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this is information on a product in full production. may 2013 docid024346 rev 1 1/42 42 LED2001 4 a monolithic step-down current source with synchronous rectification datasheet - production data features ? 3.0 v to 18 v operating input voltage range ? 850 khz fixed switching frequency ? 100 mv typ. current sense voltage drop ? pwm dimming ? 7% output current accuracy ? synchronous rectification ? 95 m hs / 69 m ls typical r ds(on) ? peak current mode architecture ? embedded compensation network ? internal current limiting ? ceramic output capacitor compliant ? thermal shutdown applications ? high brightness led driving ? halogen bulb replacement ? general lighting ? signage description the LED2001 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 4 a dc. the embedded pwm dimming circuitry features led brightness control. the regulated output current is set connecting a sensing resistor to the feedback pin. the embedded synchronous rectification and the 100 mv typical r sense voltage drop enhance the efficiency performance. the size of the overall application is minimized thanks to the high switching frequency and ceramic output capacitor compatibility. the device is fully protected against thermal overheating, overcurrent and output short-circuit. the LED2001 is available in vfqfpn 4 mm x 4 mm 8-lead package, and hsop8. figure 1. typical application circuit vfqfpn8 4x4 hsop8 &,1 &,1 &)/7 &)/7 9,1 9,1 * 1' * 1' ',0 ',0 56 56 &287 &287 /(' 9,1b$ ',0 )% $*1' 3*1' 6: 9,1b6: (3 / / $0y www.st.com
contents LED2001 2/42 docid024346 rev 1 contents 1 pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1 pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2 pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.1 power supply and voltage reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.2 voltage monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.3 soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.4 error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.5 thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 6 application notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.1 closing the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.2 g co (s) control to output transfer function . . . . . . . . . . . . . . . . . . . . . . . . 12 6.3 error amplifier compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.4 led small signal model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.5 total loop gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 6.6 dimming operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.6.1 dimming frequency vs. dimming depth . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.7 edesign studio software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 7 application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 7.1 component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 7.1.1 sensing resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 7.1.2 inductor and output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . 22 7.1.3 input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 7.2 layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
docid024346 rev 1 3/42 LED2001 contents 7.3 thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 7.4 short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 7.5 application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 8 typical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 9 ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 10 package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 11 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
list of tables LED2001 4/42 docid024346 rev 1 list of tables table 1. pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 table 2. absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 table 3. thermal data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 table 4. electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 table 5. uncompensated error amplifier characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 table 6. inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 table 7. list of ceramic capacitors for the LED2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 table 8. component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 table 9. ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 table 10. vfqfpn8 (4x4x1.08 mm) mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 table 11. hsop8 mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 table 12. document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
docid024346 rev 1 5/42 LED2001 list of figures list of figures figure 1. typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 2. pin connection (top view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 3. LED2001 block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 4. internal circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 5. block diagram of the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 6. transconductance embedded error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 7. equivalent series resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 8. load equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 9. module plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 10. phase plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 11. dimming operation example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 figure 12. led current falling edge operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 figure 13. dimming signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 14. edesign