LTM8040EV-PBF [Linear]
36V, 1A μModule LED Driver; 36V ， 1A的μModule LED驱动器
| 型号： | LTM8040EV-PBF |
| 厂家： | 
Linear |
| 描述： | 36V, 1A μModule LED Driver |
| 文件： | 总16页 (文件大小：212K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Electrical Specifications Subject to Change
LTM8040
36V, 1A µModule LED
Driver
FEATURES
DESCRIPTION
The LTM®8040 is a fixed frequency 1A step-down DC/DC
μModule™ designed to operate as a constant current
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 2.4V to 13V. Unique PWM circuitry allows a
dimming range of 400:1, avoiding the color shift normally
associated with LED current dimming.
n
True Color PWM™ Delivers Constant Color with
400:1 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
n
Open LED and Short-Circuit Protection
n
Grounded Cathode Connection
n
SmallFootprint, LowProfile(15mm ×9mm × 2.82mm)
Surface MountLGA Package
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
The LTM8040 is packaged in a thermally enhanced, com-
pact (15mm × 9mm) and low profile (2.82mm) molded
Land Grid Array (LGA) package suitable for automated
assembly by standard surface mount equipment. The
LTM8040 is Pb-free and RoHS compliant.
Architectural Detail Lighting
n
Display Backlighting
Constant Current Sources
n
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
μModule is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
TYPICAL APPLICATION
1A LED Driver μModule
Efficiency
100
90
80
70
60
50
40
30
20
V
*
IN
V
LEDA
IN
4V 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
10
0
3.3V AT 1A LEDs
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION
FOR START-UP REQUIREMENTS
0
200
400
600
800
1000
OUTPUT CURRENT (mA)
8040 TA01b
8040p
1
LTM8040
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
V ............................................................................36V
IN
PWM
BIAS..........................................................................25V
LEDA
BANK 1
BIAS + V .................................................................51V
IN
BIAS
LEDA.........................................................................15V
PWM.........................................................................10V
ADJ.............................................................................6V
SHDN........................................................................36V
SHDN
ADJ
RT
GND
BANK 2
GND
SHDN Above V .........................................................6V
IN
LPWR
V
IN
BIAS Current...............................................................1A
Internal Operating Temperature (Note 2)....–40 to 125°C
Storage Temperature Range.......................–45 to 125°C
BANK 3
T
= 125°C, θ = 250°C/W, WEIGHT = 1.083g
60 LeadJ(A15mm × 9mm × 2.82mm)
JMAX
θ
JA DERIVED FROM 6.35cm × 6.35cm 4 LAYER PCB
ORDER INFORMATION
LEAD FREE FINISH
LTM8040EV#PBF
LTM8040IV#PBF
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE (NOTE 3)
0°C to 125°C
LTM8040V
60-Lead 15mm × 9mm LGA Package
60-Lead 15mm × 9mm LGA Package
LTM8040V
–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/
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, BIAS = LPWR, VOUT = 4V, ADJ open, RT open, VPWM = 5V,
unless otherwise noted (Note 3).
PARAMETER
CONDITIONS
MIN
TYP
3.5
2.6
0.01
1
MAX
UNITS
V
l
Minimum Input Voltage
Input Quiescent Current
Shutdown Current
LEDA Current
4
4
2
Not Switching
mA
μA
SHDN =0.3V, BIAS = 0V, LEDA = 0V
ADJ open
0.98
0.965
0.49
1.02
1.035
0.51
A
A
A
A
l
l
R
ADJ
= 5.11k
0.5
0.481
0.525
ADJ Bias Current
ADJ = 0V, Current flows out of pin
24.5
5.11
500
μA
kΩ
ADJ Pull-up Resistor
Switching Frequency
SHDN Threshold
5
5.22
530
RT open
470
2.60
kHz
V
V
V
V
IH
IL
1
PWM Threshold
V
IH
V
IL
1.2
V
V
0.4
8040p
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, BIAS = LPWR, VOUT = 4V, ADJ open, RT open, 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.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency - Four 2.66V at
