LTM8040
1
8040fa
TYPICAL APPLICATION
FEATURES
APPLICATIONS
DESCRIPTION
36V, 1A µModule
LED Driver and Current Source
The LTM
®
8040 is a fi xed frequency 1A step-down DC/DC
μModule
®
regulator designed to operate as a constant cur-
rent source. Internal circuitry monitors the output current
to provide accurate current regulation, which is ideal for
driving high current LEDs. High output current accuracy
is maintained over a wide current range, from 35mA to
1A, allowing a wide dimming range over an output volt-
age range of 2.5V to 13V. Unique PWM circuitry allows a
dimming range of 250:1, avoiding the color shift normally
associated with LED current dimming.
With its wide input range of 4V to 36V, the LTM8040 regu-
lates a broad array of power sources, from 4-cell batteries
and 5V logic rails to unregulated wall transformers, lead
acid batteries and distributed power supplies.
The LTM8040 is packaged in a thermally enhanced,
compact (15mm × 9mm × 4.32mm) molded land grid
array (LGA) package suitable for automated assembly
by standard surface mount equipment. The LTM8040 is
Pb-free and RoHS compliant.
n True Color PWM™ Delivers Constant Color with
250:1 Dimming Ratio
n Wide Input Range: 4V to 36V
n Up to 1A LED Current
n Adjustable Control of LED Current
n High Output Current Accuracy is Maintained Over a
Wide Range from 35mA to 1A
n Open LED and Short-Circuit Protection
n Grounded LED Cathode Connection
n Small Footprint (15mm × 9mm × 4.32mm)
Surface Mount LGA Package
n Automotive and Avionic Lighting
n Architectural Detail Lighting
n Display Backlighting
n Constant Current Sources
GND
LTM8040
8040 TA01
VIN
SHDN
VIN*
11.5V TO 36V
1μF
215k
650kHz
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
LEDA
LPWR
TWO WHITE LEDs
6V TO 9V
1A
ADJ
PWM
RT
BIAS
Effi ciency1A μModule LED Driver
OUTPUT CURRENT (mA)
0
0
EFFICIENCY (%)
10
30
40
50
70
200 400
8040 TA01b
20
80
100
90
60
600 800 1000
VIN = 12V
TWO 3.3V AT 1A LEDs
OUTPUT CURRENT ADJUSTED
WITH ADJ VOLTAGE
L, LT, LTC, LTM, μModule, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. True Color PWM is a trademark of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
LTM8040
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ABSOLUTE MAXIMUM RATINGS
VIN ............................................................................36V
BIAS .......................................................................... 25V
BIAS + VIN ................................................................. 51V
LEDA, LPWR .............................................................15V
PWM .........................................................................10V
ADJ .............................................................................6V
RT ...............................................................................3V
SHDN ........................................................................36V
SHDN Above VIN .........................................................6V
Internal Operating Temperature (Note 2) ...–40°C to 125°C
Max Solder Temperature ....................................... 245°C
(Note 1)
PIN CONFIGURATION
LEAD FREE FINISH TRAY PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE (NOTE 3)
LTM8040EV#PBF LTM8040EV#PBF LTM8040V 66-Lead 15mm × 9mm × 4.32mm LGA Package 0°C to 125°C
LTM8040IV#PBF LTM8040IV#PBF LTM8040V 66-Lead 15mm × 9mm × 4.32mm LGA Package –40°C to 125°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based fi nish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
This product is only offered in trays. For more information go to: http://www.linear.com/packaging/
ELECTRICAL CHARACTERISTICS
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage l3.5 4 V
Input Quiescent Current Not Switching 2.6 4 mA
SHDN Current SHDN = 0.3V
SHDN = 2.65V
0.01
10.3
2μA
μA
LEDA Current ADJ Open
RADJ = 5.11k
l
l
0.98
0.965
0.49
0.481
1
0.5
1.02
1.035
0.51
0.525
A
A
A
A
ADJ Bias Current ADJ = 0V, Current Flows Out of Pin 245 μA
ADJ Pull-up Resistor 5 5.11 5.22 kΩ
Switching Frequency RT Open 470 500 530 kHz
SHDN Threshold 2.65 V
PWM Threshold VIH
VIL
1.2
0.4
V
V
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. VIN = 12V, SHDN = 5V, VPWM = 5V, unless otherwise noted (Note 3).
ORDER INFORMATION
VIN
BANK 3
LEDA
BANK 1
GND
BANK 2
PWM
SHDN
ADJ
RT
BIAS
GND
LPWR
TJMAX = 125°C, θJA = 13°C/W, θJC = 4.4°C/W, WEIGHT = 1.73g
66 LEAD (15mm × 9mm × 4.32mm)
θ
JA DERIVED FROM 6.35cm × 6.35cm 4 LAYER PCB
LTM8040
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ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: This μModule includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specifi ed maximum operating junction
temperature may impair device reliability.