studio screenshot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 15. equivalent circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 16. layout example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 17. switching losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 figure 18. constant current protection triggering hiccup mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 figure 19. demonstration board application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 figure 20. pcb layout (component side) dfn package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 figure 21. pcb layout (bottom side) dfn package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 figure 22. pcb layout (component side) hsop8 package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 figure 23. pcb layout (bottom side) hsop8 package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 figure 24. soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 figure 25. load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 figure 26. dimming operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 figure 27. led current rising edge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 figure 28. led current falling edge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 figure 29. hiccup current protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 figure 30. ocp blanking time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 figure 31. thermal shutdown protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 figure 32. vfqfpn8 (4x4x1.08 mm) package dimensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 figure 33. hsop8 package dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
pin settings LED2001 6/42 docid024346 rev 1 1 pin settings 1.1 pin connection figure 2. pin connection (top view) 1.2 pin description 1 4 5 8 vina dim fb pgnd sw vinsw nc nc vfqfpn8 4x4 hsop8 dim gnd 9 2 3 6 7 9 am12893v1 table 1. pin description package/pin type description vfqfpn8 4x4 hs0p8 1 1 vin a analog circuitry power supply connection 22 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. connect to vin a if not used (see section 6.6 ) 33 fb feedback input. connect a proper sensing resistor to set the led current 4 4 agnd analog circuitry ground connection 5 - nc not connected 6 6 vin sw power input voltage 7 7 sw regulator switching pin 8 8 pgnd power ground ep ep exposed pad connect the exposed pad to agnd
docid024346 rev 1 7/42 LED2001 maximum ratings 2 maximum ratings 3 thermal data table 2. absolute maximum ratings symbol parameter value unit v insw power input voltage -0.3 to 20 v v ina input voltage -0.3 to 20 v dim dimming voltage -0.3 to v ina v sw output switching voltage -1 to v in v pg power good -0.3 to v in v fb feedback voltage -0.3 to 2.5 i fb fb current -1 to +1 ma p tot power dissipation at t a < 60 c 2 w t op operating junction temperature range -40 to 150 c t stg storage temperature range -55 to 150 c table 3. thermal data symbol parameter value unit r thja maximum thermal resistance junction-ambient (1) 1. package mounted on demonstration board. vfqfpn8 4x4 40 c/w hsop8
electrical characteristics LED2001 8/42 docid024346 rev 1 4 electrical characteristics t j =25 c, v cc =12 v, unless otherwise specified. table 4. electrical characteristics symbol parameter test conditions value unit min. typ. max. v in operating input voltage range (1) 1. specifications referred to t j from -40 to +125 c. specifications in the -40 to +125 c temperature range are assured by design, characterization and statistical correlation. 318 v device on level 2.6 2.75 2.9 device off level 2.4 2.55 2.7 v fb feedback voltage tj=25 c 90 97 104 mv tj=125 c (1) 90 100 110 i fb v fb pin bias current (1) 600 na r dson -p high-side switch on- resistance i sw =750 ma 95 m r dson -n low-side switch on- resistance i sw =750 ma 69 m i lim maximum limiting current (2) 2. guaranteed by design. 5.6 a oscillator f sw switching frequency 0.7 0.85 1 mhz d duty cycle (2) 0100% dc characteristics i q quiescent current 1.5 2.5 ma dimming v dim dim threshold voltage switching activity 1.2 v switching activity prevented 0.4 i dim dim current 2 a soft-start t ss soft-start duration 1 ms protection t shdn thermal shutdown 150 c hystereris 15
docid024346 rev 1 9/42 LED2001 functional description 5 functional description the LED2001 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. the main internal blocks shown in the block diagram in figure 3 are: ? high-side and low-side embedded power element for synchronous rectification; ? a fully integrated sawtooth oscillator with a typical frequency of 850 khz; ? a transconductance error amplifier; ? an high-side current sense amplifier to track the inductor current; ? a pulse width modulator (pwm) comparator and the circuitry necessary to drive the internal power element; ? the soft-start circuitry to decrease the inrush current at power-up; ? the current limitation circuit based on the pulse-by-pulse current protection with frequency divider; ? the dimming circuitry for output current pwm; ? the thermal protection function circuitry. figure 3. LED2001 block diagram w s n i v a n i v osc e/a driver driver dmd otp mosfet control logic regulator dimming i_ sense comp pwm ocp ref 0.1v soft-start vsum vc ocp uvlo vdrv _p vdrv _n i2v r sense sw gndp gnda dim fb am12894v1