1A LEDs
Efficiency - Three 2.66V at
1A LEDs
Efficiency - Single 2.66V at
1A LED
90
80
70
60
50
40
30
20
10
0
100
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
12 V
24 V
12 V
IN
IN
IN
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 G01
8040 G02
8040 G03
Efficiency - Single 3.3V at
1A LED
Efficiency - Two 3.3V at
1A LEDs
Efficiency - (3 x 3.3V at
1A LEDs)
90
80
70
60
50
40
30
20
10
0
100
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
12 V
24 V
12 V
IN
IN
IN
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 G04
8040 G05
8040 G06
8040p
3
LTM8040
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Running Voltage vs
Output Voltage 2.7V at 1A LEDs
Minimum Running Voltage vs
Output Voltage 3.3V at 1A LEDs
Minimum Start Voltage vs Load
2.7V at 1A LEDs
14
12
10
8
14
12
10
8
14
12
10
8
SINGLE LED
2 LEDs
3 LEDs
4 LEDs
6
6
6
1A LOAD
0.5A LOAD
0.1A LOAD
1A LOAD
0.5A LOAD
0.1A LOAD
4
4
4
2
2
2
0
5
10
15
0
2
4
6
8
10
12
0
200
400
I
LOAD
600
(mA)
800
1000
OUTPUT VOLTAGE
OUTPUT VOLTAGE
8040 G07
8040 G08
8040 G09
Minimum Start Voltage vs Load
3.3V at 1A LEDs
BIAS Current vs Load Current
12VIN, Single 2.7V at 1A LED
SHDN Current vs Voltage
14
12
10
8
60
50
12
10
8
SINGLE LED
2 LEDs
3 LEDs
V
V
V
= 5V
= 3.2V
= 12V
BIAS
BIAS
BIAS
40
30
20
10
0
6
6
4
4
2
2
0
0
200
400
600
(mA)
800
1000
0
5
10 15 20 25 30 35 40
SHUTDOWN VOLTAGE
0
200
400
600
800
1000
I
LOAD CURRENT (mA)
LOAD
8040 G10
8040 G11
8040 G12
BIAS Current vs Load Current
24VIN, Three 3.3V at 1A LEDs
Input Current vs Input Voltage
Output Short Circuited
Temp Rise vs Load 2.7V at 1A
LEDs, 12VIN
20
18
16
14
12
10
8
120
100
32
27
V
V
V
= 5V
= 3.2V
= 12V
SINGLE LED
2 LEDs
3 LEDs
4 LEDs
BIAS
BIAS
BIAS
80
60
40
20
0
22
17
12
7
6
4
2
0
2
0
200
400
600
800
1000
0
5
10 15 20 25 30 35 40
INPUT VOLTAGE (V)
0
200
400
I
LOAD
600
(mA)
800
1000
LOAD CURRENT (mA)
8040 G13
8040 G14
8040 G15
8040p
4
LTM8040
TYPICAL PERFORMANCE CHARACTERISTICS
Temp Rise vs Load 2.7V at 1A
LEDs, 36VIN
Temp Rise vs Load 3.3V at 1A
LEDs, 12VIN
47
42
37
32
27
22
17
12
7
27
22
17
12
7
SINGLE LED
2 LEDs
3 LEDs
4 LEDs
SINGLE LED
2 LEDs
3 LEDs
2
2
0
200
400
600
(mA)
800
1000
0
200
400
I
LOAD
600
(mA)
800
1000
I
LOAD
8040 G16
8040 G17
Temp Rise vs Load 3.3V at 1A
LEDs, 36VIN
LED Current vs Adjust Voltage
37
32
27
22
17
12
7
1200
1000
SINGLE LED
2 LEDs
3 LEDs
800
600
400
200
0
2
0
200
400
I
600
(mA)
800
1000
0
200
400
600 800 1000 1200
ADJUST VOLTAGE
LOAD
8040 G18
8040 G19
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 2.4V for accurate
current regulation.
BIAS (Pin L5): The BIAS pin connects through an internal
Schottky diode to provide power to internal housekeeping
circuits.Connecttoavoltagesource(usuallyLPWRorV )
IN
greaterthan3.2V.NotethatthispinisadjacenttotheLPWR
pin to ease layout. If this pin is powered by an external
power source, a decoupling cap may be necessary.
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
LPWR (Pin K5): This is the output of the buck regulator
that sources the LED current. If the LEDA voltage is above
3.2V, connect this pin to BIAS. It is pinned out primarily
for the convenience of the user. If it is not used, leave this
to V if the SHDN function is unused.
IN
8040p
5
LTM8040
PIN FUNCTIONS
pin floating. Please refer to the Applications Information
section for details.
formula: I = 1A • ADJ/1.25V. If connecting a resistor to
LED
GND, the resistor value should be R = 5.11 • I
/(1Amp
LED
– I ), where R is in KΩ and I
is the desired current
out of LEDA in Amps. Make sure that the voltage at LEDA
LED
LED
PWM (Pin L7): Input Pin for PWM Dimming Control. A
PWMsignalabove0.9V(ONthreshold)turnstheonoutput
current source, while a PWM signal below 0.5V 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.
is at least 2.4V.
