Electrical Specifications Subject to Change LTM8040 36V, 1A µModule LED Driver FEATURES DESCRIPTION n 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 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. n n n n n n n True Color PWM™ 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 × 9mm × 2.82mm) Surface Mount LGA Package APPLICATIONS n n n n Automotive and Avionic Lighting Architectural Detail Lighting Display Backlighting Constant Current Sources , 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. 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 × 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. TYPICAL APPLICATION 1A LED Driver μModule Efficiency 100 VIN 1μF SHDN LEDA 90 LPWR 80 LTM8040 BIAS ADJ PWM RT 215k 650kHz TWO WHITE LEDs 6V TO 9V 1A GND EFFICIENCY (%) VIN* 4V TO 36V 70 60 50 40 30 20 8040 TA01 *RUNNING VOLTAGE. SEE APPLICATION INFORMATION FOR START-UP REQUIREMENTS VIN = 12V 3.3V AT 1A LEDs 10 0 0 200 800 400 600 OUTPUT CURRENT (mA) 1000 8040 TA01b 8040p 1 LTM8040 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 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 125°C Storage Temperature Range....................... –45 to 125°C PWM LEDA BANK 1 BIAS SHDN ADJ GND BANK 2 RT GND VIN LPWR BANK 3 TJMAX = 125°C, θJA = 250°C/W, WEIGHT = 1.083g 60 Lead (15mm × 9mm × 2.82mm) θJA DERIVED FROM 6.35cm × 6.35cm 4 LAYER PCB ORDER INFORMATION LEAD FREE FINISH PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE (NOTE 3) LTM8040EV#PBF LTM8040V 60-Lead 15mm × 9mm LGA Package 0°C to 125°C LTM8040IV#PBF LTM8040V 60-Lead 15mm × 9mm 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/ 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 l TYP MAX UNITS 3.5 4 V Input Quiescent Current Not Switching 2.6 4 mA Shutdown Current SHDN =0.3V, BIAS = 0V, LEDA = 0V 0.01 2 μA LEDA Current ADJ open 1 1.02 1.035 0.51 0.525 A A A A Minimum Input Voltage RADJ = 5.11k ADJ Bias Current l l 0.98 0.965 0.49 0.481 ADJ = 0V, Current flows out of pin ADJ Pull-up Resistor 24.5 5.11 5.22 kΩ 500 530 kHz RT open 470 SHDN Threshold VIH VIL 2.60 VIH VIL μA 5 Switching Frequency PWM Threshold 0.5 1 1.2 0.4 V V V V 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 LEDA Clamp Voltage TYP 13.2 Minimum BIAS Voltage 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. MAX UNITS 14.5 V 2.6 V 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 100 80 90 90 70 80 80 70 70 60 50 40 30 EFFICIENCY (%) 100 EFFICIENCY (%) EFFICIENCY (%) 90 60 50 40 30 20 0 0 200 800 400 600 OUTPUT CURRENT (mA) 50 40 20 24 VIN 12 VIN 10 0 1000 60 30 20 24 VIN 12 VIN 5 VIN 10 0 200 800 400 600 OUTPUT CURRENT (mA) 8040 G01 0 1000 0 100 90 90 70 80 80 70 70 EFFICIENCY (%) 30 EFFICIENCY (%) 100 80 40 60 50 40 30 20 0 0 200 800 400 600 OUTPUT CURRENT (mA) 8040 G04 60 50 40 20 24 VIN 12 VIN 10 1000 1000 30 20 24 VIN 12 VIN 5 VIN 10 800 400 600 OUTPUT CURRENT (mA) Efficiency - (3 x 3.3V at 1A LEDs) 90 50 200 8040 G03 Efficiency - Two 3.3V at 1A LEDs 60 24 VIN 12 VIN 10 8040 G02 Efficiency - Single 3.3V at 1A LED EFFICIENCY (%) Efficiency - Four 2.66V at 1A LEDs Efficiency - Three 2.66V at 1A LEDs Efficiency - Single 2.66V at 1A LED 0 0 200 800 400 600 OUTPUT CURRENT (mA) 24 VIN 12 VIN 10 1000 8040 G05 0 0 200 800 400 600 OUTPUT CURRENT (mA) 1000 8040 G06 8040p 3 LTM8040 TYPICAL PERFORMANCE CHARACTERISTICS Minimum Running Voltage vs Output Voltage 3.3V at 1A LEDs 14 12 12 12 10 10 10 8 6 1A LOAD 0.5A LOAD 0.1A LOAD 2 8 6 1A LOAD 0.5A LOAD 0.1A LOAD 4 2 15 5 10 OUTPUT VOLTAGE 0 INPUT VOLTAGE 14 4 2 0 4 6 8 