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. VIN = 12V, SHDN = 5V, VPWM = 5V, unless otherwise noted (Note 3).
PARAMETER CONDITIONS MIN TYP MAX UNITS
LEDA Clamp Voltage 13.2 14.5 V
Minimum BIAS Voltage 2.0 2.6 V
Note 3: The LTM8040E is guaranteed to meet performance specifi cations
from 0°C to 125°C internal. Specifi cations over the full –40°C to
125°C internal operating temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTM8040I is guaranteed to meet specifi cations over the full –40°C to 125°C
internal operating temperature range. Note that the maximum internal
temperature is determined by specifi c operating conditions in conjunction
with board layout, the rated package thermal resistance and other
environmental factors.
TYPICAL PERFORMANCE CHARACTERISTICS
Effi ciency–Single 3.3V at 1A LED Effi ciency–Two 3.3V at 1A LEDs Effi ciency–Three 3.3V at 1A LEDs
Effi ciency–Single 2.7V at 1A LED Effi ciency–Three 2.7V at 1A LEDs Effi ciency–Four 2.7V at 1A LEDs
OUTPUT CURRENT (mA)
0
50
EFFICIENCY (%)
55
65
70
75
85
200 400
8040 G01
60
80
600 800 1000
24 VIN
12 VIN
5 VIN
OUTPUT CURRENT (mA)
0
0
EFFICIENCY (%)
10
30
40
50
70
200 400
8040 G02
20
80
100
90
60
600 800 1000
24 VIN
12 VIN
OUTPUT CURRENT (mA)
0
0
EFFICIENCY (%)
10
30
40
50
70
200 400
8040 G03
20
80
100
90
60
600 800 1000
24 VIN
12 VIN
OUTPUT CURRENT (mA)
0
0
EFFICIENCY (%)
10
30
40
50
70
200 400
8040 G05
20
80
100
90
60
600 800 1000
24 VIN
12 VIN
OUTPUT CURRENT (mA)
0
0
EFFICIENCY (%)
10
30
40
50
70
200 400
8040 G04
20
80
90
60
600 800 1000
24 VIN
12 VIN
5 VIN
OUTPUT CURRENT (mA)
0
0
EFFICIENCY (%)
10
30
40
50
70
200 400
8040 G06
20
80
100
90
60
600 800 1000
24 VIN
12 VIN
(TA = 25°C, confi gured per Table 2 unless
otherwise noted)
LTM8040
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TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Start Voltage vs
Output Current (3.3V at 1A LEDs)
SHDN Current vs Voltage
BIAS Current vs Output Current
12VIN, (Single 2.7V at 1A LED)
BIAS Current vs Output Current
24VIN, (Three 3.3V at 1A LEDs)
Temperature Rise vs Output Current
(2.7V at 1A LEDs, 12VIN)
Minimum Running Voltage vs
Output Voltage
Minimum Start Voltage vs
Output Current (2.7V at 1A LEDs)
Input Current vs Input Voltage
Output Short Circuited
OUTPUT VOLTAGE (V)
0
2
INPUT VOLTAGE (V)
4
8
10
12
42
8040 G08
6
14
86 1210
1A LOAD
0.5A LOAD
0.1A LOAD
OUTPUT CURRENT (mA)
0
0
2
INPUT VOLTAGE (V)
4
8
10
12
400200
8040 G09
6
14
800600 1000
SINGLE LED
2 LEDs
3 LEDs
4 LEDs
OUTPUT CURRENT (mA)
0
0
2
INPUT VOLTAGE (V)
4
8
10
12
400200
8040 G10
6
14
800600 1000
SINGLE LED
2 LEDs
3 LEDs
SHUTDOWN PIN VOLTAGE (V)
0
0
SHUTDOWN CURRENT (μA)
10
105
8040 G11
70
60
50
40
30
20
2015 25 30 35 40
OUTPUT CURRENT (mA)
0
0
BIAS CURRENT (mA)
2
6
8
10
400200
8040 G12
4
12
800600 1000
VBIAS = 5V
VBIAS = 3.2V
VBIAS = 12V
OUTPUT CURRENT (mA)
0
0
BIAS CURRENT (mA)
2
6
8
10
12
14
16
18
400200
8040 G13
4
20
800600 1000
VBIAS = 5V
VBIAS = 3.2V
VBIAS = 12V
INPUT VOLTAGE (V)
0
0
INPUT CURRENT (mA)
20
105
8040 G14
100
80
60
40
2015 25 30 35 40
OUTPUT CURRENT (mA)
0
2
TEMPERATURE RISE (°C)
12
10
8
400200
8040 G15
22
20
18
16
14
6
4
800600 1000
SINGLE LED
2 LEDs
3 LEDs
4 LEDs
(TA = 25°C, confi gured per Table 2 unless
otherwise noted)
LTM8040
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TYPICAL PERFORMANCE CHARACTERISTICS
Temperature Rise vs Output Current
(3.3V at 1A LEDs, 36VIN)Output Current vs ADJ Voltage
Temperature Rise vs Output Current
(2.7V at 1A LEDs, 36VIN)
Temperature Rise vs Output Current
(3.3V at 1A LEDs, 12VIN)
OUTPUT CURRENT (mA)
0
2
TEMPERATURE RISE (°C)
7
400200
8040 G16
27
22
17
12
800600 1000
SINGLE LED
2 LEDs
3 LEDs
4 LEDs
OUTPUT CURRENT (mA)
0
2
TEMPERATURE RISE (°C)
7
400200
8040 G18
22
27
17
12
800600 1000
SINGLE LED
2 LEDs
3 LEDs
OUTPUT CURRENT (mA)
0
0
TEMPERATURE RISE (°C)
