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LM2794 /LM2795 Current Regulated Switched Capacitor LED Supply with Analog and
PWM Brightness Control
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FEATURES
APPLICATIONS
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1
2
•
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•
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Regulated Current Sources with ±0.5%
Matching between any Two Outputs
High Efficiency 3/2 Boost Function
Drives One, Two, Three or four White LEDs
2.7V to 5.5V Input Voltage
Up to 80mA Output Current
Analog Brightness Control
Active-Low or High Shutdown Input ('94/95)
Very Small Solution Size and no Inductor
2.3µA (typ.) Shutdown Current
325kHz Switching Frequency (min.)
Constant Frequency Generates Predictable
Noise Spectrum
Thin DSBGA Package: 2.08mm X 2.403mm X
0.600mm High
White LED Display Backlights
White LED Keypad Backlights
1-Cell Li-Ion Battery-Operated Equipment
Including PDAs, Hand-Held PCs, Cellular
Phones
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.
Basic Application Circuit
POUT
VIN
CIN
CHOLD
1PF
1PF
C1+
C1
D1
1PF
D2
C1C2+
C2
LED1
LM2794/95
1PF
D3
LED2
C2-
D4
LED3
SD
BRGT
LED4
ISET
GND
RSET
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
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DESCRIPTION (CONTINUED)
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 DSBGA CSP package.
Connection Diagram
C7
A7
E7
D6
B6
E5
A5
E3
A3
D2
E1
B2
A1
C1
Figure 1. DSBGA Package
Bottom View
PIN DESCRIPTION
Pin
(1)
Name
Function
A1
C1+
Positive terminal of C1
B2
C1−
Negative terminal of C1
C1
VIN
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
(1)
C7
ISET
B6
BRGT
Current Sense Input. Connect 1% resistor to ground to set constant current through LED
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 VIN.
A5
C2+
Positive terminal of C2
A3
POUT
Charge pump output
Variable voltage input controls output current
Note that the pin numbering scheme for the DSBGA 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
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
2
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Absolute Maximum Ratings
(1) (2)
VIN
−0.5 to 6.2V max
SD
−0.5 to (VIN+0.3V) w/ 6.2V max
−0.5 to (VIN+0.3V) w/ 6.2V max
BRGT
Continuous Power Dissipation
(3)
Internally Limited
TJMAX
(3)
135°C
θJA (3)
(4)
125°C/W
−65°C to +150°C
Storge Temperature
Lead Temp. (Soldering, 5 sec.)
ESD Rating
260°C
(5)
Human Body Model
2kV
Machine Model
(1)
200V
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.
Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
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.). D1, D2, D3 and D4 may be shorted to GND without damage. POUT may be shorted to GND for 1sec
without damage.
The value of θJA is based on a two layer evaluation board with a dimension of 2in. x1.5in.
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.
(2)
(3)
(4)
(5)
Operating Conditions
Input Voltage (VIN)
2.7V to 5.5V
Ambient Temperature (TA)
−30°C to +85°C
Junction Temperature (TJ)
−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 ≤ TJ ≤ +100°C). Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1 µF, VIN = 3.6V, BRGT
pin = 0V; RSET =124Ω ; LM2794:VSD = VIN (LM2795: VSD = 0V).
Symbol
IDX
Parameter
Available Current at Output Dx
Min
Typ
3.0V ≤ VIN ≤ 5.5V
VDX ≤ 3.8V
BRGT = 50mV
Conditions
15
16.8
2.7V ≤ VIN ≤ 3.0V
VDX ≤ 3.6V
BRGT = 0V
10
VDX ≤ 3.8V
BRGT = 200mV
20
Max
Units
mA
mA
mA
VDX
Available Voltage at Output Dx
3.0V ≤ VIN ≤ 5.5V
IDX ≤ 15mA
BRGT = 50mV
3.8
IDX
Line Regulation of Dx Output
Current
3.0V ≤ VIN ≤ 5.5V
VDX = 3.6V
14.18
15.25
16.78
mA
3.0V ≤ VIN ≤ 4.4V
VDX = 3.6V
14.18
15.25
16.32
mA
14.18
15.25
16.32
mA
V
IDX
Load Regulation of Dx Output
Current
VIN = 3.6V
3.0V ≤ VDX ≤ 3.8V
ID-MATCH
Current Matching Between Any
Two Outputs
VIN = 3.6V, VDX = 3.6V
0.5
IQ
Quiescent Supply Current
3.0V ≤ VIN ≤ 4.2V, Active, No Load
Current
RSET = OPEN
5.5
8.2
mA
ISD
Shutdown Supply Current
3.0V ≤ VIN ≤ 5.5V, Shutdown
2.3
5
µA
%
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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 ≤ TJ ≤ +100°C). Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1 µF, VIN = 3.6V, BRGT
pin = 0V; RSET =124Ω ; LM2794:VSD = VIN (LM2795: VSD = 0V).
