LTM8040EV#PBF [Linear]
LTM8040 - 36V, 1A µModule (Power Module) LED Driver and Current Source; Package: LGA; Pins: 66; Temperature Range: -40°C to 85°C;
| 型号： | LTM8040EV#PBF |
| 厂家： | 
Linear |
| 描述： | LTM8040 - 36V, 1A µModule (Power Module) LED Driver and Current Source; Package: LGA; Pins: 66; Temperature Range: -40°C to 85°C 驱动 接口集成电路 |
| 文件： | 总20页 (文件大小：205K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTM8040
36V, 1A µModule
LED Driver and Current Source
FEATURES
DESCRIPTION
The LTM®8040 is a fixed frequency 1A step-down DC/DC
μModule® regulatordesignedtooperateasaconstantcur-
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
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
Adjustable Control of LED Current
n
n
High Output Current Accuracy is Maintained Over a
Wide Range from 35mA to 1A
Open LED and Short-Circuit Protection
Grounded LED Cathode Connection
Small Footprint (15mm × 9mm × 4.32mm)
Surface MountLGA Package
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.
n
n
n
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.
APPLICATIONS
n
Automotive and Avionic Lighting
n
Architectural Detail Lighting
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
Display Backlighting
Constant Current Sources
n
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.
TYPICAL APPLICATION
1A μModule LED Driver
Efficiency
V
*
IN
100
90
80
70
60
50
40
30
V
LEDA
IN
11.5V TO 36V
SHDN
LPWR
1μF
LTM8040
TWO WHITE LEDs
6V TO 9V
1A
ADJ
PWM
RT
BIAS
GND
215k
650kHz
8040 TA01
V
= 12V
IN
20
10
0
TWO 3.3V AT 1A LEDs
OUTPUT CURRENT ADJUSTED
WITH ADJ VOLTAGE
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
0
200
400
600
800
1000
OUTPUT CURRENT (mA)
8040 TA01b
8040fa
1
LTM8040
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
V ............................................................................36V
IN
PWM
BIAS..........................................................................25V
LEDA
BANK 1
BIAS + V .................................................................51V
IN
BIAS
LEDA, LPWR.............................................................15V
PWM.........................................................................10V
ADJ.............................................................................6V
RT ...............................................................................3V
SHDN........................................................................36V
SHDN
ADJ
RT
GND
BANK 2
GND
LPWR
V
IN
SHDN Above V .........................................................6V
BANK 3
IN
Internal Operating Temperature (Note 2)...–40°C to 125°C
Max Solder Temperature....................................... 245°C
T
JMAX
= 125°C, θ = 13°C/W, θ = 4.4°C/W, WEIGHT = 1.73g
66 LEAD (15mmJ×C 9mm × 4.32mm)
JA DERIVED FROM 6.35cm × 6.35cm 4 LAYER PCB
JA
θ
ORDER INFORMATION
LEAD FREE FINISH
LTM8040EV#PBF
LTM8040IV#PBF
TRAY
PART MARKING* PACKAGE DESCRIPTION
TEMPERATURE RANGE (NOTE 3)
LTM8040EV#PBF
LTM8040IV#PBF
LTM8040V
LTM8040V
0°C to 125°C
66-Lead 15mm × 9mm × 4.32mm LGA Package
66-Lead 15mm × 9mm × 4.32mm LGA Package
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish 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 The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, SHDN = 5V, VPWM = 5V, unless otherwise noted (Note 3).
PARAMETER
CONDITIONS
MIN
TYP
3.5
2.6
MAX
UNITS
V
l
Minimum Input Voltage
Input Quiescent Current
SHDN Current
4
4
2
Not Switching
mA
SHDN = 0.3V
SHDN = 2.65V
0.01
10.3
μA
μA
LEDA Current
ADJ Open
0.98
0.965
0.49
1
1.02
1.035
0.51
A
A
A
A
l
l
R
= 5.11k
0.5
ADJ
0.481
0.525
ADJ Bias Current
ADJ Pull-up Resistor
Switching Frequency
SHDN Threshold
PWM Threshold
ADJ = 0V, Current Flows Out of Pin
RT Open
245
5.11
500
μA
kΩ
kHz
V
5
5.22
530
470
2.65
V
V
1.2
V
V
IH
IL
0.4
8040fa
2
LTM8040
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, SHDN = 5V, VPWM = 5V, unless otherwise noted (Note 3).