5
400200
8040 G17
25
20
15
10
800600 1000
SINGLE LED
2 LEDs
3 LEDs
ADJ VOLTAGE (mV)
0
0
OUTPUT CURRENT (mA)
200
400200
8040 G19
1200
1000
800
600
400
800600 1000 1200
(TA = 25°C, confi gured per Table 2 unless
otherwise noted)
LTM8040
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PIN FUNCTIONS
LEDA (Bank 1): This pin is the regulated current source
of the LTM8040. Connect the anode of the LED string to
this pin. This voltage must be at least 2.5V for accurate
current regulation.
SHDN (Pin L4): The SHDN pin is used to shut down the
switching regulator and the internal bias circuits. The
2.65V switching threshold can function as an accurate
undervoltage lockout. Pull below 0.3V to shut down the
LTM8040. Pull above 2.65V to enable the LTM8040. Tie
to VIN if the SHDN function is unused.
BIAS (Pin L5): The BIAS pin connects through an internal
Schottky diode to provide power to internal housekeeping
circuits. Connect to a voltage source (usually LPWR or VIN)
greater than 2.6V. Note that this pin is adjacent to the LPWR
pin to ease layout. If this pin is powered by an external
power source, a decoupling cap may be necessary.
LPWR (Pin K5): This is the output of the buck regulator
that sources the LED current. If the LEDA voltage is above
2.6V, connect this pin to BIAS. It is pinned out primarily
for the convenience of the user. If it is not used, leave this
pin fl oating. Please refer to the Applications Information
section for details.
PWM (Pin L7): Input Pin for PWM Dimming Control. A
PWM signal above 1.2V (ON threshold) turns on the output
current source, while a PWM signal below 0.4V shuts it
down. If the application does not require PWM dimming,
then the PWM pin can be left either open (an internal 10μA
source current pulls PWM high) or pulled up to a voltage
source between 1.2V and 10V.
VIN (Bank 3): The VIN pin supplies current to the LTM8040’s
internal power converter and other circuitry. It must be lo-
cally bypassed with a high quality (low ESR) capacitor.
ADJ (Pin L3): Use the ADJ pin to scale the LEDA output
current below 1A by either applying a voltage source or by
connecting a resistor to GND. This pin is internally pulled
up to a 1.25V reference through a 5.11k resistor, so ensure
that the voltage source can drive a 5.11k impedance. If
applying a voltage to ADJ, the LEDA current follows the
formula: ILED = 1A • ADJ/1.25V. If connecting a resistor to
GND, the resistor value should be R = 5.11 • ILED /(1Amp
– ILED), where R is in kΩ and ILED is the desired current
out of LEDA in amps. Make sure that the voltage at LEDA
is at least 2.5V.
RT (Pin L2): The RT pin is used to set the internal oscillator
frequency. An 80.6k resistor has already been installed
inside the LTM8040 to default switching frequency to
500kHz. If no modifi cation of the switching frequency is
necessary, leave this pin fl oating. Otherwise, a parallel
resistor applied from RT to GND will raise the switching
frequency. See Table 1 for details.
GND (Bank 2, Pin L1): Tie all GND pins directly to a local
ground plane. These pins serve as both signal and power
return to the LTM8040 μModule, as well as providing
the primary thermal path for heat dissipation within the
unit. See the Applications Information section for more
information about heat sinking and printed circuit board
layout.
LTM8040
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BLOCK DIAGRAM
VIN
SHDN
PWM
GND RT ADJ
80.6k
5.11k
INTERNAL
1.25V
0.1μF 4.7μF
8.2μH SENSE
RESISTOR
LPWR
LEDA
BIAS
INTERNAL
COMPENSATION
CURRENT
MODE
CONTROLLER
8040 BD
LTM8040
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OPERATION
The LTM8040 is a constant frequency, current mode
converter capable of generating a constant 1A output
intended to drive LEDs or other applications where a
constant current is required.
