LED2000 3 A monolithic step-down current source with synchronous rectification Datasheet - production data Applications High brightness LED driving Halogen bulb replacement General lighting Signage SO8 VFQFPN8 4x4 Description Features The LED2000 is an 850 kHz fixed switching frequency monolithic step-down DC-DC converter designed to operate as precise constant current source with an adjustable current capability up to 3 A DC. The embedded PWM dimming circuitry features LED brightness control.
Contents LED2000 Contents 1 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 Thermal data . . . . . . .
LED2000 Contents 7.2 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 7.3 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 7.4 Short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 7.5 Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 8 Typical characteristics . . . . . . . . . . .
List of tables LED2000 List of tables Table 1. Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. Table 8. Table 9. Table 10. Table 11. Table 12. 4/40 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Thermal data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LED2000 List of figures List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22. Figure 23. Figure 24. Figure 25. Figure 26. Figure 27. Figure 28. Figure 29. Figure 30. Figure 31. Figure 32. Figure 33. Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pin settings LED2000 1 Pin settings 1.1 Pin connection Figure 2. Pin connection (top view) SW 1 8 PGND DIM VINSW GND AGND VINA NC DIM 4 FB SO8 - BW VFQFPN 1.2 5 AM12893v1 Pin description Table 1. Pin description Package/pin Type 6/40 VFQFPN S08-BW 1 3 Description VINA Analog circuitry power supply connection 2 4 DIM Dimming control input. Logic low prevents the switching activity, logic high enables it. A square wave on this pin implements LED current PWM dimming.
LED2000 2 Maximum ratings Maximum ratings Table 2. Absolute maximum ratings Symbol Value Power input voltage -0.3 to 20 VINA Input voltage -0.3 to 20 VDIM Dimming voltage VSW Output switching voltage VPG Power Good -0.3 to VIN VFB Feedback voltage -0.3 to 2.5 IFB FB current VINSW 3 Parameter -0.
Electrical characteristics 4 LED2000 Electrical characteristics TJ = 25 °C, VCC = 12 V, unless otherwise specified. Table 4. Electrical characteristics Value Symbol Parameter Test conditions Unit Min. Operating input voltage range VIN See(1) Typ. 3 Max. 18 Device ON level 2.6 2.75 2.9 Device OFF level 2.4 2.55 2.
LED2000 5 Functional description Functional description The LED2000 device is based on a “peak current mode” architecture with fixed frequency control. As a consequence, the intersection between the error amplifier output and the sensed inductor current generates the control signal to drive the power switch.
Functional description 5.1 LED2000 Power supply and voltage reference The internal regulator circuit consists of a startup circuit, an internal voltage pre-regulator, the BandGap voltage reference and the bias block that provides current to all the blocks. The starter supplies the startup current to the entire device when the input voltage goes high and the device is enabled.
LED2000 Functional description Table 5. Uncompensated error amplifier characteristics Description Value Transconductance 250 µS Low frequency gain 96 dB CC 195 pF RC 70 K The error amplifier output is compared with the inductor current sense information to perform PWM control. 5.5 Thermal shutdown The shutdown block generates a signal that disables the power stage if the temperature of the chip goes higher than a fixed internal threshold (150 ± 10 °C typical).
Application notes LED2000 6 Application notes 6.1 Closing the loop Figure 5. Block diagram of the loop GCO(s) VIN PWM control Current sense HS switch L VOUT LC filter LS switch COUT error PWM + amplifier VCONTROL + comparator RC FB VREF RS compensation network CC α LED A O(s) 6.
LED2000 Application notes Equation 2 1 Z = ------------------------------ESR C OUT Equation 3 m C 1 – D – 0,5 1 P = -------------------------------------- + --------------------------------------------R LOAD C OUT L C OUT f SW where: Equation 4 Se m C = 1 + -----Sn S = V f pp SW e V IN – V OUT S = ----------------------------- Ri n L Sn represents the slope of the sensed inductor current, Se the slope of the external ramp (VPP peak-to-peak amplitude) that implements
Application notes LED2000 Figure 6. Transconductance embedded error amplifier E/A + COMP - FB RC CP CC V+ R0 dV C0 Gm dV RC CP CC AM12897v1 RC and CC introduce a pole and a zero in the open loop gain. CP does not significantly affect system stability but it is useful to reduce the noise at the output of the error amplifier.