OUTPUT VOLTAGE 10 8040 G07 SHUTDOWN CURRENT (mA) INPUT VOLTAGE 10 8 6 4 2 12 0 0 200 400 600 ILOAD (mA) 800 60 12 50 10 40 30 20 0 8 6 4 5 10 15 20 25 30 SHUTDOWN VOLTAGE 35 0 40 0 10 8 6 4 400 600 800 LOAD CURRENT (mA) 1000 Temp Rise vs Load 2.7V at 1A LEDs, 12VIN 120 32 100 27 TEMPERATURE RISE (°C) INPUT CURRENT (mA) 12 200 8040 G12 Input Current vs Input Voltage Output Short Circuited 14 1000 VBIAS = 5V VBIAS = 3.2V VBIAS = 12V 8040 G11 VBIAS = 5V VBIAS = 3.2V VBIAS = 12V 16 800 2 0 1000 BIAS Current vs Load Current 24VIN, Three 3.3V at 1A LEDs 18 400 600 ILOAD (mA) BIAS Current vs Load Current 12VIN, Single 2.7V at 1A LED 8040 G10 20 200 8040 G09 10 2 6 SHDN Current vs Voltage SINGLE LED 2 LEDs 3 LEDs 12 8 4 BIAS CURRENT (mA) 14 SINGLE LED 2 LEDs 3 LEDs 4 LEDs 8040 G08 Minimum Start Voltage vs Load 3.3V at 1A LEDs BIAS CURRENT (mA) Minimum Start Voltage vs Load 2.7V at 1A LEDs 14 INPUT VOLTAGE INPUT VOLTAGE Minimum Running Voltage vs Output Voltage 2.7V at 1A LEDs 80 60 40 20 SINGLE LED 2 LEDs 3 LEDs 4 LEDs 22 17 12 7 2 0 0 200 400 600 800 LOAD CURRENT (mA) 1000 8040 G13 0 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 40 8040 G14 2 0 200 400 600 ILOAD (mA) 800 1000 8040 G15 8040p 4 LTM8040 TYPICAL PERFORMANCE CHARACTERISTICS Temp Rise vs Load 3.3V at 1A LEDs, 12VIN Temp Rise vs Load 2.7V at 1A LEDs, 36VIN 47 37 TEMPERATURE RISE (°C) TEMPERATURE RISE (°C) 27 SINGLE LED 2 LEDs 3 LEDs 4 LEDs 42 32 27 22 17 12 SINGLE LED 2 LEDs 3 LEDs 22 17 12 7 7 2 0 200 400 600 ILOAD (mA) 800 2 1000 0 200 400 600 ILOAD (mA) 800 8040 G16 8040 G17 Temp Rise vs Load 3.3V at 1A LEDs, 36VIN 37 1000 27 LED CURRENT (mA) TEMPERATURE RISE (°C) LED Current vs Adjust Voltage 1200 SINGLE LED 2 LEDs 3 LEDs 32 1000 22 17 12 800 600 400 200 7 2 0 200 400 600 ILOAD (mA) 800 1000 8040 G18 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 8040p 5 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 10μA source current pulls PWM high) or pulled up to a voltage source between 1.2V and 10V. formula: ILED = 1A • ADJ/1.25V. If connecting a resistor to GND, the resistor value should be R = 5.11 • ILED /(1Amp – ILED), where R is in KΩ and ILED is the desired current out of LEDA in Amps. Make sure that the voltage at LEDA is at least 2.4V. 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. 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. 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 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 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. BLOCK DIAGRAM SENSE RESISTOR 8.2μH VIN LEDA 0.1μF 4.7μF LPWR BIAS SHDN INTERNAL COMPENSATION CURRENT MODE CONTROLLER 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. 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. Cir- cuitry 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 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: 1. Look at Table 1 and find the row that has the desired input voltage range LED string voltage range and output current. Choose resistors according to the following formula: R2 = 2.65V VUVLO − 2.65V – 10.3µA R1 2. Apply the recommended CIN, RT and RADJ values. where VUVLO is the desired UVLO Threshold 3. Connect BIAS as indicated. Suppose that the output needs to stay off until the input is above 8V. 