Operation can be best understood by referring to the Block
Diagram. The power stage is a step down converter that
regulates the output current by reading the voltage across
a power sense resistor that is in series with the output.
If the SHDN pin is tied to ground, the LTM8040 is shut
down and draws minimal current from the input source
tied to VIN. If the SHDN pin exceeds 1.5V, the internal bias
circuits turn on, including the internal regulator, reference,
and oscillator. When the SHDN pin exceeds 2.65V, the
switching regulator will begin to operate.
There are two means to dim a LED with the LTM8040.
The fi rst is to adjust the current on the LEDA output via
a voltage on the ADJ pin. The ADJ pin is internally pulled
up to a precision 1.25V reference through a 1% 5.11k
resistor. Leaving the ADJ pin fl oating sets the LEDA pin
current to 1A. Reducing the voltage below 1.25V on the
ADJ pin proportionally reduces the current fl owing out of
LEDA. This can be accomplished by connecting a resistor
from the ADJ pin to GND, forming a divider network with
the internal 5.11k resistor. LEDA pin current can also be
programmed by tying the ADJ pin directly to a voltage
source. For proper operation, make sure that LEDA is at
least 2.5V at the desired operating point.
The other means by which the LTM8040 can dim a LED
is with pulse width modulation using the PWM pin and
an optional external NFET. If the PWM pin is unconnected
or pulled high, the part operates at its nominal rating. If
the PWM pin is pulled low, the LTM8040 stops switch-
ing and the internal control circuitry is held in its present
state. Circuitry drawing current from the LPWR pin is also
disabled. This way, the LTM8040 “remembers” the current
sourced from the LEDA output until PWM is pulled high
again. This leads to a highly linear relationship between
pulse width and output light, allowing for a large and ac-
curate dimming range.
The RT pin allows programming of the switching fre-
quency. The LTM8040 is shipped with 80.6k on this pin
to GND, yielding a default switching frequency of 500kHz.
For applications requiring a faster switching frequency,
apply another resistor in parallel, from RT to GND. Refer to
Table 1 for the frequencies that correspond to the applied
external resistor values.
An external voltage is required at the BIAS pin to power
internal circuitry. For proper operation, BIAS must be at
least 2.6V. For many applications, BIAS should be tied to
LPWR; if LPWR is below 2.6V, BIAS may be tied to VIN
or some other voltage source.
The switching regulator performs frequency foldback dur-
ing overload conditions. Frequency foldback helps limit
internal power and thermal stresses.
The LTM8040 is equipped with thermal protection that
reduces the output LED current if the internal operating
temperature is too high. The junction temperature will
exceed the 125°C temperature rating of the LTM8040 when
the thermal protection is active, so continuous operation
under this operating condition may impair reliability.
LTM8040
9
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For most applications, the design process is straight
forward, summarized as follows:
1. Look at Table 2 and fi nd the row that has the desired
input voltage range LED string voltage range and output
current.
2. Apply the recommended CIN, RT and RADJ values.
3. Connect BIAS as indicated.
4. Connect LEDA to the anode of the LED string.
5. Connect the remaining pins as needed by the system
requirements.
While these component combinations have been tested
for proper operation, it is incumbent upon the user to
verify proper operation over the intended system’s line,
load and environmental conditions.
Open LED Protection
The LTM8040 has internal open LED circuit protection. If
the LED is absent or fails open, the LTM8040 clamps the
voltage on the LEDA pin to 14V. The switching regulator
then skips cycles to limit the input current.
Undervoltage Lockout
U n d e r v o l t a g e l o c k o u t ( U V LO ) i s t y p i c a l l y u s e d i n s i t u a t i o n s
where the input supply is current limited, or has high source
resistance. A switching regulator draws constant power
from the source, so the source current increases as the
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current
limit or latch low under low source voltage conditions.
UVLO prevents the regulator from operating at source volt-
ages where this might occur. An internal comparator will
force the part to stop switching when VIN falls below 3.5V.
An adjustable UVLO threshold is also realized through the
SHDN pin, as the internal comparator that performs this
function has a 2.65V threshold. If SHDN is below 0.3V all
internal circuitry is off. An internal resistor pulls 10.3μA
to ground from the SHDN pin at the UVLO threshold in
order to provide hysteresis.
Choose resistors according to the following formula:
R2=2.65V
VUVLO −2.65V
R1 – 10.3µ
A
where VUVLO is the desired UVLO threshold
Suppose that the output needs to stay off until the input
is above 8V.
V
UVLO = 8V
Let R1 = 100k
R2 =2.65V
8V −2.65V
100k – 10.3µA
=61.9k
APPLICATION INFORMATION
Figure 1. Undervoltage Lockout
GND
LTM8040
8040 F01
VIN
VIN
C1
R1
R2
SHDN
Keep the connections from the resistors to the SHDN pin
short. If high resistance values are used, the SHDN pin
should be bypassed with a 1nF capacitor to prevent cou-
pling problems from switching nodes.
