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Electrical Specifications Subject to Change
LTM8040 36V, 1A Module LED Driver FEATURES
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DESCRIPTION
The LTM(R)8040 is a fixed frequency 1A step-down DC/DC ModuleTM 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 voltage 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. With its wide input range of 4V to 36V, the LTM8040 regulates 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 x 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.
True Color PWMTM Delivers Constant Color with 400:1 Dimming Ratio Wide Input Range: 4V to 36V Up to 1A LED Current Adjustable Control of LED Current High Output Current Accuracy is Maintained Over a Wide Range from 35mA to 1A Open LED and Short-Circuit Protection Grounded Cathode Connection Small Footprint, Low Profile (15mm x 9mm x 2.82mm) Surface Mount LGA Package
APPLICATIONS
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Automotive and Avionic Lighting Architectural Detail Lighting Display Backlighting Constant Current Sources
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Module is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
1A LED Driver Module
100 VIN* 4V TO 36V 1F VIN SHDN LEDA LPWR EFFICIENCY (%) TWO WHITE LEDs 6V TO 9V 1A LTM8040 ADJ PWM RT 215k 650kHz GND BIAS 90 80 70 60 50 40 30 20
8040 TA01
Efficiency
10 0 0
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION FOR START-UP REQUIREMENTS
VIN = 12V 3.3V AT 1A LEDs 200 800 400 600 OUTPUT CURRENT (mA) 1000
8040 TA01b
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LTM8040 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
PWM LEDA BANK 1
VIN ............................................................................36V BIAS ..........................................................................25V BIAS + VIN .................................................................51V LEDA .........................................................................15V PWM .........................................................................10V ADJ .............................................................................6V SHDN ........................................................................36V SHDN Above VIN .........................................................6V BIAS Current ...............................................................1A Internal Operating Temperature (Note 2).... -40 to 125C Storage Temperature Range....................... -45 to 125C
BIAS SHDN ADJ
GND BANK 2
RT GND VIN LPWR BANK 3 TJMAX = 125C, JA = 250C/W, WEIGHT = 1.083g 60 Lead (15mm x 9mm x 2.82mm) JA DERIVED FROM 6.35cm x 6.35cm 4 LAYER PCB
ORDER INFORMATION
LEAD FREE FINISH LTM8040EV#PBF LTM8040IV#PBF PART MARKING* LTM8040V LTM8040V PACKAGE DESCRIPTION 60-Lead 15mm x 9mm LGA Package 60-Lead 15mm x 9mm LGA Package TEMPERATURE RANGE (NOTE 3) 0C to 125C -40C to 125C
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
PARAMETER Minimum Input Voltage Input Quiescent Current Shutdown Current LEDA Current Not Switching CONDITIONS
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 12V, BIAS = LPWR, VOUT = 4V, ADJ open, RT open, VPWM = 5V, unless otherwise noted (Note 3).
MIN
l
TYP 3.5 2.6 0.01
MAX 4 4 2 1.02 1.035 0.51 0.525 5.22 530 1 1.2
UNITS V mA A A A A A A k kHz V V V V
SHDN =0.3V, BIAS = 0V, LEDA = 0V ADJ open RADJ = 5.11k
l l
0.98 0.965 0.49 0.481 5
1 0.5 24.5 5.11 500
ADJ Bias Current ADJ Pull-up Resistor Switching Frequency SHDN Threshold PWM Threshold
ADJ = 0V, Current flows out of pin RT open VIH VIL VIH VIL 470 2.60
0.4
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LTM8040 ELECTRICAL CHARACTERISTICS
PARAMETER LEDA Clamp Voltage Minimum BIAS Voltage 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 Module includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. CONDITIONS
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 12V, BIAS = LPWR, VOUT = 4V, ADJ open, RT open, VPWM = 5V, unless otherwise noted (Note 3).