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 10.3μA to ground from the SHDN pin at the UVLO threshold in order to provide hysteresis. V TH = 8V Let R1 = 100k R2 = 2.65V = 61.9 8V − 2.65V – 10.3µA 100k VIN VIN R1 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 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 switching 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 (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. Table 2. RT vs Frequency RT (kΩ) Frequency (MHz) 13.0 2.00 16.0 1.84 18.7 1.70 24.9 1.50 29.4 1.37 35.8 1.25 54.9 1.00 75.0 0.90 88.7 0.85 137.0 0.75 175.0 0.68 215.0 0.65 Programming LED Current 487.0 0.57 open 0.50 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 2.2μF LTM8040 VIN LEDA SHDN LPWR VADJ BIAS PWM RT VIN 5.5V 3.3V C1 2.2μF WHITE LED VIN LEDA SHDN LPWR VADJ BIAS PWM RT GND 8040 F02a 2.7V RED LED GND 8040 F02b Figure 2b. BIAS May be Tied to XVIN if LPWR is Below 2.6V Figure 2a. Tie BIAS to LPWR if it is Greater Than 2.6V LTM8040 VIN 4V TO 36V C1 2.2μF VIN LEDA SHDN LPWR VADJ BIAS 2.7V 3.3V PWM RT GND RED LED 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 resistor from ADJ to GND as shown in Figure 3, using the following formula: RADJ = 5.11 • ILED /(1Amp – ILED), where RADJ is in kΩ 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 Figure 3. Setting ADJ with a Resistor The LEDA voltage must be at least 2.4V for proper current regulation. 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 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. PWM 60Hz TO 10kHz 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) REF LTM8040 VADJ R2 GND 8040 F04 DIM PWM LTM8040 LEDA GND 8040 F05 Figure 5. Dimming Using PWM Signal 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 8040p 10 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 = 25μs 1A =10:1 0.1A 10ms PWMRATIO = = 400:1 25µs DIMRATIO =10 • 400 = 4000:1 IRATIO = 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. 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 4.7μF 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 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, 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 125°C temperature rating of the LTM8040, so 8040p 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: 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. 1. Place the CIN capacitor as close as possible to the VIN and GND connection of the LTM8040. LEDA PWM LPWR BIAS LED STRING SHDN ADJ RT VIN GND CIN 8040 F06 VIAS TO GND PLANE Figure 6. Suggested Layout 8040p 12 LTM8040 APPLICATION INFORMATION Table 1. Recommended Configuration VIN RANGE CIN LED STRING VOLTAGE (LEDA) LED STRING CURRENT (LEDA) RTOPTIMAL fOPTIMAL RTMIN fMAX RADJ BIAS CONNECTION 4.5-36V 1μF 0805 50V 2.5-4V 35mA open 0.50M open 0.50M 154 2.8V to 25V source 6.5-36V 1μF 0805 50V 4-6V 35mA open 0.50M open 0.50M 154 LPWR 9.5-36V 1μF 0805 50V 6-9V 35mA open 0.50M open 0.50M 154 LPWR 12.5-36V 1μF 0805 50V 8-12V 35mA open 0.50M open 0.50M 154 LPWR 4.5-36V 1μF 0805 50V 2.5-4V 100mA open 0.50M open 0.50M 453 2.8V to 25V source 6.5-36V 1μF 0805 50V 4-6V 100mA open 0.50M 165k 0.70M 453 LPWR 9.5-36V 1μF 0805 50V 6-9V 100mA 487k 0.57M 137k 0.75M 453 LPWR 12.5-36V 1μF 0805 50V 8-12V 100mA 487k 0.57M 88.7k 0.85M 453 LPWR 4.8-36V 1μF 0805 50V 2.5-4V 350mA open 0.50M open 0.50M 2.87k 2.8V to 25V source 7-36V 1μF 0805 50V 4-6V 350mA open 0.50M 165k 0.70M 2.87k LPWR 10.5-36V 1μF 0805 50V 6-9V 350mA 137k 0.75M 54.9k 1.0M 2.87k LPWR 13.8-36V 1μF 0805 50V 8-12V 350mA 75k 0.90M 29.4k 1.37M 2.87k LPWR 4.8-36V 1μF 0805 50V 2.5-4V 500mA open 0.50M open 0.50M 5.11k 2.8V to 25V source 7-36V 1μF 0805 50V 4-6V 500mA open 0.50M 165k 0.70M 5.11k LPWR 10.5-36V 1μF 0805 50V 6-9V 500mA 137k 0.75M 54.9k 1.0M 5.11k LPWR 14.3-36V 1μF 0805 50V 8-12V 500mA 