functional description LED2001 10/42 docid024346 rev 1 5.1 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. the pre-regulator block supplies the bandgap cell with a pre- regulated voltage that has a very low supply voltage noise sensitivity. 5.2 voltage monitor an internal block continuously senses the v cc , v ref and v bg . if the monitored voltages are good, the regulator begins operating. there is also a hysteresis on the v cc (uvlo). figure 4. internal circuit 5.3 soft-start the startup phase is implemented ramping the reference of the embedded error amplifier in 1 ms typ. time. it minimizes the inrush current and decreases the stress of the power components at power-up. during normal operation a new soft-start cycle takes place in case of: ? thermal shutdown event; ? uvlo event. the soft-start is disabled when dim input goes high in order to maximize the dimming performance. 5.4 error amplifier the voltage error amplifier is the core of the loop regulation. it is a transconductance operational amplifier whose non-inverting input is connected to the internal voltage reference (100 mv), while the inverting input (fb) is connected to the output current sensing resistor. the error amplifier is internally compensated to minimize the size of the final application. starter preregulator ic bias bandgap vref vreg vcc d00in126 am12895v1
docid024346 rev 1 11/42 LED2001 functional description 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). the sensing element of the chip is close to the pdmos area, ensuring fast and accurate temperature detection. a 15 c typical hysteresis prevents the device from turning on and off continuously during the protection operation. table 5. uncompensated error amplifier characteristics description value transconductance 250 s low frequency gain 96 db c c 195 pf r c 70 k
application notes LED2001 12/42 docid024346 rev 1 6 application notes 6.1 closing the loop figure 5. block diagram of the loop 6.2 g co (s) control to output transfer function the accurate control to output transfer function for a buck peak current mode converter can be written as: equation 1 where r 0 represents the load resistance, r i the equivalent sensing resistor of the current sense circuitry, p the single pole introduced by the lc filter and z the zero given by the esr of the output capacitor. f h (s) accounts for the sampling effect performed by the pwm comparator on the output of the error amplifier that introduces a double pole at one half of the switching frequency. v in - + - lc filter v ref error amplifier fb compensation network pwm comparator hs switch r s l c out r c c c + pwm control current sense ls switch v out v control g co (s) a o (s) led g co s () r 0 r i ------ - 1 1 r 0 t sw ? l ---------------------- - m c 1d ? () 0.5 ? ? [] ? + ---------------------------------------------------------------------------------------- 1 s z ------ + ?? ?? 1 s p ------ + ?? ?? --------------------- - f h s () ??? =
docid024346 rev 1 13/42 LED2001 application notes equation 2 equation 3 where: equation 4 s n represents the slope of the sensed inductor current, s e the slope of the external ramp (v pp peak-to-peak amplitude) that implements the slope compensation to avoid sub- harmonic oscillations at duty cycle over 50%. the sampling effect contribution f h (s) is: equation 5 where: equation 6 and equation 7 6.3 error amplifier compensation network the LED2001 embeds (see figure 6 ) the error amplifier and a pre-defined compensation network which is effective in stabilizing the system in most application conditions. z 1 esr c out ? ------------------------------- = p 1 r load c out ? ------------------------------------- - m c 1d ? () 0.5 ? ? lc out f sw ?? --------------------------------------------- + = m c 1 s e s n ------ + = s e v pp f sw ? = s n v in v out ? l ----------------------------- - r i ? = ? ? ? ? ? ? ? ? () 1 1 s n q p ? ------------------ - s 2 n 2 ------ ++ ------------------------------------------ - = n f sw ? = q p 1 m c 1d ? () 0.5 ? ? [] ? ---------------------------------------------------------- =
application notes LED2001 14/42 docid024346 rev 1 figure 6. transconductance embedded error amplifier r c and c c introduce a pole and a zero in the open loop gain. c p does not significantly affect system stability but it is useful to reduce the noise at the output of the error amplifier. the transfer function of the error amplifier and its compensation network is: equation 8 where a vo = g m r o . the poles of this transfer function are (if c c >> c 0 +c p ): equation 9 equation 10 whereas the zero is defined as: equation 11 + - c p r c c c fb comp dv r 0 g m dv v + e/a r c c c c p c 0 am12897v1 a 0 s () a v0 1s + r c c c ?? () ? s 2 r 0 c 0 c p + () r c c c sr 0 c c ? r 0 c 0 c p + () r c c c ? + ? + () 1 + ? + ?? ?? ------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------- = f p lf 1 2 r 0 c c ?? ? --------------------------------- - = f p hf 1 2 r c c 0 c p + () ?? ? ---------------------------------------------------- = f z 1 2 r c c c ?? ? --------------------------------- =
docid024346 rev 1 15/42 LED2001 application notes the embedded compensation network is r c =70 k, c c =195 pf while c p and c o can be considered as negligible. the error amplifier output resistance is 240 m , so the relevant singularities are: equation 12 6.4 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. the led manufacturer typically provides the equivalent dynamic resistance of the led biased at different dc currents. this parameter is required to study the behavior of the system in the small signal analysis. for instance, the equivalent dynamic resistance of the luxeon iii star from lumiled measured with different biasing current level is reported below: equation 13 if the led datasheet does not report the equivalent resistor value, it can be simply derived as the tangent to the diode i-v characteristic in the present working point (see figure 7 ). f z 11 6 khz , = f p lf 34 hz , = r led 1.3 i led 350ma = 0.9 i led 700ma = ? ? ?