Setting the Switching Frequency
The LTM8040 uses a constant frequency architecture that
can be programmed over a 500kHz to 2MHz range with a
single external timing resistor from the RT pin to ground.
The current that fl ows into the timing resistor is used to
charge an internal oscillator capacitor. The LTM8040 is
confi gured such that the default frequency is 500kHz
without adding a resistor. Many applications use this value.
LTM8040
10
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If another frequency is desired, Table 1 shows suggested
RT selections for a variety of switching frequencies.
Table 1. RT vs Frequency
RT (kΩ) FREQUENCY (MHz)
13 2.00
16 1.84
18.7 1.70
24.9 1.50
29.4 1.37
35.7 1.25
54.9 1.00
75 0.90
88.7 0.85
137 0.75
174 0.68
215 0.65
487 0.57
OPEN 0.50
GND
3.3V
WHITE
LED
LTM8040
8040 F02a
VIN
SHDN
ADJ
PWM
RT
VIN
4V TO 36V
C1
2.2μF
LEDA
LPWR
BIAS
Figure 2a. Tie BIAS to LPWR if LPWR is Greater Than 2.6V
GND
2.5V
RED
LED
LTM8040
8040 F02b
VIN
SHDN
ADJ
PWM
RT
VIN
5.5V
C1
2.2μF
LEDA
LPWR
BIAS
GND
3.3V
2.5V
1μF
RED
LED
LTM8040
8040 F02c
VIN
SHDN
ADJ
PWM
RT
VIN
4V TO 36V
C1
2.2μF
LEDA
LPWR
BIAS
Figure 2b. BIAS May be Tied to VIN if LPWR is Below 2.6V
Figure 2c. Tie BIAS to an External Power Source if Neither VIN
Nor LPWR are Suitable
APPLICATION INFORMATION
BIAS Pin Considerations
For proper operation, the BIAS pin must be powered by
at least 2.6V. Figure 2 shows three ways to arrange the
circuit. For outputs of 2.6V or higher, the standard circuit
(Figure 2a) is recommended. For output voltages below
2.6V, the BIAS pin can be tied to the input (Figure 2b) at
the cost of some effi ciency. Finally, the BIAS pin can be
tied to another source that is at least 2.6V (Figure 2c).
For example, if a 3.3V source is on whenever the LED is
on, the BIAS pin can be connected to the 3.3V output. In
all cases, be sure that the maximum voltage at the BIAS
pin is both less than 25V and the sum of VIN and BIAS is
less than 51V. If BIAS is powered by a source other than
LPWR, a local decoupling capacitor may be necessary.
The value of the decoupling capacitor is dependent upon
the source and PCB layout.
LTM8040
11
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Programming LED Current
The LED current can be set by adjusting the voltage on the
ADJ pin. The ADJ pin is internally pulled up to a precision
1.25V voltage source through a 5.11k 1% resistor. This
resistor makes it easy to adjust the LED current with a
single external resistor. For a 1A LED current, leave the
ADJ pin fl oating. For lower output currents, apply a re-
sistor from ADJ to GND as shown in Figure 3, using the
following formula:
R
ADJ = 5.11k • ILED /(1A – ILED),
where ILED is the desired current out of LEDA.
In order to have accurate LED current, a precision 1%
resistor is recommended.
Dimming Control
There are several different types of dimming control
circuits. Analog dimming control (Figure 4) changes the
voltage on the ADJ pin by tying a low on-resistance FET to
the resistor. This allows the selection of two different LED
currents. For reliable operation, program an LED current of
no less than 35mA. The maximum current dimming ratio
(IRATIO) can be calculated from the maximum LED current
(IMAX) and the minimum LED current (IMIN) as follows:
IMAX
IMIN
=IRATIO
Figure 4. Dimming with an NFET and Resistor
GND
LTM8040
8040 F04
R2
ADJ
DIM
APPLICATION INFORMATION
The LEDA voltage must be at least 2.5V for proper current
regulation. See Table 2 for recommended RADJ values.
Figure 3. Setting ADJ with a Resistor
GND
LTM8040
8040 F03
RADJ
ADJ
LTM8040
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Example: IMAX = 1A, IMIN = 0.1A, tMAX = 10ms,
tMIN = 40μs
IRATIO =1A
0.1A
=10:1
PWMRATIO =10ms
40µs
=250:1
DIMRATIO =10 • 250 =2500:1
Minimum Input Voltage
The LTM8040 is a step-down converter, so a minimum
amount of headroom is required to keep the output in
regulation. For most applications at full load, the input
needs to be at least 1.5V above the desired output. In
addition, it takes more input voltage to initially start than
is required for continuous operation. This start voltage is
also dependent on whether turn-on is controlled by the
LTM8040’s SHDN pin or UVLO (that is, the SHDN pin is
tied to VIN). See Typical Performance Characteristics for
details.