MIN 13.2 2.0 TYP MAX 14.5 2.6 UNITS V V
Note 3: The LTM8040E is guaranteed to meet performance specifications from 0C to 125C internal. Specifications over the full -40C to 125C 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 -40C to 125C 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 - Single 2.66V at 1A LED
90 80 70 EFFICIENCY (%) EFFICIENCY (%) 60 50 40 30 20 10 0 0 24 VIN 12 VIN 5 VIN 200 800 400 600 OUTPUT CURRENT (mA) 1000
8040 G01
Efficiency - Three 2.66V at 1A LEDs
100 90 80 EFFICIENCY (%) 24 VIN 12 VIN 0 200 800 400 600 OUTPUT CURRENT (mA) 1000
8040 G02
Efficiency - Four 2.66V at 1A LEDs
100 90 80 70 60 50 40 30 20 10 0 0 200 24 VIN 12 VIN 800 400 600 OUTPUT CURRENT (mA) 1000
8040 G03
70 60 50 40 30 20 10 0
Efficiency - Single 3.3V at 1A LED
90 80 70 EFFICIENCY (%) EFFICIENCY (%) 60 50 40 30 20 10 0 0 24 VIN 12 VIN 5 VIN 200 800 400 600 OUTPUT CURRENT (mA) 1000
8040 G04
Efficiency - Two 3.3V at 1A LEDs
100 90 80 EFFICIENCY (%) 24 VIN 12 VIN 0 200 800 400 600 OUTPUT CURRENT (mA) 1000
8040 G05
Efficiency - (3 x 3.3V at 1A LEDs)
100 90 80 70 60 50 40 30 20 10 0 0 200 24 VIN 12 VIN 800 400 600 OUTPUT CURRENT (mA) 1000
8040 G06
70 60 50 40 30 20 10 0
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LTM8040 TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Running Voltage vs Output Voltage 2.7V at 1A LEDs
14 12 INPUT VOLTAGE INPUT VOLTAGE 10 8 6 4 2 0 5 10 OUTPUT VOLTAGE 1A LOAD 0.5A LOAD 0.1A LOAD 15
8040 G07
Minimum Running Voltage vs Output Voltage 3.3V at 1A LEDs
14 12 INPUT VOLTAGE 10 8 6 4 2 0 2 4 6 8 OUTPUT VOLTAGE 1A LOAD 0.5A LOAD 0.1A LOAD 10 12
8040 G08
Minimum Start Voltage vs Load 2.7V at 1A LEDs
14 12 10 8 6 4 2 0 200 400 600 ILOAD (mA) 800 SINGLE LED 2 LEDs 3 LEDs 4 LEDs
1000
8040 G09
Minimum Start Voltage vs Load 3.3V at 1A LEDs
14 12 INPUT VOLTAGE 10 8 6 4 2 0 200 400 600 ILOAD (mA) 800 SINGLE LED 2 LEDs 3 LEDs 60 50 SHUTDOWN CURRENT (mA) 40 30 20 10 0
SHDN Current vs Voltage
12 10 BIAS CURRENT (mA) 8 6 4 2 0
BIAS Current vs Load Current 12VIN, Single 2.7V at 1A LED
VBIAS = 5V VBIAS = 3.2V VBIAS = 12V
1000
8040 G10
0
5
10 15 20 25 30 SHUTDOWN VOLTAGE
35
40
0
200
400 600 800 LOAD CURRENT (mA)
1000
8040 G12
8040 G11
BIAS Current vs Load Current 24VIN, Three 3.3V at 1A LEDs
20 18 16 BIAS CURRENT (mA) 14 12 10 8 6 4 2 0 0 200 400 600 800 LOAD CURRENT (mA) 1000
8040 G13
Input Current vs Input Voltage Output Short Circuited
120 100 80 60 40 20 0 0 5 10 TEMPERATURE RISE (C) INPUT CURRENT (mA) 32 27 22 17 12 7 2
Temp Rise vs Load 2.7V at 1A LEDs, 12VIN
SINGLE LED 2 LEDs 3 LEDs 4 LEDs
VBIAS = 5V VBIAS = 3.2V VBIAS = 12V
15 20 25 30 INPUT VOLTAGE (V)
35
40
0
200
400 600 ILOAD (mA)
800
1000
8040 G15
8040 G14
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LTM8040 TYPICAL PERFORMANCE CHARACTERISTICS
Temp Rise vs Load 2.7V at 1A LEDs, 36VIN
47 42 TEMPERATURE RISE (C) 37 32 27 22 17 12 7 2 0 200 400 600 ILOAD (mA) 800 1000
8040 G16
Temp Rise vs Load 3.3V at 1A LEDs, 12VIN
27 SINGLE LED 2 LEDs 3 LEDs
TEMPERATURE RISE (C)
SINGLE LED 2 LEDs 3 LEDs 4 LEDs
22
17
12
7
2 0 200 400 600 ILOAD (mA) 800
1000
8040 G17
Temp Rise vs Load 3.3V at 1A LEDs, 36VIN
37 32 TEMPERATURE RISE (C) 27 22 17 12 7 2 0 200 400 600 ILOAD (mA) 800 SINGLE LED 2 LEDs 3 LEDs 1200 1000 LED CURRENT (mA) 1000
8040 G18
LED Current vs Adjust Voltage
800 600 400 200 0 0 200 400 600 800 ADJUST VOLTAGE
1000 1200
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. 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 3.2V. 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 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
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LTM8040 PIN FUNCTIONS