application notes LED2001 16/42 docid024346 rev 1 figure 7. equivalent series resistor figure 8 shows the equivalent circuit of the led constant current generator. figure 8. load equivalent circuit 1 0.1 1 2 3 [v] 4 [a] working point am12898v1 vin rd1 cout l l rd2 rs dled1 rs d1 d cout vin l dled2 am12899v1
docid024346 rev 1 17/42 LED2001 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 6.5 total loop gain in summary, the open loop gain can be expressed as: equation 15 example 1 design specification: v in =12 v, v fw_led =3.5 v, n led = 2, r led = 1.1 , i led = 4 a, i led ripple = 2% the inductor and capacitor value are dimensioned in order to meet the i led ripple specification (see section 7.1.2 for output capacitor and inductor selection guidelines): l=2.2 h, c out =2.2 f mlcc (negligible esr) accordingly, with section 7.1.1 the sensing resistor value is: equation 16 equation 17 the gain and phase margin bode diagrams are plotted respectively in figure 9 and figure 10 . led n led () r sense n led r led r sense + ? --------------------------------------------------------- - = gs () g co s () a 0 s () led n led () ?? = r s 100 mv 4a -------------------- 25 m ? = led n led () r sense n led r led r sense + ? --------------------------------------------------------- - = 25 m 21.1 ? 25 m + --------------------------------------------- - 0.011 ==
application notes LED2001 18/42 docid024346 rev 1 figure 9. module plot figure 10. phase plot the cut-off frequency and the phase margin are: equation 18 6.6 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. $0y [ [ [ [ [ (;7(51$//22302'8/( )uhtxhqf\>+]@ 0rgxoh>g%@ $0y [ [ [ [ [ (;7(51$//223*$,13+$6( )uhtxhqf\>+]@ 3kdvh f c 14 khz = pm 120 =
docid024346 rev 1 19/42 LED2001 application notes figure 11. dimming operation example in fact, when dimming enables the switching activity, a small capacitor value is fast charged with low inductor value. as a consequence, the leds current rising edge time is improved and the inductor current oscillation reduced. an oversized output capacitor value requires extra current for fast charge so generating certain inductor current oscillations the switching activity is prevented as soon as the dimming signal goes low. nevertheless, the led current drops to zero only when the voltage stored in the output capacitor goes below a minimum voltage determined by the selected leds. as a consequence, a big capacitor value makes the led current falling time worse than a smaller one. the LED2001 embeds dedicated circuitry to improve led current falling time. as soon as the dimming input goes low, the low-side is kept enabled to discharge c out until the led current drops to 60% of the nominal current. a negative current limitation (-1 a typical) protects the device during this operation (see figure 12 ). am12902v1
application notes LED2001 20/42 docid024346 rev 1 figure 12. led current falling edge operation 6.6.1 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 (t rise ) and the selected output capacitor value (t rise and t fall ). the dimming performance depends on the minimum current pulse shape specification of the final application. the ideal minimum current pulse has rectangular shape, however, it degenerates into a trapezoid or, at worst, into a triangle, depending on the ratio (t rise + t fall )/ t dim . equation 19 the small signal response in figure 11 and figure 12 is considered as an example. equation 20 assuming the minimum current pulse shape specification as: equation 21: it is possible to calculate the maximum dimming depth given the dimming frequency or vice versa. am12903v1 rec gle tan t rise t fall + t dim -------------------------------------------- - 1 ? trapezoid t rise t fall + t dim -------------------------------------------- - 1 < triangle t rise t fall + t dim -------------------------------------------- - 1 = t rise 20 s ? t fall 5 s ? ? ? ? + 0.5 t min_pulse ? 0.5 d min t dimming ?? ==
docid024346 rev 1 21/42 LED2001 application notes figure 13. dimming signal 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 f dim . the LED2001 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 c out value are crucial for the best performance. 6.7 edesign studio software the LED2001 is supported by the edesign software which can be viewed online at www.st.com. figure 14. edesign studio screenshot the software easily supports the component sizing according to the technical information given in this datasheet (see section 6 and section 7 ). the end user is requested to fill in the requested information such as the input voltage range, the selected led parameters and the number of leds composing the row. am12904v1