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 2 for details.
V (Bank3):TheV pinsuppliescur-renttotheLTM8040’s
IN
IN
internal power converter and other circuitry. It must be lo-
cally bypassed with a high quality (low ESR) capacitor.
GND (Bank 2): Tie all GND pins directly to a local ground
plane. These pins serve as both signal and power return
totheLTM8040μModule, aswellasprovidingtheprimary
thermal path for heat dissipation within the unit. See the
Applications Information section for more information
about heat-sinking and printed circuit board layout.
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
BLOCK DIAGRAM
SENSE
RESISTOR
8.2μH
V
LEDA
IN
0.1μF
4.7μF
LPWR
BIAS
SHDN
CURRENT
INTERNAL
MODE
CONTROLLER
COMPENSATION
PWM
5.11k
INTERNAL
1.25V
80.6k
8040 BD
GND
RT
ADJ
8040p
6
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.
cuitrydrawingcurrentfromtheLPWRpinisalsodisabled.
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 accurate
dimming range.
Operation can be best understood by referring to the
Block Diagram. The power stage is step down converter
that regulates the output current by reading the voltage
across a power sense resistor that is in series with the
output.
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 2 for the frequencies that correspond to the applied
external resistor values.
If the SHDN pin is tied to ground, the LTM8040 is shut
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.
An external voltage is required at the BIAS pin to power
internal circuitry. For proper operation, BIAS must be at
least 3.2V. For many applications, BIAS should be tied to
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 LED 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. LED 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 2.4V at the desired operating point.
LWPR; if LWPR is below 3.2V, BIAS may be tied to V
or some other voltage source.
IN
The switching regulator performs frequency foldback
during overload conditions. An amplifier senses when
LWPR is less than 2V and begins decreasing the oscillator
frequencydownfromfullfrequencyto20%ofthenominal
frequency when V
= 0V. The LPWR pin is less than 2V
OUT
during startup, short circuit, and overload conditions, so
the BIAS pin will be below the specified limit for efficient
operation if the two pins are tied together. Frequency
foldback helps limit internal power and thermal stresses
under these conditions.
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 nominally. If the PWM
pin is pulled low, the LTM8040 stops switching and the
internal control circuitry is held in its present state. Cir-
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.
8040p
7
LTM8040
APPLICATION INFORMATION
For most applications, the design process is straight
forward, summarized as follows:
Choose resistors according to the following formula:
2.65V
UVLO −2.65V
R2=
1. Look at Table 1 and find the row that has the desired
inputvoltagerangeLEDstringvoltagerangeandoutput
current.
V
–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
TH
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
8V − 2.65V
R2=
= 61.9
–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
thenskipscyclestolimittheinputcurrent.Theinputcurrent
andoutputvoltageduringanopenLEDconditionisshown
in the Typical Performance Characteristics section.
C1
GND
8040 F01
Figure 1. Undervoltage Lockout
Undervoltage Lockout
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.
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.
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
without adding any resistor. Many applications use this
value.Ifanotherfrequencyisdesired,agraphforselecting
UVLO prevents the regulator from operating at source
voltages where this might occur. An internal comparator
will force the par t into shutdown when V fallsbelow3.5V.
An adjustable UVLO threshold is also realized through
the SHDN pin, as the internal comparator that performs
this function has a 2.65V threshold. An internal resistor
pulls 10.3μA to ground from the SHDN pin at the UVLO
threshold in order to provide hysteresis.
IN
8040p
8
LTM8040
APPLICATION INFORMATION
the value of RT for a given operating frequency is shown
in the Typical Performance Characteristics section. Table
2 shows suggested RT selections for a variety of switch-
ing frequencies.
BIAS Pin Considerations
For proper operation, The BIAS pin must be powered by
at least 3.2V. Figure 2 shows three ways to arrange the
circuit. For outputs of 3.2V or higher, the standard circuit
(Figure2a)isbest,becausethecircuit’sefficiencyisbetter
for lower voltages above 3.2V. For output voltages below
3.2V, 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 3.2V (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 2. RT vs Frequency
R (kΩ)
Frequency (MHz)
2.00
T
13.0
16.0
18.7
24.9
29.4
35.8
54.9
75.0
88.7
137.0
175.0
215.0
487.0
open
1.84
1.70
1.50
1.37
1.25
pin is both less than 25V and the sum of V and BIAS is
IN
1.00
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 cap is dependent upon the
source and PCB layout.