pin floating. Please refer to the Applications Information section for details. PWM (Pin L7): Input Pin for PWM Dimming Control. A PWM signal above 0.9V (ON threshold) turns the on output 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 10A source current pulls PWM high) or pulled up to a voltage source between 1.2V and 10V. VIN (Bank 3): The VIN pin supplies cur-rent to the LTM8040's internal power converter and other circuitry. It must be locally 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 K and ILED is the desired current out of LEDA in Amps. Make sure that the voltage at LEDA is at least 2.4V. 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 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. GND (Bank 2): Tie all GND pins directly to a local ground plane. These pins serve as both signal and power return to the LTM8040 Module, 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.
BLOCK DIAGRAM
8.2H VIN 0.1F 4.7F SENSE RESISTOR LEDA LPWR
BIAS
SHDN INTERNAL COMPENSATION PWM 5.11k 80.6k INTERNAL 1.25V CURRENT MODE CONTROLLER
8040 BD
GND
RT
ADJ
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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. 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. 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 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. 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. 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 accurate dimming range. The RT pin allows programming of the switching frequency. 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 2 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 3.2V. For many applications, BIAS should be tied to LWPR; if LWPR is below 3.2V, BIAS may be tied to VIN or some other voltage source. The switching regulator performs frequency foldback during overload conditions. An amplifier senses when LWPR is less than 2V and begins decreasing the oscillator frequency down from full frequency to 20% of the nominal frequency when VOUT = 0V. The LPWR pin is less than 2V 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 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 125C temperature rating of the LTM8040, so continuous operation under this operating condition may impair reliability.
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LTM8040 APPLICATION INFORMATION
For most applications, the design process is straight forward, summarized as follows: 1. Look at Table 1 and find 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. The input current and output voltage during an open LED condition is shown in the Typical Performance Characteristics section. Undervoltage Lockout Undervoltage lockout (UVLO) is typically used in situations 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 voltages where this might occur. An internal comparator will force the part into shutdown 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. An internal resistor pulls 10.3A 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 - 10.3A R1
where VUVLO is the desired UVLO Threshold Suppose that the output needs to stay off until the input is above 8V. V TH = 8V Let R1 = 100k R2 = 2.65V = 61.9 8V - 2.65V - 10.3A 100k
VIN R1 VIN LTM8040 SHDN C1 R2 GND
8040 F01
Figure 1. 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 coupling 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 without adding any resistor. Many applications use this value. If another frequency is desired, a graph for selecting
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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 switching frequencies.