application information LED2001 22/42 docid024346 rev 1 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 7.1.1 sensing resistor in closed loop operation the LED2001 feedback pin voltage is 100 mv, so the sensing resistor calculation is expressed as: equation 22 since the main loop (see section 6.1 ) regulates the sensing resistor voltage drop, the average current is regulated into the leds. the integration period is at minimum 5*t sw since the system bandwidth can be dimensioned up to f sw /5 at maximum. the system performs the output current regulation over a period which is at least five times longer than the switching frequency. the output current regulation neglects the ripple current contribution and its reliance on external parameters like input voltage and output voltage variations (line transient and led forward voltage spread). this performance can not be achieved with simpler regulation loops such as a hysteretic control. for the same reason, the switching frequency is constant over the application conditions, which helps to tune the emi filtering and to guarantee the maximum led current ripple specification in the application range. this performance can not be achieved using constant on/off-time architecture. 7.1.2 inductor and output capacitor selection the output capacitor filters the inductor current ripple that, given the application condition, depends on the inductor value. as a consequence, the led current ripple, that is the main specification for a switching current source, depends on the inductor and output capacitor selection. r s 100 mv i led -------------------- =
docid024346 rev 1 23/42 LED2001 application information figure 15. equivalent circuit the led ripple current can be calculated as the inductor ripple current ratio flowing into the output impedance using the laplace transform (see figure 11 ): equation 23 where the term 8/ 2 represents the main harmonic of the inductor current ripple (which has a triangular shape) and i l is the inductor current ripple. equation 24 so l value can be calculated as: equation 25 where t off is the off-time of the embedded high switch, given by 1-d. as a consequence, the lower the inductor value (so the higher the current ripple), the higher the c out value would be to meet the specification. a general rule to dimension l value is: equation 26 finally, the required output capacitor value can be calculated equalizing the led current ripple specification with the module of the fourier transformer (see equation 23 ) calculated at f sw frequency. '&5 '&5 &287 5v 9,1 / (65 5g 9,1 / / (65 'ohg ' ' &287 ' ' 5v 5gq 'ohgq $0y i ripple s () 8 2 ----- - i l 1 s esr c out ?? + () ?? 1sr s esr n led r led ? ++ () c out ?? + ---------------------------------------------------------------------------------------------------------- - = i l v out l -------------- t off ? n led v fw_led 100mv + ? l ----------------------------------------------------------------- - t off ? == l n led v fw_led 100mv + ? i l ----------------------------------------------------------------- - t off ? n led v fw_led 100mv + ? i l ----------------------------------------------------------------- - 1 n led v fw_led 100mv + ? v in ----------------------------------------------------------------- - ? ?? ?? ? == i l i led ----------- 0.5
application information LED2001 24/42 docid024346 rev 1 equation 27 example (see section example 1 ): v in =12 v, i led =700 ma, iled /i led =2%, v fw_led =3.5 v, n led =2. a lower inductor value maximizes the inductor current slew rate for better dimming performance. equation 26 becomes: equation 28 which is satisfied selecting a10 h inductor value. the output capacitor value must be dimensioned according to equation 27 . finally, given the selected inductor value, a 2.2 f ceramic capacitor value keeps the led current ripple ratio lower than the 2% of the nominal current. an output ceramic capacitor type (negligible esr) is suggested to minimize the ripple contribution given a fixed capacitor value. 