAX

(Note 2) 135˚C

θ

JA

(Notes 2, 3) 125˚C/W

Storge Temperature −65˚C to +150˚C

Lead Temp. (Soldering, 5 sec.) 260˚C

ESD Rating (Note 4)

Human Body Model 2kV

Machine Model 200V

Operating Conditions

Input Voltage (VIN) 2.7V to 5.5V

Ambient Temperature (T

A

) −30˚C to +85˚C

Junction Temperature (T

J

) −30˚C to +100˚C

Electrical Characteristics

Limits in standard typeface are for TJ= 25˚C and limits in boldface type apply over the full Operating Junction Temperature
Range (−30˚C ≤ T

J

≤ +100˚C). Unless otherwise specified, C1 = C2 = CIN=C

HOLD

= 1 µF, VIN= 3.6V, BRGT pin = 0V; R

SET

=124Ω ; LM2794:VSD=VIN(LM2795: VSD= 0V).

Symbol Parameter Conditions Min Typ Max Units

I

DX

Available Current at Output Dx 3.0V ≤ VIN≤ 5.5V

V

DX

≤ 3.8V

BRGT = 50mV

15 16.8 mA

2.7V ≤ V

IN

≤ 3.0V

V

DX

≤ 3.6V

BRGT = 0V

10

mA

V

DX

≤ 3.8V

BRGT = 200mV

20

mA

V

DX

Available Voltage at Output Dx 3.0V ≤ VIN≤ 5.5V

I

DX

≤ 15mA

BRGT = 50mV

3.8 V

I

DX

Line Regulation of Dx Output
Current

3.0V ≤ VIN≤ 5.5V
V

DX

= 3.6V

14.18 15.25 16.78 mA

3.0V ≤ V

IN

≤ 4.4V

V

DX

= 3.6V

14.18 15.25 16.32 mA

I

DX

Load Regulation of Dx Output
Current

VIN= 3.6V

3.0V ≤ V

DX

≤ 3.8V

14.18 15.25 16.32 mA

I

D-MATCH

Current Matching Between Any
Two Outputs

VIN= 3.6V, VDX= 3.6V 0.5 %

I

Q

Quiescent Supply Current 3.0V ≤ VIN≤ 4.2V, Active, No

Load Current
R

SET

= OPEN

5.5 8.2 mA

I

SD

Shutdown Supply Current 3.0V ≤ VIN≤ 5.5V, Shutdown 2.3 5 µA

I

PULL-SD

Shutdown Pull-Up Current
(LM2795)

VIN= 3.6V 1.5 µA

V

CP

Input Charge-Pump Mode To
Pass Mode Threshold

4.7 V

V

CPH

Input Charge-Pump Mode To
Pass Mode Hysteresis

(Note 5) 250 mV

V

IH

SD Input Logic High (LM2794) 3.0V ≤ VIN≤ 5.5V 1.0 V

SD Input Logic High (LM2795) 0.8V

IN

V

IL

SD Input Logic Low (LM2794) 3.0V ≤ VIN≤ 5.5V 0.2 V

SD Input Logic Low (LM2795) 0.2V

IN

I

LEAK-SD

SD Input Leakage Current 0V ≤ VSD≤ V

IN

100 nA

R

BRGT

BRGT Input Resistance 240 kΩ

I

SET

I

SET

Pin Output Current IDX/10 mA

LM2794/LM2795

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Electrical Characteristics (Continued)

Limits in standard typeface are for TJ= 25˚C and limits in boldface type apply over the full Operating Junction Temperature
Range (−30˚C ≤ T

J

≤ +100˚C). Unless otherwise specified, C1 = C2 = CIN=C

HOLD

= 1 µF, VIN= 3.6V, BRGT pin = 0V; R

SET

=124Ω ; LM2794:VSD=VIN(LM2795: VSD= 0V).

Symbol Parameter Conditions Min Typ Max Units

f

SW

Switching Frequency (Note 6) 3.0V ≤ VIN≤ 4.4V 325 515 675 kHz

Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device
beyond its rated operating conditions.

Note 2: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at T

J

=150˚C (typ.) and disengages at

T

J

=140˚C (typ.). D1, D2, D3 and D4 may be shorted to GND without damage. P

OUT

may be shorted to GND for 1sec without damage.