Symbol
Parameter
Conditions
IPULL-SD
Shutdown Pull-Up Current
(LM2795)
VCP
Input Charge-Pump Mode To Pass
Mode Threshold
VCPH
Input Charge-Pump Mode To Pass
Mode Hysteresis
VIH
SD Input Logic High (LM2794)
(1)
3.0V ≤ VIN ≤ 5.5V
SD Input Logic High (LM2795)
VIL
SD Input Logic Low (LM2794)
Min
VIN = 3.6V
Typ
SD Input Leakage Current
RBRGT
ISET
fSW
(1)
(2)
4
Units
µA
4.7
V
250
mV
1.0
V
0.8VIN
3.0V ≤ VIN ≤ 5.5V
0.2
SD Input Logic Low (LM2795)
ILEAK-SD
Max
1.5
V
0.2VIN
0V ≤ VSD ≤ VIN
100
nA
BRGT Input Resistance
240
kΩ
ISET Pin Output Current
IDX/10
mA
Switching Frequency
(2)
3.0V ≤ VIN ≤ 4.4V
325
515
675
kHz
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.
The output switches operate at one eigth of the oscillator frequency, fOSC = 1/8fSW.
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Typical Performance Characteristics
Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN = 3.6V, BRGT pin = 0V, RSET = 124Ω.
20
IDIODE
vs
VIN
IDIODE
vs
BRGT
Figure 2.
Figure 3.
IDIODE
vs
VIN
BRGT = 3V
IDIODE
vs
RSET
Figure 4.
Figure 5.
IDIODE
vs
RSET
VBRGT = 0V
IDIODE
vs
VDIODE
18
16
IDX (mA)
14
12
10
8
6
4
2
0
100 300 500 700 900 1100 130015001700 1900 2100
RSET(Ÿ)
Figure 6.
Figure 7.
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Typical Performance Characteristics (continued)
Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN = 3.6V, BRGT pin = 0V, RSET = 124Ω.
VSET
vs
VBRGT
RSET = 1KΩ
Duty Cycle
vs.
Led Current (LM2794)
IDIODE 1- 4 = 15mA
Figure 8.
Figure 9.
Supply Current
vs
VIN
IDIODE 1-4 = 15mA
Supply Current
vs
VIN
IDIODE 1-4 = Open
120
100°C
-30°C
80
25°C
60
40
I
SUPPLY
(mA)
100
20
0
2.7
3.2
3.7
4.2
4.7
5.2
5.7
V IN (V)
Figure 10.
Figure 11.
Shutdown Supply Current
vs
VIN
Shutdown Threshold
vs
VIN
5
SHUTDOWN SUPPLY CURRENT (µA)
25°C
-30°C
4
3
2
1
100°C
0
2.7
3.2
3.7
4.2
4.7
5.2
5.7
V IN (V)
Figure 12.
6
Figure 13.
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Typical Performance Characteristics (continued)
Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN = 3.6V, BRGT pin = 0V, RSET = 124Ω.
Start-Up Response @ VIN = 2.7V (LM2794)
Start-Up Response @ VIN = 2.7V (LM2795)
Figure 14.
Figure 15.
Start-Up Response @ VIN = 3.6V (LM2794)
Start-Up Response @ VIN = 3.6V (LM2795)
Figure 16.
Figure 17.
Start-Up Response @ VIN = 4.2V (LM2794)
Start-Up Response @ VIN = 4.2V (LM2795)
Figure 18.
Figure 19.
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Typical Performance Characteristics (continued)
Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN = 3.6V, BRGT pin = 0V, RSET = 124Ω.
8
Available Additional Current @ POUT
IDIODE 1− 4 = 15mA, RSET = 124 Ω
Switching Frequency
Figure 20.
Figure 21.