7.1.3 input capacitor the input capacitor must be able to support the maximum input operating voltage and the maximum rms input current. since step-down converters draw current from the input in pulses, the input current is squared and the height of each pulse is equal to the output current. the input capacitor must absorb all this switching current, whose rms value can be up to the load current divided by two (worst case, with duty cycle of 50%). for this reason, the quality of these capacitors must be very high to minimize the power dissipation generated by the internal esr, thereby improving system reliability and efficiency. the critical parameter is usually the rms current rating, which must be higher than the rms current flowing through the capacitor. the maximum rms input current (flowing through the input capacitor) is: equation 29 where is the expected system efficiency, d is the duty cycle and i o is the output dc current. considering = 1 this function reaches its maximum value at d = 0.5 and the table 6. inductor selection manufacturer series inductor value ( h) saturation current (a) wurth elektronik we-hci 7040 1 to 4.7 20 to 7 we-hci 7050 4.9 to 10 20 to 4.0 coilcraft xpl 7030 2.2 to 10 29 to 7.2 i ripple s=j ? () i ripple_spec = i l i led ----------- 0.5 = i rms i o d 2d 2 ? -------------- - ? d 2 2 ------ - + ? =
docid024346 rev 1 25/42 LED2001 application information equivalent rms current is equal to i o divided by 2. the maximum and minimum duty cycles are: equation 30 and equation 31 where v f is the free-wheeling diode forward voltage and v sw the voltage drop across the internal pdmos. considering the range d min to d max , it is possible to determine the max. i rms going through the input capacitor. capacitors that can be considered are: ? electrolytic capacitors: these are widely used due to their low price and their availability in a wide range of rms current ratings. the only drawback is that, considering ripple current rating requirements, they are physically larger than other capacitors. ? ceramic capacitors: if available for the required value and voltage rating, these capacitors usually have a higher rms current rating for a given physical dimension (due to very low esr). the drawback is the considerably high cost. ? tantalum capacitors: small tantalum capacitors with very low esr are becoming more widely available. however, they can occasionally burn if subjected to very high current during charge. therefore, it is suggested to avoid this type of capacitor for the input filter of the device as they may be stressed by a high surge current when connected to the power supply. if the selected capacitor is ceramic (so neglecting the esr contribution), the input voltage ripple can be calculated as: equation 32 table 7. list of ceramic capacitors for the LED2001 manufacturer series capacitor value (c) rated voltage (v) taiyo yuden umk325bj106mm-t 10 50 murata grm42-2 x7r 475k 50 4.7 50 d max v out v f + v inmin v sw ? ------------------------------------ - = d min v out v f + v inmax v sw ? -------------------------------------- = v in pp i o c in f sw ? ----------------------- 1 d --- - ? ?? ?? d ? d --- - 1d ? () ? + ? =
application information LED2001 26/42 docid024346 rev 1 7.2 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. a layout example is provided in figure 16 . the input and output loops are minimized to avoid radiation and high frequency resonance problems. the feedback pin to the sensing resistor path must be designed as short as possible to avoid pick-up noise. another important issue is the ground plane of the board. as the package has an exposed pad, it is very important to connect it to an extended ground plane in order to reduce the thermal resistance junction-to-ambient. to increase the design noise immunity, different signal and power ground should be implemented in the layout (see section 7.5: application circuit ). the signal ground serves the small signal components, the device analog ground pin, the exposed pad and a small filtering capacitor connected to the vcc pin. the power ground serves the device ground pin and the input filter. the different grounds are connected underneath the output capacitor. neglecting the current ripple contribution, the current flowing through this component is constant during the switching activity and so this is the cleanest ground point of the buck application circuit. figure 16. layout example
docid024346 rev 1 27/42 LED2001 application information 7.3 thermal considerations the dissipated power of the device is tied to three different sources: ? conduction losses due to the r dson , which are equal to: equation 33 where d is the duty cycle of the application. note that the duty cycle is theoretically given by the ratio between v out (n led ? v led + 100 mv) and v in , but in practice it is substantially higher than this value to compensate for the losses in the overall application. for this reason, the conduction losses related to the r dson increase compared to an ideal case. ? switching losses due to turn-on and turn-off. these are derived using the following equation: equation 34 where t rise and t fall represent the switching times of the power element that cause the switching losses when driving an inductive load (see figure 17 ). t sw is the equivalent switching time. figure 17. switching losses ? quiescent current losses. equation 35 example (see section example 1 ): p on r rdson_hs i out () ? 2 d ? = p off r rdson_ls i out () ? 2 1d ? () ? = p sw v in i out t rise t fall + () 2 ---------------------------------------- - f sw v in = i out t sw_eq f sw ?? ? ?? ? = am12908v1 p q v in i q ? =