Note 3: The value of θ

JA

is based on a two layer evaluation board with a dimension of 2in. x1.5in.

Note 4: In the test circuit, all capacitors are 1.0µF, 0.3Ω maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce output
voltage and efficiency.

Note 5: Voltage at which the device switches from charge-pump mode to pass mode or pass mode to charge-pump mode. For example, during pass mode the
device output (Pout) follows the input voltage.

Note 6: The output switches operate at one eigth of the oscillator frequency, f

OSC

= 1/8fSW.

LM2794/LM2795

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Typical Performance Characteristics Unless otherwise specified, C1 = C2 = C

IN=CHOLD

= 1µF,

V

IN

= 3.6V, BRGT pin = 0V, R

SET

= 124Ω.

I

DIODE

vs V

IN

I

DIODE

vs BRGT

20028512

20028509

I

DIODE

vs V

IN

BRGT = 3V I

DIODE

vs R

SET

20028507

20028508

I

DIODE

vs R

SET

V

BRGT

=0V I

DIODE

vs V

DIODE

20028541

20028524

LM2794/LM2795

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Typical Performance Characteristics Unless otherwise specified, C1 = C2 = C

IN=CHOLD

= 1µF, V

IN

= 3.6V, BRGT pin = 0V, R

SET

= 124Ω. (Continued)

V

SET

vs V

BRGT

R

SET

=1KΩ

Duty Cycle vs. Led Current (LM2794)

I

DIODE

1- 4 = 15mA

20028506

20028532

Supply Current vs V

IN

I

DIODE

1-4 = 15mA

Supply Current vs V

IN

I

DIODE

1-4 = Open

20028514

20028515

Shutdown Supply Current vs V

IN

Shutdown Threshold vs V

IN

20028513

20028505

LM2794/LM2795

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Typical Performance Characteristics Unless otherwise specified, C1 = C2 = C

IN=CHOLD

= 1µF, V

IN

= 3.6V, BRGT pin = 0V, R

SET

= 124Ω. (Continued)

Start-Up Response

@

VIN= 2.7V (LM2794) Start-Up Response@VIN= 2.7V (LM2795)

20028517 20028520

Start-Up Response@VIN= 3.6V (LM2794) Start-Up Response@VIN= 3.6V (LM2795)

20028518 20028522

Start-Up Response@VIN= 4.2V (LM2794) Start-Up Response@VIN= 4.2V (LM2795)

20028519 20028521

LM2794/LM2795

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Typical Performance Characteristics Unless otherwise specified, C1 = C2 = C

IN=CHOLD

= 1µF, V

IN

= 3.6V, BRGT pin = 0V, R

SET

= 124Ω. (Continued)

Available Additional Current

@

P

OUT

I

DIODE

1− 4 = 15mA, R

SET

= 124 Ω Switching Frequency

20028531

20028516

LM2794/LM2795

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Functional Block Diagram

20028530

LM2794/LM2795

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Application Information

CIRCUIT DESCRIPTION

The LM2794/5 is a 1.5x/1x CMOS charge pump with four
matched constant current outputs, each capable of driving
up to 20mA through White LEDs. This device operates over
the extended Li-Ion battery range from 2.7V to 5.5V. The
LM2794/5 has four regulated current sources connected to
the device’s 1.5x charge pump output (P

OUT

). At input volt­ages below 4.7V (typ.), the charge-pump provides the
needed voltage to drive high forward voltage drop White
LEDs. It does this by stepping up the P

OUT

voltage 1.5 times
the input voltage. The charge pump operates in Pass Mode,
providing a voltage on P

OUT

equal to the input voltage, when
the input voltage is at or above 4.7V (typ.). The device can
drive up to 80mA through any combination of LEDs con­nected to the constant current outputs D

1-D4

.

To set the LED drive current, the device uses a resistor
connected to the I

SET

pin to set a reference current. This
reference current is then multiplied and mirrored to each
constant current output. The LED brightness can then be
controlled by analog and/or digital methods. Applying an
analog voltage in the range of 0V to 3.0V to the Brightness
pin (BRGT) adjusts the dimming profile of the LEDs. The
digital technique uses a PWM (Pulse Width Modulation)
signal applied to the Shutdown pin (SD). (see I

SET

and

BRGT PINS section).