PWM dimming control (Figure 5) uses the PWM pin and
an ex ternal NFET tied to the cathode of the LED. When the
PWM signal goes low, the NFET turns off, disconnecting
the LED from the internal current source and “freezing”
the state of LTM8040 internal control and drive circuitry,
but leaving the output capacitor of the internal step-down
converter charged. When the PWM pin goes high again,
the LED current returns rapidly to its previous on state.
This fast settling time allows the LTM8040 to maintain
LED current regulation with PWM pulse widths as short
as 40μs. It is also possible to not use an external NFET,
but the output capacitor of the internal regulator will be
discharged by the LED(s) and have to be charged up again
when the current source turns back on. This will lengthen
the minimum dimming pulse width.
The maximum PWM dimming ratio (PWMRATIO) can
be calculated from the maximum PWM period (tMAX)
and minimum PWM pulse width (tMIN) as follows:
tMAX
tMIN
=PWMRATIO
Total dimming ratio (DIMRATIO) is the product of the PWM
dimming ratio and the current dimming ratio.
APPLICATION INFORMATION
Figure 5. Dimming Using PWM Signal
GND
LTM8040
8040 F05
PWM
PWM
60Hz TO
10kHz
LEDA
LTM8040
13
8040fa
APPLICATION INFORMATION
Capacitor Selection Considerations
The CIN capacitor values in Table 2 are the minimum rec-
ommended values for the associated operating conditions.
Applying capacitor values below those indicated in Table 2
is not recommended, and may result in undesirable
operation. Using larger values is generally acceptable, and
can yield improved per formance, if it is necessar y. Again,
it is incumbent upon the user to verify proper operation
over the intended system’s line, load and environmental
conditions.
Ceramic capacitors are small, robust and have very low
ESR. However, not all ceramic capacitors are suitable. X5R
and X7R types are stable over temperature and applied
voltage and give dependable service. Other types, includ-
ing Y5V and Z5U have very large temperature and voltage
coeffi cients of capacitance. In an application circuit they
may have only a small fraction of their nominal capacitance
resulting in much higher voltage ripple than expected.
A fi nal precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8040. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit. If
the LTM8040 circuit is plugged into a live supply, the input
voltage can ring to twice its nominal value, possibly ex-
ceeding the device’s rating. This situation is easily avoided
by introducing a small series damping resistance into the
circuit. This is most often taken care of by the presence
of an electrolytic bulk capacitor in the board.
High Temperature Considerations
The internal operating temperature of the LTM8040 must
be lower than the 125°C rating, so care should be taken
in the layout of the circuit to ensure good heat sinking of
the LTM8040. To estimate the junction temperature, ap-
proximate the power dissipation within the LTM8040 by
applying the typical effi ciency stated in this data sheet to
the desired output power, or, if you have an actual module,
by taking a power measurement. Then calculate the tem-
perature rise of the LTM8040 junction above the surface
of the printed circuit board by multiplying the module’s
power dissipation by the thermal resistance θJA. The actual
thermal resistance of the LTM8040 to the printed circuit
board depends on the layout of the circuit board, but the
thermal resistance given in the Pin Confi guration, which
is based upon a 40.3cm2 4 layer FR4 PC board, can be
used as a guide.
The LTM8040 is equipped with thermal protection that
reduces the output LED current if the internal operating
temperature is too high. This thermal protection is active
above the 125°C temperature rating of the LTM8040, so
continuous operation under this operating condition may
impair reliability.
LTM8040
14
8040fa
APPLICATION INFORMATION
Figure 6. Suggested Layout
LEDA
LED
STRING
GND
VIAS TO GND PLANE
SHDN
LPWR BIAS
ADJ
RT
PWM
VIN
CIN
8040 F06
Layout Hints
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8040. The LTM8040 is neverthe-
less a switching power supply, and care must be taken to
minimize EMI and ensure proper operation. Even with the
high level of integration, you may fail to achieve specifi ed
operation with a haphazard or poor layout. See Figure 6
for a suggested layout. Ensure that the grounding and heat
sinking are acceptable. A few rules to keep in mind are:
1. Place the CIN capacitor as close as possible to the VIN
and GND connection of the LTM8040.
2. Connect all of the GND connections to as large a cop-
per pour or plane area as possible on the top layer.
Avoid breaking the ground connection between the
external components and the LTM8040.
3. Use vias to connect the GND copper area to the board’s
internal ground plane. Liberally distribute these GND
vias to provide both a good ground connection and
thermal path to the internal planes of the printed circuit
board.
4. If the application requires BIAS to be connected to the
input voltage potential, tie BIAS to VIN, but be careful
not to break up the ground plane.