LED2000 Application notes The embedded compensation network is RC = 70 K, CC = 195 pF while CP and CO can be considered as negligible. The error amplifier output resistance is 240 Mso the relevant singularities are: Equation 12 f Z = 11 6 kHz 6.4 f P LF = 3 4 Hz LED small signal model Once the system reaches the working condition, the LEDs composing the row are biased and their equivalent circuit can be considered as a resistor for frequencies << 1 MHz.
Application notes LED2000 Figure 7. Equivalent series resistor [A] 1 working point 0.1 2 1 3 [V] 4 AM12898v1 Figure 8 shows the equivalent circuit of the LED constant current generator. Figure 8.
LED2000 Application notes As a consequence, the LED equivalent circuit gives the LED(s) term correlating the output voltage with the high impedance FB input: Equation 14 R SENSE LED n LED = ---------------------------------------------------------n LED r LED + R SENSE 6.5 Total loop gain In summary, the open loop gain can be expressed as: Equation 15 G s = G CO s A 0 s LED n LED Example 1 Design specification: VIN = 12 V, VFW_LED = 3.5 V, nLED = 2, rLED = 1.
Application notes LED2000 Figure 9. Module plot (;7(51$/ /223 02'8/( 0RGXOH >G%@ )UHTXHQF\ >+]@ $0 Y Figure 10.
LED2000 6.6 Application notes Dimming operation The dimming input disables the switching activity, masking the PWM comparator output. The inductor current dynamic when dimming input goes high depends on the designed system response. The best dimming performance is obtained maximizing the bandwidth and phase margin, when it is possible. As a general rule, the output capacitor minimization improves the dimming performance. Figure 11.
Application notes LED2000 Figure 12. LED current falling edge operation AM12903v1 Dimming frequency vs. dimming depth As seen in Section 6.6, the LEDs current rising and falling edge time mainly depends on the system bandwidth (TRISE) and the selected output capacitor value (TRISE and TFALL). The dimming performance depends on the minimum current pulse shape specification of the final application.
LED2000 Application notes Figure 13. Dimming signal AM12904v1 For example, assuming a 1 kHz dimming frequency the maximum dimming depth is 5% or, given a 2% dimming depth, it follows a 200 Hz maximum fDIM. The LED2000 dimming performance is strictly dependent on the system small signal response. As a consequence, an optimized compensation (good phase margin and bandwidth maximized) and minimized COUT value are crucial for the best performance. 6.
Application information LED2000 The software calculates external components according to the internal database. It is also possible to define new components and ask the software to use them. Bode plots, estimated efficiency and thermal performance are provided. Finally, the user can save the design and print all the information including the bill of material of the board. 7 Application information 7.1 Component selection 7.1.
LED2000 Application information Figure 15.
Application information LED2000 Equation 27 I RIPPLE s=j = I RIPPLE_SPEC Example 2(see Example 1): VIN = 12 V, ILED = 700 mA, ILED/ILED = 2%, VFW_LED = 3.5 V, nLED = 2. A lower inductor value maximizes the inductor current slew rate for better dimming performance. Equation 26 becomes: Equation 28 I L ----------- = 0,5 I LED which is satisfied selecting a10 H inductor value. The output capacitor value must be dimensioned according to Equation 27.
LED2000 Application information where is the expected system efficiency, D is the duty cycle and IO is the output DC current. Considering = 1 this function reaches its maximum value at D = 0.5 and the equivalent RMS current is equal to IO divided by 2.
Application information 7.2 LED2000 Layout considerations The layout of switching DC-DC converters is very important to minimize noise and interference. Power-generating portions of the layout are the main cause of noise and so high switching current loop areas should be kept as small as possible and lead lengths as short as possible. High impedance paths (in particular the feedback connections) are susceptible to interference, so they should be as far as possible from the high current paths.
LED2000 7.3 Application information Thermal considerations The dissipated power of the device is tied to three different sources: Conduction losses due to the RDSON, which are equal to: Equation 33 2 P ON = R RDSON_HS I OUT D 2 P OFF = R RDSON_LS I OUT 1 – D where D is the duty cycle of the application.