SOFT START

Soft start is implemented internally by ramping the reference
voltage more slowly than the applied voltage. During soft
start, the current through the LED outputs will ramp up in
proportion to the rate that the reference voltage is being
ramped up.

SHUTDOWN MODE

The shutdown pin (SD) disables the part and reduces the
quiescent current to 2.3µA (typ.).

The LM2795 has an active-high shutdown pin (HIGH =
shutdown, LOW = operating). An internal pull-up is con­nected between SD and V

IN

of the LM2795. This allows the
use of open-drain logic control of the LM2795 shutdown, as
shown in Figure 1. The LM2795 SD pin can also be driven
with a rail-to-rail CMOS logic signal.

The LM2794 has an active-low shutdown pin (LOW = shut­down, HIGH = operating). The LM2794 SD pin can be driven
with a low-voltage CMOS logic signal (1.5V logic, 1.8V logic,
etc). There is no internal pull-up or pull-down on the SD pin
of the LM2794.

CAPACITOR SELECTION

The LM2794/5 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, ≤15mΩ
typ.). Tantalum capacitors, OS-CON capacitors, and alumi­num electrolytic capacitors are generally not recommended
for use with the LM2794/5 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
LM2794/5. 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. Capaci­tors with Y5V or Z5U temperature characteristic are gener­ally not recommended for use with the LM2794/5. Capaci­tors with these temperature characteristics typically have
wide capacitance 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 LM2794/5. Table 1 lists suggested
capacitor suppliers for the typical application circuit.

TABLE 1. Ceramic Capacitor Manufacturers

Manufacturer Contact

TDK www.component.tdk.com

Murata www.murata.com

Taiyo Yuden www.t-yuden.com

LED SELECTION

The LM2794/5 is designed to drive LEDs with a forward
voltage of about 3.0V to 4.0V. The typical and maximum
diode forward voltage depends highly on the manufacturer
and their technology. Table 2 lists two suggested manufac­turers. Forward current matching is assured over the LED
process variations due to the constant current output of the
LM2794/5.

TABLE 2. White LED Selection

Manufacturer Contact

Osram www.osram-os.com

Nichia www.nichia.com

I

SET

AND BRGT PINS

An external resistor, R

SET

, is connected to the I

SET

pin to set
the current to be mirrored in each of the LED outputs. The
internal current mirror sets each LED output current with a
10:1 ratio to the current through R

SET

. The current mirror

circuitry matches the current through each LED to within

0.5%.
In addition to R

SET

, a voltage may be applied to the V

BRGT

pin to vary the LED current. By adjusting current with the
Brightness pin (BRGT), the brightness of the LEDs can be
smoothly varied.

20028536

FIGURE 1. Open-Drain Logic Shutdown Control

LM2794/LM2795

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Application Information (Continued)

Applying a voltage on BRGT between 0 to 3 volts will linearly
vary the LED current. Voltages above 3V do not increase the
LED current any further. The voltage on the V

BRGT

pin is fed
into an internal resistor network with a ratio of 0.385. The
resulting voltage is then summed with a measured offset
voltage of 0.188V, which comes from the reference voltage
being fed through a resistor network (See Functional Block
Diagram). The brightness control circuitry then uses the
summed voltage to control the voltage across R

SET

.An

equation for approximating the LED current is:

20028540

I

LED

CURRENT SELECTION PROCEDURES

The following procedures illustrate how to set and adjust
output current levels. For constant brightness or analog
brightness control, go to “Brightness control using BRGT”.
Otherwise refer to “Brightness control using PWM”.

Brightness Control Using PWM

1. Set the BRGT pin to 0V.

2. Determine the maximum desired I

LED

current. Use the

I

LED

equation to calculate R

SET

by setting BRGT to 0V or

use Table 3 to select a value for R

SET

when BRGT

equals 0V.

3. Brightness control can be implemented by pulsing a

signal at the SD pin. LED brightness is proportional to
the duty cycle (D) of the PWM signal. For linear bright­ness control over the full duty cycle adjustment range,
the PWM frequency (f) should be limited to accommo­date the turn-on time (T

ON

= 100µs) of the device.