75k 0.90M 29.4k 1.37M 5.11k LPWR 5-36V 1μF 0805 50V 2.5-4V 700mA open 0.50M open 0.50M 11.8k 2.8V to 25V source 7.7-36V 1μF 0805 50V 4-6V 700mA 487k 0.57M 165k 0.70M 11.8k LPWR 11-36V 1μF 0805 50V 6-9V 700mA 165k 0.70M 54.9k 1.0M 11.8k LPWR 14.8-36V 1μF 0805 50V 8-12V 700mA 75k 0.90M 29.4k 1.37M 11.8k LPWR 5.5-36V 1μF 0805 50V 2.5-4V 1A open 0.50M open 0.50M open 2.8V to 25V source 8-36V 1μF 0805 50V 4-6V 1A open 0.50M 137k 0.75M open LPWR 11.5-36V 1μF 0805 50V 6-9V 1A 215k 0.65M 54.9k 1.0M open LPWR 15.5-36V 1μF 0805 50V 8-12V 1A 137k 0.75M 29.4k 1.37M open LPWR 8040p 13 LTM8040 TYPICAL APPLICATIONS Step Down 1A Drive with Single Red or White LED VIN* 5.5V TO 25V VIN 1μF LTM8040 LEDA SHDN LPWR 2.5V TO 4V 1A BIAS ADJ 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 VIN 1μF LTM8040 LEDA SHDN LPWR BIAS 2.87k 6V TO 9V 350mA 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 VIN* 15.5V TO 36V VIN LTM8040 LEDA SHDN LPWR 1μF BIAS 8V TO 12V 1A ADJ PWM RT 137k 750kHz GND 8040 TA04 *RUNNING VOLTAGE. SEE APPLICATION INFORMATION FOR START-UP REQUIREMENTS 8040p 14 1.270 3.810 2.540 1.270 0.000 0.9525 1.5875 2.540 SUGGESTED PCB LAYOUT TOP VIEW 1.270 PACKAGE TOP VIEW 0.000 X 5.080 9.000 BSC Y aaa Z 2.40 – 2.60 DETAIL A MOLD CAP Z 0.27 – 0.37 SUBSTRATE 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 4 SYMBOL TOLERANCE 0.15 aaa 0.10 bbb 6. THE TOTAL NUMBER OF PADS: 66 5. PRIMARY DATUM -Z- IS SEATING PLANE LAND DESIGNATION PER JESD MO-222, SPP-010 AND SPP-020 3 2. ALL DIMENSIONS ARE IN MILLIMETERS TRAY PIN 1 BEVEL COMPONENT PIN 1 3 PADS SEE NOTES 1.270 BSC 0.605 – 0.665 7.620 BSC NOTES: 1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994 DETAIL A PACKAGE SIDE VIEW 2.670 – 2.970 bbb Z aaa Z 3.810 4 0.9525 1.270 1.5875 PAD 1 CORNER 6.350 15.000 BSC L K H F E PACKAGE BOTTOM VIEW G D C LGA 66 0407 REV Ø PACKAGE IN TRAY LOADING ORIENTATION LTMXXXXXX μModule J 12.700 BSC 0.605 – 0.665 B A PAD 1 C (0.30) 1 2 3 4 5 6 7 LTM8040 PACKAGE DESCRIPTION LGA Package 66-Lead (15mm × 9mm × 2.82mm) (Reference LTC DWG # 05-08-1810 Rev Ø) 8040p 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. 15 6.350 5.080 3.810 2.540 2.540 3.810 LTM8040 TYPICAL APPLICATION Step Down 1A Drive with Four Series Red LEDs VIN* 15.5V TO 36V VIN LTM8040 LEDA SHDN LPWR 1μF BIAS 8V TO 12V 1A ADJ PWM RT 137k 750kHz GND 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 and Remote Sensing Synchronizable, PolyPhase® Operation, LTM4601-1 Version Has No Remote Sensing LTM4602 6A DC/DC μModule Pin-Compatible with the LTM4600 LTM4603 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 4A Low VIN DC/DC μModule 2.375V ≤ VIN ≤ 5V, 0.8V ≤ VOUT ≤ 5V, 9mm × 15mm × 2.3mm LGA LTM4605 5A to 12A Buck-Boost μModule High Efficiency, Adjustable Frequency, 4.5V ≤ VIN ≤ 20V, 0.8V ≤ VOUT ≤ 16V, 15mm × 15mm × 2.8mm LTM4607 5A to 12A Buck-Boost μModule High Efficiency, Adjustable Frequency, 4.5V ≤ VIN ≤ 36V, 0.8V ≤ VOUT ≤ 25V, 15mm × 15mm × 2.8mm LTM4608 8A Low VIN DC/DC μModule 2.375V ≤ VIN ≤ 5V, 0.8V ≤ VOUT ≤ 5V, 9mm × 15mm × 2.8mm LGA LTM8020 36V, 200mA DC/DC μModule 4V ≤ VIN ≤ 36V, 1.25V ≤ VOUT ≤ 5V, 6.25mm × 6.25mm × 2.3mm LGA LTM8022 1A, 36V DC/DC μModule Adjustable Frequency, 0.8V ≤ VOUT ≤ 5V, 11.25mm × 9mm × 2.82mm, Pin-Compatible to the LTM8023 LTM8023 2A, 36V DC/DC μModule Adjustable Frequency, 0.8V ≤ VOUT ≤ 5V, 11.25mm × 9mm × 2.82mm, Pin-Compatible to the LTM8022 PolyPhase is a trademark of Linear Technology Corporation 8040p 16 Linear Technology Corporation LT 0808 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008