0.90
0.85
0.75
0.68
Programming LED Current
0.65
0.57
The LED current can be set by adjusting the voltage on
the ADJ pin. The ADJ pin is internally pulled up to a pre-
0.50
LTM8040
LTM8040
V
V
5.5V
IN
IN
3.3V
V
LEDA
LPWR
BIAS
2.7V
V
LEDA
LPWR
BIAS
IN
IN
4V TO 36V
C1
2.2μF
SHDN
C1
2.2μF
SHDN
WHITE
LED
RED
LED
V
V
ADJ
ADJ
PWM
RT
PWM
RT
GND
GND
8040 F02a
8040 F02b
Figure 2a. Tie BIAS to LPWR if it is Greater Than 2.6V
Figure 2b. BIAS May be Tied to XVIN if LPWR is Below 2.6V
LTM8040
V
IN
2.7V
V
LEDA
LPWR
BIAS
IN
4V TO 36V
C1
2.2μF
SHDN
3.3V
V
ADJ
PWM
RT
RED
LED
GND
8040 F02c
Figure 2c. Tie BIAS to an External Power Source if Neither VIN Nor
LPWR are Suitable
8040p
9
LTM8040
APPLICATION INFORMATION
cision 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:
can be calculated from the maximum LED current
(I ) and the minimum LED current (I ) as follows:
MAX
MIN
IMAX
=IRATIO
IMIN
Another dimming control circuit (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, dis-
connecting 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, in turn
reducing the PWM dimming frequency.
R
= 5.11 • I
/(1Amp – I ),
ADJ
LED LED
where R
of LEDA.
is in kΩ and I
is the desired current out
ADJ
LED
In order to have accurate LED current, a precision 1%
resistor is recommended.
REF
LTM8040
ADJ
GND
R
ADJ
8040 F03
Figure 3. Setting ADJ with a Resistor
The LEDA voltage must be at least 2.4V for proper current
regulation.
PWM
60Hz TO
10kHz
PWM
LTM8040
LEDA
Dimming Control
There are several different types of dimming control
circuits. One dimming control circuit (Figure 4) changes
the voltage on the ADJ pin by tying a low on-resis-
tance FET to the resistor divider string. 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)
GND
8040 F05
Figure 5. Dimming Using PWM Signal
The maximum PWM dimming ratio (PWM
) can
MAX
RATIO
be calculated from the maximum PWM period (t
)
REF
LTM8040
and minimum PWM pulse width (t ) as follows:
MIN
V
ADJ
GND
R2
tMAX
8040 F04
=PWMRATIO
tMIN
DIM
Figure 4. Dimming with an NFET and Resistor
8040p
10
LTM8040
APPLICATION INFORMATION
Total dimming ratio (DIM
) is the product of the PWM
Ceramic capacitors are also piezoelectric. While the
LTM8040isafixedfrequencydevice,theinternalregulators
may skip cycles at light loads and extend the switching
cycle on time as the input voltage falls towards the to
output.Undereitheroftheseconditions,theLTM8040can
exciteaceramiccapacitorataudiofrequencies,generating
audible noise.
RATIO
dimming ratio and the current dimming ratio.
Example: I
MIN
= 1A, I
= 0.1A, t
= 1.0ms,
MAX
MAX
MIN
t
= 25μs
1A
0.1A
IRATIO
=
=10:1
Ifthisaudiblenoiseisunacceptable,useahighperformance
electrolytic capacitor at the output. The input capacitor
can be a parallel combination of a 4.7μF ceramic capacitor
and a low cost electrolytic capacitor.
10ms
PWMRATIO
=
=400:1
25µs
DIMRATIO =10 • 400=4000:1
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, possi-
bly exceeding 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.
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 V ). See Typical Performance Characteristics for
IN
High Temperature Considerations
details.
The internal operating temperature of the LTM8040 must
be lower than 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 datasheet to
thedesiredoutputpower,or,ifyouhaveandactualmodule,
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. 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 on page 2, which is based upon
Capacitor Selection Considerations
The C and capacitor values in tables 1 and 2 are the
IN
minimum recommended values for the associated oper-
ating conditions. Applying capacitor values below those
indicated in table 1 is not recommended, and may result
in undesirable operation. Using larger values is generally
acceptable, 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.
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 ap-
plied voltage and give dependable service. Other types,
including Y5V and Z5U have very large temperature and
voltage coefficients of capacitance. In an application cir-
cuit they may have only a small fraction of their nominal
capacitanceresultinginmuchhigheroutputvoltageripple
than expected.
2
a 40.3cm 4 layer FR4 PC board, can be used 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
8040p
11
LTM8040
APPLICATION INFORMATION
continuous operation under this operating condition may
impair reliability.