D x (1/f)

>

T

ON

f

MAX=DMIN÷TON

If the PWM frequency is much less than 100Hz, flicker
may be seen in the LEDs. For the LM2794, zero duty
cycle will turn off the LEDs and a 50% duty cycle will
result in an average I

LED

being half of the programmed

LED current. For example, if R

SET

is set to program

15mA, a 50% duty cycle will result in an average I

LED

of

7.5mA. For the LM2795 however, 100% duty cycle will
turn off the LEDs and a 50% duty cycle will result in an
average I

LED

being half the programmed LED current.

Brightness Control Using BRGT

1. Choose the maximum I

LED

desired and determine the
max voltage to be applied to the BRGT pin. For constant
brightness, set BRGT to a fixed voltage between 0V to
3V.

2. Use Table 3 to determine the value of R

SET

required or

use the I

LED

equation above to calculate R

SET

.

3. Use Table 4 as a reference for the dimming profile of the
LEDs, when BRGT ranges from 0V to 3V.

TABLE 3. R

SET

Values

LED Current

BRGT 5mA 10mA 15mA 20mA

0.0V 374Ω 187Ω 124Ω 93.1Ω

0.5V 768Ω 383Ω 255Ω 191Ω

1.0V 1.15KΩ 576Ω 383Ω 287Ω

1.5V 1.54KΩ 768Ω 511Ω 383Ω

2.0V 1.91KΩ 953Ω 624Ω 475Ω

2.5V 2.32KΩ 1.15KΩ 768Ω 576Ω

3.0V 2.67KΩ 1.33KΩ 909Ω 665Ω

R

SET

values are rounded off to the nearest 1% standard values.

TABLE 4. LED Current

R

SET

Values

BRGT 2.67KΩ 1.33KΩ 909Ω 665Ω

0.0V 0.7mA 1.4mA 2.1mA 2.8mA

0.5V 1.4mA 2.9mA 4.2mA 5.7mA

1.0V 2.1mA 4.3mA 6.3mA 8.6mA

1.5V 2.9mA 5.8mA 8.4mA 11.5mA

2.0V 3.6mA 7.2mA 10.5mA 14.4mA

2.5V 4.3mA 8.7mA 12.7mA 17.3mA

3.0V 5.0mA 10.1mA 14.8mA 20.2mA

CHARGE PUMP OUTPUT (P

OUT

)

The LM2794/5 charge pump is an unregulated switched
capacitor converter with a gain of 1.5. The voltage at the
output of the pump (the P

OUT

pin) is nominally 1.5 x VIN. This
rail can be used to deliver additional current to other circuitry.
Figure 2 shows how to connect additional LEDs to P

OUT

.A
ballast resistor sets the current through each LED, and LED
current matching is dependent on the LED forward voltage
matching. Because of this, LEDs driven by P

OUT

are recom­mended for functions where brightness matching is not criti­cal, such as keypad backlighting.

Since P

OUT

is unregulated, driving LEDs directly off P

OUT

is
usually practical only with a fixed input voltage. If the input
voltage is not fixed (Li-Ion battery, for example), using a
linear regulator between the P

OUT

pin and the LEDs is
recommended. National Semiconductor’s LP3985-4.5V low­dropout linear regulator is a good choice for such an appli­cation.

The voltage at P

OUT

is dependent on the input voltage
supplied to the LM2794/5, the total LM2794/5 output current,
and the output resistance (R

OUT

) of the LM2794/5 charge
pump. Output resistance is a model of the switching losses
of the charge pump. Resistances of the internal charge
pump switches (MOS transistors) are a primary component
of the LM2794/5 output resistance. Typical LM2794/5 output
resistance is 3.0Ω. For worst-case design calculations, using
an output resistance of 3.5Ω is recommended. (Worst-case
recommendation accounts for parameter shifts from part-to­part variation and applies over the full operating temperature
range).

LM2794/LM2795

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Application Information (Continued)

20028535

FIGURE 2. Keypad LEDs Connected to P

OUT

LM2794/LM2795

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Application Information (Continued)

Output resistance results in droop in the P

OUT

voltage pro­portional to the amount of current delivered by the pump.
The P

OUT

voltage is an important factor in determining the
total output current capability of an application. Taking total
output current to be the sum of all D

X

output currents plus

the current delivered through the P

OUT

pin, the voltage at

P

OUT

can be predicted with the following equations:

I

TOTAL=ID1+ID2+ID3+ID4+IPOUT

V

POUT

=1.5xVIN−I

TOTALxROUT

LED HEADROOM VOLTAGE (VHR)