LTM8040
15
8040fa
APPLICATION INFORMATION
Table 2. Recommended Confi guration
VIN RANGE CIN
LED STRING
VOLTAGE
(LEDA)
LED STRING
CURRENT
(LEDA) RADJ RT(OPTIMAL) fOPTIMAL RT(MIN) fMAX BIAS CONNECTION
4.5V to 36V 1μF 0805 50V 2.5V to 4V 35mA 154 Open 0.50M Open 0.50M 2.6V to 25V Source
6.5V to 36V 1μF 0805 50V 4V to 6V 35mA 154 Open 0.50M Open 0.50M LPWR
9.5V to 36V 1μF 0805 50V 6V to 9V 35mA 154 Open 0.50M Open 0.50M LPWR
12.5V to 36V 1μF 0805 50V 8V to 12V 35mA 154 Open 0.50M Open 0.50M LPWR
4.5V to 36V 1μF 0805 50V 2.5V to 4V 100mA 453 Open 0.50M Open 0.50M 2.6V to 25V Source
6.5V to 36V 1μF 0805 50V 4V to 6V 100mA 453 Open 0.50M 165k 0.70M LPWR
9.5V to 36V 1μF 0805 50V 6V to 9V 100mA 453 487k 0.57M 137k 0.75M LPWR
12.5V to 36V 1μF 0805 50V 8V to 12V 100mA 453 487k 0.57M 88.7k 0.85M LPWR
4.8V to 36V 1μF 0805 50V 2.5V to 4V 350mA 2.87k Open 0.50M Open 0.50M 2.6V to 25V Source
7V to 36V 1μF 0805 50V 4V to 6V 350mA 2.87k Open 0.50M 165k 0.70M LPWR
10.5V to 36V 1μF 0805 50V 6V to 9V 350mA 2.87k 137k 0.75M 54.9k 1.0M LPWR
13.8V to 36V 1μF 0805 50V 8V to 12V 350mA 2.87k 75k 0.90M 29.4k 1.37M LPWR
4.8V to 36V 1μF 0805 50V 2.5V to 4V 500mA 5.11k Open 0.50M Open 0.50M 2.6V to 25V Source
7V to 36V 1μF 0805 50V 4V to 6V 500mA 5.11k Open 0.50M 165k 0.70M LPWR
10.5V to 36V 1μF 0805 50V 6V to 9V 500mA 5.11k 137k 0.75M 54.9k 1.0M LPWR
14.3V to 36V 1μF 0805 50V 8V to 12V 500mA 5.11k 75k 0.90M 29.4k 1.37M LPWR
5V to 36V 1μF 0805 50V 2.5V to 4V 700mA 11.8k Open 0.50M Open 0.50M 2.6V to 25V Source
7.7V to 36V 1μF 0805 50V 4V to 6V 700mA 11.8k 487k 0.57M 165k 0.70M LPWR
11V to 36V 1μF 0805 50V 6V to 9V 700mA 11.8k 165k 0.70M 54.9k 1.0M LPWR
14.8V to 36V 1μF 0805 50V 8V to 12V 700mA 11.8k 75k 0.90M 29.4k 1.37M LPWR
5.5V to 36V 1μF 0805 50V 2.5V to 4V 1A Open Open 0.50M Open 0.50M 2.6V to 25V Source
8V to 36V 1μF 0805 50V 4V to 6V 1A Open Open 0.50M 137k 0.75M LPWR
11.5V to 36V 1μF 0805 50V 6V to 9V 1A Open 215k 0.65M 54.9k 1.0M LPWR
15.5V to 36V 1μF 0805 50V 8V to 12V 1A Open 137k 0.75M 29.4k 1.37M LPWR
Note: Input bulk capacitor assumed.
Do not allow VIN + BIAS to exceed the Absolute Maximum Rating of 51V.
LTM8040
16
8040fa
TYPICAL APPLICATIONS
Step Down 350mA Drive with Three Series Red LEDs
GND
LTM8040
8040 TA03
VIN*
10.5V TO 36V
1μF
2.87k
137k
750kHz
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
LEDA
LPWR
BIAS
6V TO 9V
350mA
VIN
SHDN
ADJ
PWM
RT
Step Down 1A Drive with Single Red or White LED
GND
LTM8040
8040 TA02
VIN*
5.5V TO 24V
1μF
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
LEDA
LPWR
2.5V TO 4V
1A
VIN
SHDN
BIAS
ADJ
PWM
RT
LTM8040
17
8040fa
TYPICAL APPLICATIONS
Step Down 1A Drive with Four Series Red LEDs
GND
LTM8040
8040 TA05
VIN*
15.5V TO 36V
1μF
137k
750kHz
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
LEDA
LPWR
BIAS
8V TO 12V
1A
VIN
SHDN
ADJ
PWM
RT
Step Down 1A Drive with Three White Series LEDs
GND
LTM8040
8040 TA04
VIN*
15.5V TO 36V
1μF
137k
750kHz
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
LEDA
LPWR
BIAS
8V TO 12V
1A
VIN
SHDN
ADJ
PWM
RT
LTM8040
18
8040fa
PACKAGE DESCRIPTION
NOTES:
1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994
2. ALL DIMENSIONS ARE IN MILLIMETERS
LAND DESIGNATION PER JESD MO-222, SPP-010 AND SPP-020
5. PRIMARY DATUM -Z- IS SEATING PLANE
6. THE TOTAL NUMBER OF PADS: 66
4
3
DETAILS OF PAD #1 IDENTIFIER ARE OPTIONAL,
BUT MUST BE LOCATED WITHIN THE ZONE INDICATED.