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.
Layout Hints
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of in-
tegration of the LTM8040. The LTM8040 is nevertheless a
switchingpowersupply,andcaremustbetakentominimize
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 for a sug-
gested layout. Ensure that the grounding and heatsinking
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
input voltage potential, tie BIAS to V , but be careful
IN
not to break up the ground plane.
1. Place the C capacitor as close as possible to the V
IN
IN
and GND connection of the LTM8040.
LEDA
PWM
LPWR BIAS
LED
STRING
SHDN
ADJ
RT
V
IN
GND
8040 F06
C
IN
VIAS TO GND PLANE
Figure 6. Suggested Layout
8040p
12
LTM8040
APPLICATION INFORMATION
Table 1. Recommended Configuration
LED STRING LED STRING
V
RANGE
C
VOLTAGE
(LEDA)
CURRENT
(LEDA)
RT
f
RT
f
MAX
RADJ
BIAS CONNECTION
IN
IN
OPTIMAL
OPTIMAL
MIN
4.5-36V
6.5-36V
9.5-36V
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
2.5-4V
4-6V
35mA
35mA
35mA
35mA
100mA
100mA
100mA
100mA
350mA
350mA
350mA
350mA
500mA
500mA
500mA
500mA
700mA
700mA
700mA
700mA
1A
open
open
open
open
open
open
487k
487k
open
open
137k
75k
0.50M
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
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
154
154
2.8V to 25V source
LPWR
6-9V
154
LPWR
12.5-36V
4.5-36V
6.5-36V
9.5-36V
12.5-36V
4.8-36V
7-36V
8-12V
2.5-4V
4-6V
154
LPWR
453
2.8V to 25V source
LPWR
453
6-9V
453
LPWR
8-12V
2.5-4V
4-6V
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
2.8V to 25V source
LPWR
10.5-36V
13.8-36V
4.8-36V
7-36V
6-9V
LPWR
8-12V
2.5-4V
4-6V
1.37M
0.50M
0.70M
1.0M
LPWR
open
open
137k
75k
2.8V to 25V source
LPWR
10.5-36V
14.3-36V
5-36V
6-9V
LPWR
8-12V
2.5-4V
4-6V
1.37M
0.50M
0.70M
1.0M
LPWR
open
487k
165k
75k
2.8V to 25V source
LPWR
7.7-36V
11-36V
6-9V
LPWR
14.8-36V
5.5-36V
8-36V
8-12V
2.5-4V
4-6V
1.37M
0.50M
0.75M
1.0M
LPWR
open
open
215k
137k
2.8V to 25V source
LPWR
1A
11.5-36V
15.5-36V
6-9V
1A
LPWR
8-12V
1A
1.37M
LPWR
8040p
13
LTM8040
TYPICAL APPLICATIONS
Step Down 1A Drive with Single Red or White LED
LTM8040
V
*
IN
V
LEDA
IN
5.5V TO 25V
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
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
8040p
14
LTM8040
PACKAGE DESCRIPTION
LGA Package
66-Lead (15mm × 9mm × 2.82mm)
(Reference LTC DWG # 05-08-1810 Rev Ø)
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
8040p
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.
15
LTM8040
TYPICAL APPLICATION
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
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTM4600
10A DC/DC μModule
Basic 10A DC/DC μModule, 15mm × 15mm × 2.8mm LGA
LTM4600HVMPV
Military Plastic 10A DC/DC μModule
–55°C to 125°C Operation, 15mm × 15mm × 2.8mm LGA
LTM4601/
LTM4601A
12A DC/DC μModule with PLL, Output Tracking/Margining Synchronizable, PolyPhase® Operation, LTM4601-1 Version Has No
and Remote Sensing
Remote Sensing
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 trademark of Linear Technology Corporation
8040p
LT 0808 • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
© LINEAR TECHNOLOGY CORPORATION 2008
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
相关型号：
LTM8040IV#PBF
LTM8040 - 36V, 1A µModule (Power Module) LED Driver and Current Source; Package: LGA; Pins: 66; Temperature Range: -40°C to 85°C
Linear
LTM8042EV#PBF
LTM8042/LTM8042-1 - µModule (Power Module) Boost LED Driver and Current Source; Package: LGA; Pins: 77; Temperature Range: -40°C to 85°C
Linear
LTM8042IV#PBF
LTM8042/LTM8042-1 - µModule (Power Module) Boost LED Driver and Current Source; Package: LGA; Pins: 77; Temperature Range: -40°C to 85°C
Linear
©2020 ICPDF网 联系我们和版权申明