PARAMETER
CONDITIONS
MIN
TYP
MAX
14.5
2.6
UNITS
LEDA Clamp Voltage
Minimum BIAS Voltage
13.2
V
V
2.0
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 specified maximum operating junction
temperature may impair device reliability.
Note 3: The LTM8040E is guaranteed to meet performance specifications
from 0°C to 125°C internal. Specifications 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 specifications over the full –40°C to 125°C
internal operating temperature range. Note that the maximum internal
temperature is determined by specific operating conditions in conjunction
with board layout, the rated package thermal resistance and other
environmental factors.
(TA = 25°C, configured per Table 2 unless
TYPICAL PERFORMANCE CHARACTERISTICS
otherwise noted)
Efficiency–Single 2.7V at 1A LED
Efficiency–Three 2.7V at 1A LEDs
Efficiency–Four 2.7V at 1A LEDs
100
90
80
70
60
50
40
30
20
10
0
85
80
75
70
65
60
55
50
100
90
80
70
60
50
40
30
20
10
0
24 V
12 V
IN
IN
IN
24 V
IN
12 V
IN
24 V
IN
12 V
IN
5 V
0
200
400
600
800
1000
400
800
1000
0
200
600
0
200
400
600
800
1000
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
8040 G01
8040 G02
8040 G03
Efficiency–Single 3.3V at 1A LED
Efficiency–Two 3.3V at 1A LEDs
Efficiency–Three 3.3V at 1A LEDs
100
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
24 V
12 V
IN
IN
IN
24 V
IN
12 V
IN
24 V
IN
12 V
IN
5 V
0
200
400
600
800
1000
0
200
400
600
800
1000
0
200
400
600
800
1000
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
8040 G05
8040 G04
8040 G06
8040fa
3
LTM8040
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, configured per Table 2 unless
Minimum Start Voltage vs
otherwise noted)
Minimum Running Voltage vs
Output Voltage
Minimum Start Voltage vs
Output Current (2.7V at 1A LEDs)
Output Current (3.3V at 1A LEDs)
14
12
10
8
14
12
10
8
14
12
10
8
SINGLE LED
2 LEDs
3 LEDs
SINGLE LED
2 LEDs
3 LEDs
4 LEDs
6
6
6
4
4
1A LOAD
0.5A LOAD
0.1A LOAD
4
2
2
0
0
2
0
200
400
600
800
1000
0
200
400
600
800
1000
0
2
4
6
8
10
12
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
8040 G10
8040 G09
8040 G08
BIAS Current vs Output Current
24VIN, (Three 3.3V at 1A LEDs)
BIAS Current vs Output Current
12VIN, (Single 2.7V at 1A LED)
SHDN Current vs Voltage
20
18
16
14
12
10
8
70
60
50
40
30
20
10
0
12
10
8
V
V
V
= 5V
= 3.2V
= 12V
V
V
V
= 5V
= 3.2V
= 12V
BIAS
BIAS
BIAS
BIAS
BIAS
BIAS
6
4
6
4
2
2
0
0
0
200
400
600
800
1000
0
5
10 15 20 25 30 35 40
SHUTDOWN PIN VOLTAGE (V)
0
200
400
600
800
1000
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
8040 G13
8040 G11
8040 G12
Input Current vs Input Voltage
Output Short Circuited
Temperature Rise vs Output Current
(2.7V at 1A LEDs, 12VIN)
22
100
80
60
40
20
0
SINGLE LED
2 LEDs
3 LEDs
4 LEDs
20
18
16
14
12
10
8
6
4
2
0
5
10 15 20 25 30 35 40
INPUT VOLTAGE (V)
0
200
400
600
800
1000
OUTPUT CURRENT (mA)
8040 G14
8040 G15
8040fa
4
LTM8040
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C, configured per Table 2 unless
otherwise noted)
Temperature Rise vs Output Current
(2.7V at 1A LEDs, 36VIN)
Temperature Rise vs Output Current
(3.3V at 1A LEDs, 12VIN)
27
22
17
12
7
25
20
15
10
5
SINGLE LED
2 LEDs
3 LEDs
SINGLE LED
2 LEDs
3 LEDs
4 LEDs
0
2
0
200
400
600
800
1000
0
200
400
600
800
1000
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
8040 G17
8040 G16
Temperature Rise vs Output Current
(3.3V at 1A LEDs, 36VIN)
Output Current vs ADJ Voltage
27
22
17
12
7
1200
1000
SINGLE LED
2 LEDs
3 LEDs
800
600
400
200
0
2
0
200
400
600
800
1000
0
200
400
600 800 1000 1200
OUTPUT CURRENT (mA)
ADJ VOLTAGE (mV)
8040 G18
8040 G19
8040fa
5
LTM8040
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.