Application information LED2000 Example 3(see Example 1): VIN = 12 V, VFW_LED = 3.5 V, nLED = 2, ILED = 700 mA The typical output voltage is: Equation 36 V OUT = n LED V FW_LED + V FB = 7,1V RDSON_HS has a typical value of 95 m and RDSON_LS is 69 m at 25 °C. For the calculation we can estimate RDSON_HS = 140 m and RDSON_LS = 100 mas a consequence of TJ increase during the operation. TSW_EQ is approximately 12 ns. IQ has a typical value of 1.5 mA at VIN = 12 V.
LED2000 Application information In overcurrent condition, the duty cycle is strongly reduced and, in most applications, this is enough to limit the switch current to the current threshold.
Application information LED2000 higher than the nominal value thanks to the increased voltage drop across stray components. In most of the application conditions the pulse-by-pulse current limitation is effective to limit the inductor current. Whenever the current escalates, a second level current protection called “Hiccup mode” is enabled.
LED2000 Application information Table 8. Component list Reference Part number Description Manufacturer 100 nF 50 V (size 0805) C1 C2 GRM31CR61E106KA12L 10 F 25 V (size 1206) Murata C3 GRM21BR71E225KA73L 2.2 F 25 V (size 0805) Murata R1 4.7 K5% (size 0603) R2 Not mounted Rs ERJ14BSFR15U 0.15 1% (size 1206) Panasonic L1 XAL6060-223ME 22 H ISAT = 5.6 A (30% drop) IRMS = 6.9 A (40 C rise) (size 6.36 x 6.56 x 6.1 mm) Coilcraft Figure 20.
Application information LED2000 Figure 21. PCB layout (bottom side) VFQFPN package Figure 22. PCB layout (component side) SO8 package It is strongly recommended that the input capacitors are to be put as close as possible to the pins, see C1 and C2.
LED2000 Application information Figure 23.
Typical characteristics 8 LED2000 Typical characteristics Figure 24. Soft-start Figure 25. Load regulation Vin 12V Vled 7V AM12913v1 Figure 26. Dimming operation AM12914v1 Figure 27.
LED2000 Typical characteristics Figure 28. LED current falling edge Figure 29. Hiccup current protection To maximize the dimming performan ce the embedded LS discharges C O UT w hen D IM goes lo w. (D IM = 0 & & V F B > 60mV ): the low side is enabled as long as I L > -1A (implements negative current limitation) AM12918v1 AM12917v1 Figure 30. Efficiency vs. IOUT (VIN 32 V) Figure 31. Thermal shutdown protection 130 ns typ.
Package information 9 LED2000 Package information In order to meet environmental requirements, ST offers these devices in different grades of ECOPACK® packages, depending on their level of environmental compliance. ECOPACK specifications, grade definitions and product status are available at: www.st.com. ECOPACK is an ST trademark. Figure 32. VFQFPN8 (4 x 4 x 1.
LED2000 Package information Table 9. VFQFPN8 (4 x 4 x 1.08 mm) mechanical data Dimensions (mm) Symbol Min. Typ. Max. 0.80 0.90 1.00 A1 0.02 0.05 A3 0.20 A b 0.23 0.30 0.38 D 3.90 4.00 4.10 D2 2.82 3.00 3.23 E 3.90 4.00 4.10 E2 2.05 2.20 2.30 e L 0.80 0.40 DocID023432 Rev 4 0.50 0.
Package information LED2000 Figure 33. SO8 package outline 0016023_Rev_E Table 10. SO8 mechanical data Dimensions (mm) Symbol Min. Typ. A 1.75 A1 0.10 0.25 A2 1.25 b 0.28 0.48 c 0.17 0.23 D 4.80 4.90 5.00 E 5.80 6.00 6.20 E1 3.80 3.90 4.00 e 1.27 h 0.25 0.50 L 0.40 1.27 L1 k 1.04 0° 8° ccc 38/40 Max. 0.
LED2000 10 Ordering information Ordering information Table 11. Ordering information 11 Order code Package Packaging LED2000PUR VFQFPN 4 x 4 8L Tape and reel LED2000DR SO8-BW Tape and reel Revision history Table 12. Document revision history Date Revision 11-Jul-2012 1 Initial release. 27-Jul-2012 2 Document status promoted form preliminary to production data. 16-Oct-2012 3 Figure 22 and Figure 23 have been added. 4 Unified package names in the whole document.
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