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FUNCTIONAL BLOCK DIAGRAM
C1
C2
1 …F
1 …F
LM2794/95
VIN
2.7 - 5.5V
CIN
1 …F
POUT
(3/2)x
Charge Pump
*
*(Only on
LM2795)
CPOUT
10PA
1 …F
SD
Brightness
Control
Voltage
Reference
330 k:
BRGT
130 k:
110 k:
GND
RSET
D1
D2
D3
D4
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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
(POUT). 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 POUT voltage 1.5 times the input voltage. The charge
pump operates in Pass Mode, providing a voltage on POUT 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 D1-D4.
To set the LED drive current, the device uses a resistor connected to the ISET 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 ISET AND BRGT PINS).
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 VIN of the LM2795. This allows the use of open-drain logic control of the LM2795
shutdown, as shown in Figure 22. The LM2795 SD pin can also be driven with a rail-to-rail CMOS logic signal.
LM2795
VIN
*
Shutdown
Control
SD
*Only on
LM2795
10PA
Figure 22. Open-Drain Logic Shutdown Control
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.
10
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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
ISET AND BRGT PINS
An external resistor, RSET, is connected to the ISET 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 RSET.
The current mirror circuitry matches the current through each LED to within 0.5%.
In addition to RSET, a voltage may be applied to the VBRGT pin to vary the LED current. By adjusting current with
the Brightness pin (BRGT), the brightness of the LEDs can be smoothly varied.
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 VBRGT 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 RSET. An equation for approximating
the LED current is:
§ VOFFSET + VBRGT * 0. 385
¨
R SET
©
I LED = ¨
§ 0.188 + V BRGT * 0. 385
¨
R SET
©
I LED = ¨
·
¸¸ * MirrorRati o
¹
· 10
Amps
¸¸ *
¹ 1
ILED 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.
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2. Determine the maximum desired ILED current. Use the ILED equation to calculate RSET by setting BRGT to 0V
or use Table 3 to select a value for RSET 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 (TON = 100µs) of the device.
D × (1/f) > TON
fMAX = 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 ILED being half of the programmed
LED current. For example, if RSET is set to program 15mA, a 50% duty cycle will result in an average ILED 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 ILED being half the programmed LED current.
Brightness Control Using BRGT
1. Choose the maximum ILED 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 RSET required or use the ILED equation above to calculate RSET.
3. Use Table 4 as a reference for the dimming profile of the LEDs, when BRGT ranges from 0V to 3V.
Table 3. RSET 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
Table 4. LED Current
RSET 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 (POUT)
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 POUT pin) is nominally 1.5 × VIN. This rail can be used to deliver additional current to
other circuitry. Figure 23 shows how to connect additional LEDs to POUT. 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 POUT are recommended for functions where brightness matching is not critical, such as
keypad backlighting.
Since POUT is unregulated, driving LEDs directly off POUT 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 POUT pin and the
LEDs is recommended. Texas Instruments LP3985-4.5V low-dropout linear regulator is a good choice for such
an application.
12
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The voltage at POUT is dependent on the input voltage supplied to the LM2794/5, the total LM2794/5 output
current, and the output resistance (ROUT) 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 worstcase design calculations, using an output resistance of 3.5Ω is recommended. (Worst-case recommendation
accounts for parameter shifts from part-to-part variation and applies over the full operating temperature range).
C1
1 …F
3.0V
VIN
C1-
C2
1 …F
C1+ C2-
C2+
(3/2) VIN = approx. 4.5V
Keypad LEDs
POUT
CIN
1 …F
CPOUT
1 …F
LM2794/95
DK1
DK2
DKX
5mA
BRGT
RSET
D1
D2
D3
GND
D4
*Optional
Independent
Shutdown
15mA each
= 60 mA
Total
124:
Figure 23. Keypad LEDs Connected to POUT
Output resistance results in droop in the POUT voltage proportional to the amount of current delivered by the
pump. The POUT 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 DX output currents plus the current delivered through the POUT pin,
the voltage at POUT can be predicted with the following equations:
ITOTAL = ID1 + ID2 + ID3 + ID4 + IPOUT
VPOUT = 1.5 × VIN − ITOTAL × ROUT
(1)
(2)
LED HEADROOM VOLTAGE (VHR)
Four current sources are connected internally between POUT and D1-D4. The voltage across each current source,
(VPOUT − 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:
VHR-MIN = kHR × IDX
(3)
The parameter kHR, typically 20mV/mA in the LM2794/5, is a proportionality constant that represents the ONresistance of the internal current mirror transistors. For worst-case design calculations, using a kHR 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 24 shows how output current of the LM2794/5 varies
with respect to headroom voltage.