V (Bank3):TheV pinsuppliescurrenttotheLTM8040’s
IN
IN
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
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 V if the SHDN function is unused.
formula: I = 1A • ADJ/1.25V. If connecting a resistor to
IN
LED
GND, the resistor value should be R = 5.11 • I
– I ), where R is in kΩ and I
out of LEDA in amps. Make sure that the voltage at LEDA
is at least 2.5V.
/(1Amp
LED
BIAS (Pin L5): The BIAS pin connects through an internal
is the desired current
LED
LED
Schottky diode to provide power to internal housekeeping
circuits.Connecttoavoltagesource(usuallyLPWRorV )
IN
greaterthan2.6V.NotethatthispinisadjacenttotheLPWR
pin to ease layout. If this pin is powered by an external
power source, a decoupling cap may be necessary.
RT(PinL2):TheRTpinisusedtosettheinternaloscillator
frequency. An 80.6k resistor has already been installed
inside the LTM8040 to default switching frequency to
500kHz. If no modification of the switching frequency is
necessary, leave this pin floating. Otherwise, a parallel
resistor applied from RT to GND will raise the switching
frequency. See Table 1 for details.
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 floating. Please refer to the Applications Information
section 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.
PWM (Pin L7): Input Pin for PWM Dimming Control. A
PWMsignalabove1.2V(ONthreshold)turnsontheoutput
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.
8040fa
6
LTM8040
BLOCK DIAGRAM
SENSE
RESISTOR
8.2μH
V
LEDA
IN
0.1μF
4.7μF
LPWR
BIAS
SHDN
CURRENT
MODE
CONTROLLER
INTERNAL
COMPENSATION
PWM
5.11k
INTERNAL
1.25V
80.6k
8040 BD
GND
RT
ADJ
8040fa
7
LTM8040
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.
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.Thisway,theLTM8040“remembers”thecurrent
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.
OperationcanbebestunderstoodbyreferringtotheBlock
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
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,
applyanotherresistorinparallel, fromRTtoGND. Referto
Table 1 for the frequencies that correspond to the applied
external resistor values.
down and draws minimal current from the input source
tied to V . If the SHDN pin exceeds 1.5V, the internal bias
IN
circuitsturnon,includingtheinternalregulator,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 first 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 floating sets the LEDA pin
current to 1A. Reducing the voltage below 1.25V on the
ADJ pin proportionally reduces the current flowing 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.
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 V
or some other voltage source.
IN
Theswitchingregulatorperformsfrequencyfoldbackdur-
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
exceedthe125°CtemperatureratingoftheLTM8040when
the thermal protection is active, so continuous operation
under this operating condition may impair reliability.
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
8040fa
8
LTM8040
APPLICATION INFORMATION
For most applications, the design process is straight
forward, summarized as follows:
Choose resistors according to the following formula:
2.65V
R2=
1. Look at Table 2 and find the row that has the desired
inputvoltagerangeLEDstringvoltagerangeandoutput
current.
VUVLO −2.65V
–10.3µA
R1
where V
is the desired UVLO threshold
2. Apply the recommended C , R and R values.
ADJ
UVLO
IN
T
Suppose that the output needs to stay off until the input
is above 8V.