Four current sources are connected internally between P

OUT

and D1-D4. The voltage across each current source, (V

POUT

−VDX), is referred to as headroom voltage (VHR). The cur­rent sources require a sufficient amount of headroom voltage
to be present across them in order to regulate properly.
Minimum required headroom voltage is proportional to the
current flowing through the current source, as dictated by the
equation:

V

HR-MIN=kHRxIDX

The parameter kHR, typically 20mV/mA in the LM2794/5, is a
proportionality constant that represents the ON-resistance of
the internal current mirror transistors. For worst-case design
calculations, using a k

HR

of 25mV/mA is recommended.
(Worst-case recommendation accounts for parameter shifts
from part-to-part variation and applies over the full operating
temperature range). Figure 3 shows how output current of
the LM2794/5 varies with respect to headroom voltage.

On the flat part of the graph, the currents regulate properly
as there is sufficient headroom voltage for regulation. On the
sloping part of the graph the headroom voltage is too small,
the current sources are squeezed, and their current drive
capability is limited. Changes in headroom voltage from one
output to the next, possible with LED forward voltage mis­match, will result in different output currents and LED bright­ness mismatch. Thus, operating the LM2794/5 with insuffi­cient headroom voltage across the current sources should
be avoided.

OUTPUT CURRENT CAPABILITY

The primary constraint on the total current capability is the
headroom voltage requirement of the internal current
sources. Combining the V

POUT

and VHRequations from the
previous two sections yields the basic inequality for deter­mining the validity of an LM2794/5 LED-drive application:

V

POUT

=1.5xVIN−I

TOTALxROUT

V

HR-MIN=kHRxIDX

V

POUT−VDX

≥ V

HR-MIN

1.5xVIN−I

TOTALxROUT−VDX

≥ (kHRxIDX)

Rearranging this inequality shows the estimated total output
current capability of an application:

I

TOTAL

≤ [(1.5 x V

IN-MIN

)−V

DX-MAX

−(kHRxIDX)]÷R

OUT

Examining the equation above, the primary limiting factors
on total output current capability are input and LED forward
voltage. A low input voltage combined with a high LED
voltage may result in insufficient headroom voltage across
the current sources, causing them to fall out of regulation.
When the current sources are not regulated, LED currents
will be below desired levels and brightness matching will be
highly dependent on LED forward voltage matching.

Typical LM2794/5 output resistance is 3.0Ω. For worst-case
design calculations, using an output resistance of 3.5Ω is
recommended. LM2794/5 has a typical k

HR

constant of

20mV/mA. For worst-case design calculations, use k

HR

=
25mV/mA. (Worst-case recommendations account for pa­rameter shifts from part-to-part variation and apply over the
full operating temperature range). R

OUT

and kHRincrease
slightly with temperature, but losses are typically offset by
the negative temperature coefficient properties of LED for­ward voltages. Power dissipation and internal self-heating
may also limit output current capability but is discussed in a
later section.

PARALLEL Dx OUTPUTS FOR INCREASED CURRENT
DRIVE

Outputs D

1

through D4may be connected together in any
combination to drive higher currents through fewer LEDs.
For example in Figure 4, outputs D

1

and D2are connected

together to drive one LED while D

3

and D4are connected

together to drive a second LED.

20028539

FIGURE 3. I

LED

vs V

HR

4 LEDs, VIN= 3.0V

20028533

FIGURE 4. Two Parallel Connected LEDs

LM2794/LM2795

www.national.com13

Application Information (Continued)

With this configuration, two parallel current sources of equal
value provide current to each LED. R

SET

and V

BRGT

should
therefore be chosen so that the current through each output
is programmed to 50% of the desired current through the
parallel connected LEDs. For example, if 30mA is the de­sired drive current for 2 parallel connected LEDs , R

SET

and

V

BRGT

should be selected so that the current through each of
the outputs is 15mA. Other combinations of parallel outputs
may be implemented in similar fashions, such as in Figure 5.

Connecting outputs in parallel does not affect internal opera­tion of the LM2794/95 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 4-LED application circuit.

THERMAL PROTECTION

When the junction temperature exceeds 150˚C (typ.), the
LM2794/5 internal thermal protection circuitry disables the
part. This feature protects the device from damage due to
excessive power dissipation. The device will recover and
operate normally 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.