Table 2. RT vs Frequency
RT (k) 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 Frequency (MHz) 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
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 (Figure 2a) is best, because the circuit's efficiency is better 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 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 cap is dependent upon the source and PCB layout. 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 pre-
LTM8040 VIN 4V TO 36V C1 2.2F VIN SHDN VADJ PWM RT GND LEDA LPWR BIAS WHITE LED 3.3V VIN 5.5V C1 2.2F
LTM8040 VIN SHDN VADJ PWM RT GND LEDA LPWR BIAS RED LED 2.7V
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
VIN 4V TO 36V C1 2.2F
VIN SHDN VADJ PWM RT GND
LEDA LPWR BIAS
2.7V 3.3V RED LED
8040 F02c
Figure 2c. Tie BIAS to an External Power Source if Neither VIN Nor LPWR are Suitable
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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 resistor from ADJ to GND as shown in Figure 3, using the following formula: RADJ = 5.11 * ILED /(1Amp - ILED), where RADJ is in k and ILED is the desired current out of LEDA. In order to have accurate LED current, a precision 1% resistor is recommended.
REF LTM8040 ADJ RADJ GND
8040 F03
can be calculated from the maximum LED current (IMAX) and the minimum LED current (IMIN) as follows: IMAX =I IMIN RATIO 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, 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 40s. 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.
PWM 60Hz TO 10kHz
Figure 3. Setting ADJ with a Resistor
The LEDA voltage must be at least 2.4V for proper current regulation. 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-resistance 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)
PWM LTM8040 LEDA GND
8040 F05
Figure 5. Dimming Using PWM Signal
REF LTM8040 VADJ R2 DIM GND
8040 F04
The maximum PWM dimming ratio (PWMRATIO) can be calculated from the maximum PWM period (tMAX) and minimum PWM pulse width (tMIN) as follows: tMAX = PWMRATIO tMIN
Figure 4. Dimming with an NFET and Resistor
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LTM8040 APPLICATION INFORMATION
Total dimming ratio (DIMRATIO) is the product of the PWM dimming ratio and the current dimming ratio. Example: IMAX = 1A, IMIN = 0.1A, t MAX = 1.0ms, tMIN = 25s IRATIO = 1A =10:1 0.1A 10ms PWMRATIO = = 400:1 25s DIMRATIO =10 * 400 = 4000:1 Ceramic capacitors are also piezoelectric. While the LTM8040 is a fixed frequency device, the internal regulators may skip cycles at light loads and extend the switching cycle on time as the input voltage falls towards the to output. Under either of these conditions, the LTM8040 can excite a ceramic capacitor at audio frequencies, generating audible noise. If this audible noise is unacceptable, use a high performance electrolytic capacitor at the output. The input capacitor can be a parallel combination of a 4.7F ceramic capacitor and a low cost electrolytic capacitor. 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 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. High Temperature Considerations The internal operating temperature of the LTM8040 must be lower than 125C 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, approximate the power dissipation within the LTM8040 by applying the typical efficiency stated in this datasheet to the desired output power, or, if you have and actual module, by taking a power measurement. Then calculate the temperature 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 a 40.3cm2 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 125C temperature rating of the LTM8040, so
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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. Capacitor Selection Considerations The CIN and capacitor values in tables 1 and 2 are the minimum recommended values for the associated operating 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 applied voltage and give dependable service. Other types, including Y5V and Z5U have very large temperature and voltage coefficients of capacitance. In an application circuit they may have only a small fraction of their nominal capacitance resulting in much higher output voltage ripple than expected.
11
LTM8040 APPLICATION INFORMATION
continuous operation under this operating condition may impair reliability. 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 nevertheless 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 for a suggested layout. Ensure that the grounding and heatsinking 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 copper 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.