3. Connect BIAS as indicated.
4. Connect LEDA to the anode of the LED string.
V
= 8V
UVLO
5. Connect the remaining pins as needed by the system
requirements.
Let R1 = 100k
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.
2.65V
R2=
= 61.9k
8V − 2.65V
–10.3µA
100k
Open LED Protection
V
V
IN
IN
R1
R2
LTM8040
SHDN
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.
C1
GND
8040 F01
Undervoltage Lockout
Figure 1. Undervoltage Lockout
Under voltage lockout (UVLO) is t ypically used in situations
wheretheinputsupplyiscurrentlimited,orhashighsource
resistance. A switching regulator draws constant power
from the source, so the source current increases as the
sourcevoltagedrops. Thislookslikeanegativeresistance
load to the source and can cause the source to current
limit or latch low under low source voltage conditions.
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 flows into the timing resistor is used to
charge an internal oscillator capacitor. The LTM8040 is
configured such that the default frequency is 500kHz
withoutaddingaresistor.Manyapplicationsusethisvalue.
UVLOpreventstheregulatorfromoperatingatsourcevolt-
ages where this might occur. An internal comparator will
force the part to stop switching when V falls below 3.5V.
IN
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.
8040fa
9
LTM8040
APPLICATION INFORMATION
If another frequency is desired, Table 1 shows suggested
T
BIAS Pin Considerations
R selections for a variety of switching frequencies.
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 efficiency. 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
Table 1. RT vs Frequency
R (kΩ)
T
FREQUENCY (MHz)
13
16
2.00
1.84
1.70
1.50
1.37
1.25
1.00
0.90
0.85
0.75
0.68
0.65
0.57
0.50
18.7
24.9
29.4
35.7
54.9
75
pin is both less than 25V and the sum of V and BIAS is
IN
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.
88.7
137
174
215
487
OPEN
LTM8040
LTM8040
V
V
5.5V
IN
IN
3.3V
V
LEDA
LPWR
BIAS
2.5V
V
LEDA
LPWR
BIAS
IN
IN
4V TO 36V
C1
2.2μF
SHDN
ADJ
C1
2.2μF
SHDN
ADJ
WHITE
LED
RED
LED
PWM
RT
PWM
RT
GND
GND
8040 F02a
8040 F02b
Figure 2a. Tie BIAS to LPWR if LPWR is Greater Than 2.6V
Figure 2b. BIAS May be Tied to VIN if LPWR is Below 2.6V
LTM8040
V
IN
2.5V
V
LEDA
LPWR
BIAS
IN
4V TO 36V
C1
2.2μF
SHDN
ADJ
3.3V
1μF
PWM
RT
RED
LED
GND
8040 F02c
Figure 2c. Tie BIAS to an External Power Source if Neither VIN
Nor LPWR are Suitable
8040fa
10
LTM8040
APPLICATION INFORMATION
Programming LED Current
Dimming Control
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 floating. For lower output currents, apply a re-
sistor from ADJ to GND as shown in Figure 3, using the
following formula:
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.Forreliableoperation,programanLEDcurrentof
no less than 35mA. The maximum current dimming ratio
(I
(I
) can be calculated from the maximum LED current
RATIO
) and the minimum LED current (I ) as follows:
MAX
MIN
R
= 5.11k • I
/(1A – I ),
ADJ
LED LED
IMAX
IMIN
=IRATIO
where I
is the desired current out of LEDA.
LED
In order to have accurate LED current, a precision 1%
resistor is recommended.
LTM8040
ADJ
GND
LTM8040
ADJ
R2
8040 F04
DIM
GND
R
ADJ
8040 F03
Figure 4. Dimming with an NFET and Resistor
Figure 3. Setting ADJ with a Resistor
The LEDA voltage must be at least 2.5V for proper current
regulation. See Table 2 for recommended R
values.
ADJ
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11
LTM8040
APPLICATION INFORMATION
PWM dimming control (Figure 5) uses the PWM pin and
an external 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.