THE PAD #1 IDENTIFIER MAY BE EITHER A MOLD OR A
MARKED FEATURE
SYMBOL
aaa
bbb
eee
TOLERANCE
0.15
0.10
0.05
9.000
BSC
PACKAGE TOP VIEW
LGA 66 0407 REV A
15.000
BSC
4
PAD 1
CORNER
3
PADS
SEE NOTES
XY
aaa Z
aaa Z
1.270
BSC
0.605 – 0.665
0.605 – 0.665
12.700
BSC
7.620
BSC
PAD 1
C (0.30)
HBADC
6
7
5
1
2
3
4
EF
PACKAGE BOTTOM VIEW
PACKAGE IN TRAY LOADING ORIENTATION
GLJK
2.540
2.540
1.270
5.080
5.080
6.350
6.350
3.810
3.810
0.000
1.270
3.810
3.810
2.540
2.540
1.270
1.270
0.000
1.5875
0.9525
SUGGESTED PCB LAYOUT
TOP VIEW
LTMXXXXXX
μModule
TRAY PIN 1
BEVEL
COMPONENT
PIN 1
0.9525
1.5875
4.22 – 4.42
DETAIL B
DETAIL B
SUBSTRATE
MOLD
CAP
0.27 – 0.37
3.95 – 4.05
Z
DETAIL A
DETAIL A
0.635 p0.025 SQ. 66x
SYXeee
bbb Z
LGA Package
66-Lead (15mm × 9mm × 4.32mm)
(Reference LTC DWG # 05-08-1810 Rev A)
LTM8040
19
8040fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represent a-
t i o n t h a t t h e i n t e r c o n n e c t i o n o f i t s c i r c u i t s a s d e s c r i b e d h e r e i n w i l l n o t i n f r i n g e o n e x i s t i n g p a t e n t r i g h t s .
PACKAGE DESCRIPTION
PIN NAME PIN NAME PIN NAME PIN NAME PIN NAME PIN NAME
A1 GND B1 GND C1 GND D1 GND E1 GND F1 GND
A2 GND B2 GND C2 GND D2 GND E2 GND F2 GND
A3 GND B3 GND C3 GND D3 GND E3 GND F3 GND
A4 GND B4 GND C4 GND D4 GND E4 GND F4 GND
A5 LEDA B5 LEDA C5 LEDA D5 LEDA E5 LEDA F5 LEDA
A6 LEDA B6 LEDA C6 LEDA D6 LEDA E6 LEDA F6 LEDA
A7 LEDA B7 LEDA C7 LEDA D7 GND E7 GND F7 GND
PIN NAME PIN NAME PIN NAME PIN NAME PIN NAME
G1 - H1 VIN J1 VIN K1 - L1 GND
G2 - H2 VIN J2 VIN K2 - L2 RT
G3 - H3 - J3 VIN K3 - L3 ADJ
G4 - H4 - J4 - K4 - L4 SHDN
G5 LEDA H5 LEDA J5 GND K5 LPWR L5 BIAS
G6 LEDA H6 LEDA J6 GND K6 GND L6 GND
G7 GND H7 GND J7 GND K7 GND L7 PWM
Pin Assignment Table
(Arranged by Pin Number)
LTM8040
20
8040fa
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2009
LT 0809 REV A • PRINTED IN USA
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PART NUMBER DESCRIPTION COMMENTS
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25V, 15mm × 15mm × 2.8mm
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LTM8022 1A, 36V DC/DC μModule Adjustable Frequency, 0.8V ≤ VOUT ≤ 5V, 11.25mm × 9mm × 2.82mm,
Pin Compatible to the LTM8023
LTM8023 2A, 36V DC/DC μModule Adjustable Frequency, 0.8V ≤ VOUT ≤ 5V, 11.25mm × 9mm × 2.82mm,
Pin Compatible to the LTM8022
PolyPhase is a registered trademark of Linear Technology Corporation
PACKAGE PHOTOGRAPH