LEDA PWM
LPWR LED STRING
BIAS
SHDN ADJ
RT
VIN
GND
8040 F06
CIN VIAS TO GND PLANE
Figure 6. Suggested Layout
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LTM8040 APPLICATION INFORMATION
Table 1. Recommended Configuration
VIN RANGE 4.5-36V 6.5-36V 9.5-36V 12.5-36V 4.5-36V 6.5-36V 9.5-36V 12.5-36V 4.8-36V 7-36V 10.5-36V 13.8-36V 4.8-36V 7-36V 10.5-36V 14.3-36V 5-36V 7.7-36V 11-36V 14.8-36V 5.5-36V 8-36V 11.5-36V 15.5-36V CIN 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V 1F 0805 50V LED STRING VOLTAGE (LEDA) 2.5-4V 4-6V 6-9V 8-12V 2.5-4V 4-6V 6-9V 8-12V 2.5-4V 4-6V 6-9V 8-12V 2.5-4V 4-6V 6-9V 8-12V 2.5-4V 4-6V 6-9V 8-12V 2.5-4V 4-6V 6-9V 8-12V LED STRING CURRENT (LEDA) 35mA 35mA 35mA 35mA 100mA 100mA 100mA 100mA 350mA 350mA 350mA 350mA 500mA 500mA 500mA 500mA 700mA 700mA 700mA 700mA 1A 1A 1A 1A RTOPTIMAL open open open open open open 487k 487k open open 137k 75k open open 137k 75k open 487k 165k 75k open open 215k 137k fOPTIMAL 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 RTMIN 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 fMAX 0.50M 0.50M 0.50M 0.50M 0.50M 0.70M 0.75M 0.85M 0.50M 0.70M 1.0M 1.37M 0.50M 0.70M 1.0M 1.37M 0.50M 0.70M 1.0M 1.37M 0.50M 0.75M 1.0M 1.37M RADJ 154 154 154 154 453 453 453 453 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 BIAS CONNECTION 2.8V to 25V source LPWR LPWR LPWR 2.8V to 25V source LPWR LPWR LPWR 2.8V to 25V source LPWR LPWR LPWR 2.8V to 25V source LPWR LPWR LPWR 2.8V to 25V source LPWR LPWR LPWR 2.8V to 25V source LPWR LPWR LPWR
8040p
13
LTM8040 TYPICAL APPLICATIONS
Step Down 1A Drive with Single Red or White LED
VIN* 5.5V TO 25V 1F LTM8040 LEDA LPWR 2.5V TO 4V 1A
VIN BIAS ADJ
SHDN
PWM RT GND
8040 TA02
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION FOR START-UP REQUIREMENTS
Step Down 350mA Drive with Three Series Red LEDs
VIN* 10.5V TO 36V 1F 2.87k ADJ PWM RT 137k 750kHz GND LTM8040 LEDA LPWR BIAS 6V TO 9V 350mA
VIN
SHDN
8040 TA03
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION FOR START-UP REQUIREMENTS
Step Down 1A Drive with Three White Series LEDs
LTM8040 LEDA LPWR BIAS ADJ PWM RT 137k 750kHz GND 8V TO 12V 1A
VIN* 15.5V TO 36V 1F
VIN
SHDN
8040 TA04
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION FOR START-UP REQUIREMENTS
8040p
14
aaa Z
15.000 BSC
X Y
2.670 - 2.970 0.605 - 0.665
12.700 BSC
7 6 0.605 - 0.665 9.000 BSC 0.27 - 0.37 2.40 - 2.60
Z
PACKAGE DESCRIPTION
5 4 3 2
MOLD CAP SUBSTRATE 7.620 BSC
4 aaa Z
bbb Z
PAD 1 CORNER DETAIL A 1.270 BSC
1 PADS SEE NOTES L K J DETAIL A PACKAGE SIDE VIEW
3
PACKAGE TOP VIEW
H
G
F
E PACKAGE BOTTOM VIEW
D
C
B
A
PAD 1 C (0.30)