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18
16
14
R SET = 124Ω
I DX (mA)
12
10
8
R SET = 475Ω
6
4
R SET = 2.67kΩ
2
0
0.05
0.20
0.35
0.50
0.65
0.80
VHR (V)
Figure 24. ILED vs VHR
4 LEDs, VIN = 3.0V
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 VPOUT and VHR equations from the previous two sections yields the basic inequality for
determining the validity of an LM2794/5 LED-drive application:
VPOUT = 1.5 × VIN − ITOTAL × ROUT
VHR-MIN = kHR × IDX
VPOUT − VDX ≥ VHR-MIN
1.5 × VIN − ITOTAL × ROUT − VDX ≥ (kHR × IDX)
(4)
(5)
(6)
(7)
Rearranging this inequality shows the estimated total output current capability of an application:
ITOTAL ≤ [(1.5 × VIN-MIN) − VDX-MAX − (kHR × IDX)] ÷ ROUT
(8)
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 kHR constant of 20mV/mA. For worst-case design calculations,
use kHR = 25mV/mA. (Worst-case recommendations account for parameter shifts from part-to-part variation and
apply over the full operating temperature range). ROUT and kHR increase 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 D1 through D4 may be connected together in any combination to drive higher currents through fewer
LEDs. For example in Figure 25, outputs D1 and D2 are connected together to drive one LED while D3 and D4
are connected together to drive a second LED.
14
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C2
1 …F
C1
1 …F
3.0V
VIN
C1-
C1+ C2-
C2+
POUT
CPOUT
1 …F
CIN
1 …F
LM2794/95
BRGT
RSET
D1
GND
D4
D2 D3
15mA
124:
30mA
Figure 25. Two Parallel Connected LEDs
With this configuration, two parallel current sources of equal value provide current to each LED. RSET and VBRGT
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 , RSET and VBRGT 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 26.
C2
1 …F
C1
1 …F
3.0V
VIN
C1-
C1+ C2-
C2+
POUT
CPOUT
1 …F
CIN
1 …F
LM2794/95
BRGT
RSET
D1
D2
D3
GND
D4
15mA
124:
60mA
Figure 26. One Parallel Connected LED
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.
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POWER EFFICIENCY
Figure 27 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 (PLED) to the power
drawn from the input source (PIN). In the equations below, IQ is the quiescent current of the LM2794/5, ILED is the
current flowing through one LED, VLED 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.
PLED = N × VLED × ILED
PIN = VIN × IIN
PIN = VIN × (1.5 × N × ILED + IQ)
E = (PLED ÷ PIN)
(9)
(10)
(11)
(12)
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 (VIN x IIN) be evaluated rather than power efficiency. Figure 28
shows the power consumption of the LM2794/5 Typical Application Circuit.
Figure 27. Efficiency vs VIN
4 LEDs, VLED = 3.6V, ILED = 15mA
450
430
410
PIN (mW)
390
370
350
330
310
290
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
V IN (V)
Figure 28. PIN vs VIN
4 LEDs, 2.5 ≤ VDX ≤ 3.9V, IDX = 15mA
16
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POWER DISSIPATION
The power dissipation (PDISSIPATION) and junction temperature (TJ) can be approximated with the equations
below. PIN is the power generated by the 1.5x charge pump, PLED is the power consumed by the LEDs, PPOUT is
the power provided through the POUT pin, TAis the ambient temperature, and θJA is the junction-to-ambient
thermal resistance for the DSBGA package. VIN is the input voltage to the LM2794/5, VDX is the LED forward
voltage, IDX is the programmed LED current, and IPOUT is the current drawn through POUT.
PDISSIPATION = PIN - PLED − PPOUT
= [1.5×VIN×(4IDX + IPOUT)] − (VDX×4IDX) − (1.5×VIN×IPOUT)
TJ = TA + (PDISSIPATION × θJA)
(13)
(14)
(15)
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.
DSBGA MOUNTING
The LM2794/5 is a 14-bump DSBGA with a bump size of 300 micron diameter. The DSBGA package requires
specific mounting techniques detailed in Application Note (AN -1112 SNVA009). 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 DSBGA device. DSBGA is a wafer level chip size package, which means
the dimensions of the package are equal to the die size. As such, the DSBGA 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.
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REVISION HISTORY
Changes from Revision K (May 2013) to Revision L
•
18
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 17
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM2794TL/NOPB
ACTIVE
DSBGA
YPA
14
250
RoHS & Green
SNAGCU
Level-1-260C-UNLIM
-30 to 85
LOG
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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