Example: I
MIN
= 1A, I
= 0.1A, t
= 10ms,
MAX
MAX
MIN
t
= 40μs
1A
IRATIO
=
=10:1
0.1A
10ms
PWMRATIO
=
=250:1
40µs
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
The maximum PWM dimming ratio (PWM
) can
MAX
RATIO
be calculated from the maximum PWM period (t
)
and minimum PWM pulse width (t ) as follows:
MIN
tMAX
tied to V ). See Typical Performance Characteristics for
IN
=PWMRATIO
tMIN
details.
Total dimming ratio (DIM
) is the product of the PWM
RATIO
dimming ratio and the current dimming ratio.
PWM
60Hz TO
10kHz
PWM
LTM8040
LEDA
GND
8040 F05
Figure 5. Dimming Using PWM Signal
8040fa
12
LTM8040
APPLICATION INFORMATION
Capacitor Selection Considerations
circuit. This is most often taken care of by the presence
of an electrolytic bulk capacitor in the board.
The C capacitor values in Table 2 are the minimum rec-
IN
ommendedvaluesfortheassociatedoperatingconditions.
ApplyingcapacitorvaluesbelowthoseindicatedinTable2
is not recommended, and may result in undesirable
operation.Usinglargervaluesisgenerallyacceptable,and
can yield improved performance, if it is necessary. Again,
it is incumbent upon the user to verify proper operation
over the intended system’s line, load and environmental
conditions.
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 efficiency 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
Ceramic capacitors are small, robust and have very low
ESR.However,notallceramiccapacitorsaresuitable.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
coefficients of capacitance. In an application circuit they
mayhaveonlyasmallfractionoftheirnominalcapacitance
resulting in much higher voltage ripple than expected.
powerdissipationbythethermalresistanceθ .Theactual
JA
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 Configuration, which
2
is based upon a 40.3cm 4 layer FR4 PC board, can be
A final 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-
ceedingthedevice’srating. Thissituationiseasilyavoided
by introducing a small series damping resistance into the
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.
8040fa
13
LTM8040
APPLICATION INFORMATION
Layout Hints
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.
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 specified
operation with a haphazard or poor layout. See Figure 6
forasuggestedlayout. Ensurethatthegroundingandheat
sinking are acceptable. A few rules to keep in mind are:
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
1. Place the C capacitor as close as possible to the V
input voltage potential, tie BIAS to V , but be careful
IN
IN
IN
and GND connection of the LTM8040.
not to break up the ground plane.
LEDA
PWM
LPWR BIAS
LED
STRING
SHDN
ADJ
RT
V
IN
GND
8040 F06
C
IN
VIAS TO GND PLANE
Figure 6. Suggested Layout
8040fa
14
LTM8040
APPLICATION INFORMATION
Table 2. Recommended Configuration
LED STRING LED STRING