POWER EFFICIENCY

Figure 6 shows the efficiency of the LM2794/5. The change
in efficiency shown by the graph comes from the transition
from Pass Mode to a gain of 1.5.

Efficiency (E) of the LM2794/5 is defined here as the ratio of
the power consumed by LEDs (P

LED

) to the power drawn

from the input source (P

IN

). In the equations below, IQis the

quiescent current of the LM2794/5, I

LED

is the current flowing

through one LED, V

LED

is the forward voltage at that LED
current, and N is the number of LEDs connected to the
regulated current outputs. In the input power calculation, the

1.5 represents the switched capacitor gain configuration of
the LM2794/5.

P

LED

=NxV

LEDxILED

PIN=VINxI

IN

PIN=VINx(1.5xNxI

LED+IQ

)

E=(P

LED÷PIN

)

Efficiency, as defined here, is in part dependent 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 analysis, it is rec­ommended that power consumed by the circuit (V

INxIIN

)be
evaluated rather than power efficiency. Figure 7 shows the
power consumption of the LM2794/5 Typical Application Cir­cuit.

20028534

FIGURE 5. One Parallel Connected LED

20028537

FIGURE 6. Efficiency vs V

IN

4 LEDs, V

LED

= 3.6V, I

LED

= 15mA

LM2794/LM2795

www.national.com 14

Application Information (Continued)

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 charge pump, P

LED

is the

power consumed by the LEDs, P

POUT

is the power provided

through the P

OUT

pin, TAis the ambient temperature, and θ

JA

is the junction-to-ambient thermal resistance for the micro
SMD-14 package. V

IN

is the input voltage to the LM2794/5,

V

DX

is the LED forward voltage, IDXis the programmed LED

current, and I

POUT

is the current drawn through P

OUT

.

P

DISSIPATION=PIN-PLED−PPOUT

= [1.5xVINx(4IDX+I

POUT

)]−(VDXx4IDX) − (1.5xVINxI

POUT

)

T

J=TA

+(P

DISSIPATION

x θJA)

The junction temperature rating takes precedence over the
ambient temperature rating. The LM2794/5 may be operated
outside the ambient temperature rating, so long as the junc­tion temperature of the device does not exceed the maxi­mum operating rating of 100˚C. The maximum ambient tem­perature rating must be derated in applications where high
power dissipation and/or poor thermal resistance causes the
junction temperature to exceed 100˚C.

MICRO SMD MOUNTING

The LM2794/5 is a 14-bump micro SMD with a bump size of
300 micron diameter. The micro SMD package requires
specific mounting techniques detailed in National Semicon­ductor Application Note (AN -1112). NSMD (non-solder mask
defined) layout pattern is recommended over the SMD (sol­der mask defined) since the NSMD requires larger solder
mask openings over the pad size as opposed to the SMD.
This reduces stress on the PCB and prevents possible
cracking at the solder joint. For best results during assembly,
alignment ordinals on the PC board should be used to
facilitate placement of the micro SMD device. Micro SMD is
a wafer level chip size package, which means the dimen­sions of the package are equal to the die size. As such, the
micro SMD package lacks the plastic encapsulation charac­teristics of the larger devices and is sensitive to direct expo­sure to light sources such as infrared, halogen, and sun light.
The wavelengths of these light sources may cause unpre­dictable operation.

20028538

FIGURE 7. PINvs V

IN

4 LEDs, 2.5 ≤ VDX≤ 3.9V, IDX= 15mA

LM2794/LM2795

www.national.com15

Physical Dimensions inches (millimeters) unless otherwise noted

Standard Micro SMD Package

For Ordering, Refer to Ordering Information Table

NS Package Number BLP14

The dimensions for X1, X2, X3 are given as:

X1 = 2.08mm

±

0.03mm

X2 = 2.403mm

±

0.03mm

X3 = 0.845mm

±

0.01mm

LM2794/LM2795

www.national.com 16

Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

Thin Micro SMD Package

For Ordering, Refer to Ordering Information Table

NS Package Number TLP14

The dimensions for X1, X2, X3 are given as:

X1 = 2.08mm

±

0.03mm

X2 = 2.403mm

±

0.03mm

X3 = 0.600mm

±

0.075mm

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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.

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www.national.com

LM2794/LM2795 Current Regulated Switched Capacitor LED Supply with Analog and PWM

Brightness Control

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.