6.350
5.080
3.810
2.540
1.270
0.000
0.9525 1.270 1.5875 2.540
3.810
5.080
6.350
NOTES: 1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994 2. ALL DIMENSIONS ARE IN MILLIMETERS 3 LAND DESIGNATION PER JESD MO-222, SPP-010 AND SPP-020 4
3.810
LGA Package 66-Lead (15mm x 9mm x 2.82mm)
2.540
(Reference LTC DWG # 05-08-1810 Rev O)
1.270
1.5875
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
0.9525
5. PRIMARY DATUM -Z- IS SEATING PLANE 6. THE TOTAL NUMBER OF PADS: 66 SYMBOL TOLERANCE 0.15 aaa 0.10 bbb
COMPONENT PIN 1 TRAY PIN 1 BEVEL LTMXXXXXX Module
0.000
1.270
2.540
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PACKAGE IN TRAY LOADING ORIENTATION
LGA 66 0407 REV O
3.810
SUGGESTED PCB LAYOUT TOP VIEW
LTM8040
15
8040p
LTM8040 TYPICAL APPLICATION
Step Down 1A Drive with Four Series Red LEDs
VIN* 15.5V TO 36V 1F ADJ PWM RT 137k 750kHz GND VIN LTM8040 LEDA LPWR BIAS 8V TO 12V 1A
SHDN
8040 TA05
*RUNNING VOLTAGE. SEE APPLICATION INFORMATION FOR START-UP REQUIREMENTS
RELATED PARTS
PART NUMBER LTM4600 LTM4600HVMPV LTM4601/ LTM4601A LTM4602 LTM4603 LTM4604 LTM4605 LTM4607 LTM4608 LTM8020 LTM8022 LTM8023 DESCRIPTION 10A DC/DC Module Military Plastic 10A DC/DC Module 12A DC/DC Module with PLL, Output Tracking/Margining and Remote Sensing 6A DC/DC Module 6A DC/DC Module with PLL and Output Tracking/ Margining and Remote Sensing 4A Low VIN DC/DC Module 5A to 12A Buck-Boost Module 5A to 12A Buck-Boost Module 8A Low VIN DC/DC Module 36V, 200mA DC/DC Module 1A, 36V DC/DC Module 2A, 36V DC/DC Module COMMENTS Basic 10A DC/DC Module, 15mm x 15mm x 2.8mm LGA -55C to 125C Operation, 15mm x 15mm x 2.8mm LGA Synchronizable, PolyPhase(R) Operation, LTM4601-1 Version Has No Remote Sensing Pin-Compatible with the LTM4600 Synchronizable, PolyPhase Operation, LTM4603-1 Version Has No Remote Sensing, Pin-Compatible with the LTM4601 2.375V VIN 5V, 0.8V VOUT 5V, 9mm x 15mm x 2.3mm LGA High Efficiency, Adjustable Frequency, 4.5V VIN 20V, 0.8V VOUT 16V, 15mm x 15mm x 2.8mm High Efficiency, Adjustable Frequency, 4.5V VIN 36V, 0.8V VOUT 25V, 15mm x 15mm x 2.8mm 2.375V VIN 5V, 0.8V VOUT 5V, 9mm x 15mm x 2.8mm LGA 4V VIN 36V, 1.25V VOUT 5V, 6.25mm x 6.25mm x 2.3mm LGA Adjustable Frequency, 0.8V VOUT 5V, 11.25mm x 9mm x 2.82mm, Pin-Compatible to the LTM8023 Adjustable Frequency, 0.8V VOUT 5V, 11.25mm x 9mm x 2.82mm, Pin-Compatible to the LTM8022
PolyPhase is a trademark of Linear Technology Corporation
8040p
16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 0808 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2008

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