VOLTAGE
(LEDA)
CURRENT
(LEDA)
V
RANGE
C
R
R
f
R
f
MAX
BIAS CONNECTION
2.6V to 25V Source
LPWR
IN
IN
ADJ
T(OPTIMAL)
OPTIMAL
T(MIN)
4.5V to 36V
6.5V to 36V
9.5V to 36V
12.5V to 36V
4.5V to 36V
6.5V to 36V
9.5V to 36V
12.5V to 36V
4.8V to 36V
7V to 36V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
1μF 0805 50V
2.5V to 4V
4V to 6V
6V to 9V
8V to 12V
2.5V to 4V
4V to 6V
6V to 9V
8V to 12V
2.5V to 4V
4V to 6V
6V to 9V
8V to 12V
2.5V to 4V
4V to 6V
6V to 9V
8V to 12V
2.5V to 4V
4V to 6V
6V to 9V
8V to 12V
2.5V to 4V
4V to 6V
6V to 9V
8V to 12V
35mA
35mA
35mA
35mA
100mA
100mA
100mA
100mA
350mA
350mA
350mA
350mA
500mA
500mA
500mA
500mA
700mA
700mA
700mA
700mA
1A
154
154
Open
Open
Open
Open
Open
Open
487k
487k
Open
Open
137k
75k
0.50M
Open
Open
Open
Open
Open
165k
137k
88.7k
Open
165k
54.9k
29.4k
Open
165k
54.9k
29.4k
Open
165k
54.9k
29.4k
Open
137k
54.9k
29.4k
0.50M
0.50M
0.50M
0.50M
0.50M
0.70M
0.75M
0.85M
0.50M
0.70M
1.0M
0.50M
0.50M
0.50M
0.50M
0.50M
0.57M
0.57M
0.50M
0.50M
0.75M
0.90M
0.50M
0.50M
0.75M
0.90M
0.50M
0.57M
0.70M
0.90M
0.50M
0.50M
0.65M
0.75M
154
LPWR
154
LPWR
453
2.6V to 25V Source
LPWR
453
453
LPWR
453
LPWR
2.87k
2.87k
2.87k
2.87k
5.11k
5.11k
5.11k
5.11k
11.8k
11.8k
11.8k
11.8k
Open
Open
Open
Open
2.6V to 25V Source
LPWR
10.5V to 36V
13.8V to 36V
4.8V to 36V
7V to 36V
LPWR
1.37M
0.50M
0.70M
1.0M
LPWR
Open
Open
137k
75k
2.6V to 25V Source
LPWR
10.5V to 36V
14.3V to 36V
5V to 36V
LPWR
1.37M
0.50M
0.70M
1.0M
LPWR
Open
487k
165k
75k
2.6V to 25V Source
LPWR
7.7V to 36V
11V to 36V
14.8V to 36V
5.5V to 36V
8V to 36V
LPWR
1.37M
0.50M
0.75M
1.0M
LPWR
Open
Open
215k
137k
2.6V to 25V Source
LPWR
1A
11.5V to 36V
15.5V to 36V
1A
LPWR
1A
1.37M
LPWR
Note: Input bulk capacitor assumed.
Do not allow V + BIAS to exceed the Absolute Maximum Rating of 51V.
IN
8040fa
15
LTM8040
TYPICAL APPLICATIONS
Step Down 1A Drive with Single Red or White LED
LTM8040
V
*
IN
V
LEDA
IN
5.5V TO 24V
SHDN
BIAS
ADJ
LPWR
1μF
2.5V TO 4V
1A
PWM
RT
GND
8040 TA02
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
Step Down 350mA Drive with Three Series Red LEDs
LTM8040
V
*
IN
V
LEDA
LPWR
BIAS
IN
10.5V TO 36V
SHDN
6V TO 9V
350mA
1μF
2.87k
ADJ
PWM
RT
GND
137k
750kHz
8040 TA03
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
8040fa
16
LTM8040
TYPICAL APPLICATIONS
Step Down 1A Drive with Three White Series LEDs
LTM8040
V
*
IN
V
LEDA
LPWR
BIAS
IN
15.5V TO 36V
SHDN
8V TO 12V
1A
1μF
ADJ
PWM
RT
GND
137k
750kHz
8040 TA04
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
Step Down 1A Drive with Four Series Red LEDs
LTM8040
V
*
IN
V
LEDA
LPWR
BIAS
IN
15.5V TO 36V
SHDN
8V TO 12V
1A
1μF
ADJ
PWM
RT
GND
137k
750kHz
8040 TA05
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
8040fa
17
LTM8040
PACKAGE DESCRIPTION
Z
b b b
Z
6 . 3 5 0
5 . 0 8 0
3 . 8 1 0
2 . 5 4 0
1 . 5 8 7 5
0 . 9 5 2 5
1 . 2 7 0
0 . 0 0 0
1 . 2 7 0
2 . 5 4 0
3 . 8 1 0
5 . 0 8 0
6 . 3 5 0
a a a
Z
8040fa
18
LTM8040
PACKAGE DESCRIPTION
Pin Assignment Table
(Arranged by Pin Number)
PIN NAME
A1 GND
PIN NAME
PIN NAME
C1 GND
PIN NAME
D1 GND
PIN NAME
E1 GND
PIN NAME
F1 GND
B1 GND
B2 GND
B3 GND
B4 GND
B5 LEDA
B6 LEDA
B7 LEDA
A2 GND
A3 GND
A4 GND
A5 LEDA
A6 LEDA
A7 LEDA
C2 GND
C3 GND
C4 GND
C5 LEDA
C6 LEDA
C7 LEDA
D2 GND
D3 GND
D4 GND
D5 LEDA
D6 LEDA
D7 GND
E2 GND
E3 GND
E4 GND
E5 LEDA
E6 LEDA
E7 GND
F2 GND
F3 GND
F4 GND
F5 LEDA
F6 LEDA
F7 GND
PIN NAME
PIN NAME
PIN NAME
PIN NAME
PIN NAME
L1 GND
L2 RT
G1
G2
G3
G4
-
-
-
-
H1
H2
H3
H4
V
V
-
J1
J2
J3
J4
V
IN
V
IN
V
IN
-
K1
K2
K3
K4
-
-
-
-
IN
IN
L3 ADJ
L4 SHDN
L5 BIAS
L6 GND
L7 PWM
-
G5 LEDA
G6 LEDA
G7 GND
H5 LEDA
H6 LEDA
H7 GND
J5 GND
J6 GND
J7 GND
K5 LPWR
K6 GND
K7 GND
8040fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However,noresponsibilityisassumedforitsuse.LinearTechnologyCorporationmakesnorepresenta-
t ion t h a t t he in ter c onne c t ion o f i t s cir cui t s a s de s cr ib e d her ein w ill no t in fr inge on ex is t ing p a ten t r igh t s.
19
LTM8040
PACKAGE PHOTOGRAPH
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTM4600
10A DC/DC μModule
Basic 10A DC/DC μModule, 15mm × 15mm × 2.8mm LGA
–55°C to 125°C Operation, 15mm × 15mm × 2.8mm LGA
LTM4600HVMPV
Military Plastic 10A DC/DC μModule
LTM4601/
LTM4601A
12A DC/DC μModule with PLL, Output Tracking/Margining Synchronizable, PolyPhase® Operation, LTM4601-1 Version Has No
and Remote Sensing
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LTM4602
LTM4603
6A DC/DC μModule
Pin Compatible with the LTM4600
6A DC/DC μModule with PLL and Output Tracking/
Margining and Remote Sensing
Synchronizable, PolyPhase Operation, LTM4603-1 Version Has No
Remote Sensing, Pin Compatible with the LTM4601
LTM4604
LTM4605
4A Low V DC/DC μModule
2.375V ≤ V ≤ 5V, 0.8V ≤ V
≤ 5V, 9mm × 15mm × 2.3mm LGA
IN
IN
OUT
5A to 12A Buck-Boost μModule
High Efficiency, Adjustable Frequency, 4.5V ≤ V ≤ 20V, 0.8V ≤ V
16V, 15mm × 15mm × 2.8mm
≤
≤
IN
OUT
LTM4607
5A to 12A Buck-Boost μModule
High Efficiency, Adjustable Frequency, 4.5V ≤ V ≤ 36V, 0.8V ≤ V
25V, 15mm × 15mm × 2.8mm
IN
OUT
LTM4608
LTM8020
LTM8022
8A Low V DC/DC μModule
2.375V ≤ V ≤ 5V, 0.8V ≤ V
≤ 5V, 9mm × 15mm × 2.8mm LGA
IN
IN
OUT
36V, 200mA DC/DC μModule
1A, 36V DC/DC μModule
4V ≤ V ≤ 36V, 1.25V ≤ V
≤ 5V, 6.25mm × 6.25mm × 2.3mm LGA
OUT
IN
Adjustable Frequency, 0.8V ≤ V
Pin Compatible to the LTM8023
≤ 5V, 11.25mm × 9mm × 2.82mm,
OUT
LTM8023
2A, 36V DC/DC μModule
Adjustable Frequency, 0.8V ≤ V
≤ 5V, 11.25mm × 9mm × 2.82mm,
OUT
Pin Compatible to the LTM8022
PolyPhase is a registered trademark of Linear Technology Corporation
8040fa
LT 0809 REV A • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
20
●
●
© LINEAR TECHNOLOGY CORPORATION 2009
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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