application information LED2001 28/42 docid024346 rev 1 v in =12 v, v fw_led =3.5 v, n led =2, i led =700 ma the typical output voltage is: equation 36 r dson_hs has a typical value of 95 m and r dson_ls is 69 m @ 25 c. for the calculation we can estimate r dson_hs = 140 m and r dson_ls = 100 m as a consequence of tj increase during the operation. t sw_eq is approximately 12 ns. i q has a typical value of 1.5 ma @ v in = 12 v. the overall losses are: equation 37 equation 38 the junction temperature of the device is: equation 39 where t a is the ambient temperature and rth j-a is the thermal resistance junction-to- ambient. the junction-to-ambient (rth j-a ) thermal resistance of the device assembled in the hso8 package and mounted on the board is about 40 c/w. assuming the ambient temperature is around 40 c, the estimated junction temperature is: 7.4 short-circuit protection in overcurrent protection mode, when the peak current reaches the current limit threshold, the device disables the power element and it is able to reduce the conduction time down to the minimum value (approximately 100 nsec typical) to keep the inductor current limited. this is the pulse-by-pulse current limitation to implement the constant current protection feature. 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. v out n led v fw_led v fb + ? 7.1v == 2 p tot r dson_hs i out () ? 2 dr dson_ls i out () ? 2 1d ? () ? v in i out f sw t sw ??? v in i q ? +++ ? = p tot 0.14 0.7 2 0.6 0.1 0.7 2 0.4 ?? 12 + 0.71210 9 ? 850 10 3 12 1.5 10 3 ? ?? + ??? ? ? + ?? 205mw ? = t j t a rth ja ? p tot ? + = t j 60 0.205 40 68 c ? ? + =
docid024346 rev 1 29/42 LED2001 application information the inductor current ripple during on and off phases can be written as: ? on phase equation 40 ? off phase equation 41 where dcr l is the series resistance of the inductor. the pulse-by-pulse current limitation is effective to implement constant current protection when: equation 42 from equation 40 and equation 41 it can be seen that the implementation of the constant current protection becomes more critical the lower the v out and the higher the v in . in fact, in short-circuit condition the voltage applied to the inductor during the off-time becomes equal to the voltage drop across parasitic components (typically the dcr of the inductor and the r dson of the low-side switch) since vout is negligible, while during t on the voltage applied at the inductor is maximized and is approximately equal to v in . in general, the worst case scenario is heavy short-circuit at the output with maximum input voltage. equation 40 and equation 41 in overcurrent conditions can be simplified to: equation 43 considering t on which has already been reduced to its minimum. equation 44 where t sw =1/f sw and considering the nominal f sw . at higher input voltage i l ton may be higher than i l toff and so the inductor current can escalate. as a consequence, the system typically meets equation 42 at a current level 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. hiccup protection offers an additional protection against heavy short-circuit conditions at very high input voltage even considering the spread i l ton v in v out ? dcr l r dson hs + () i ? ? l ----------------------------------------------------------------------------------------------- t on () = i l ton v out dcr l r dson ls + () i ? + () ? l --------------------------------------------------------------------------------------- - t off () = i l ton i l toff = i l ton v in dcr l r dson hs + () i ? ? l ------------------------------------------------------------------------ t on min () v in l -------- - 90ns () ? = i l toff dcr l r dson ls + () ? i ? l ------------------------------------------------------------- - t sw 90ns ? () dcr l r dson ls + () ? i ? l ------------------------------------------------------------- - 1.18 s () ? =
application information LED2001 30/42 docid024346 rev 1 of the minimum conduction time of the power element. if the hiccup current level (6.2 a typical) is triggered, the switching activity is prevented for 12 cycles. figure 18 shows the operation of the constant current protection when a short-circuit is applied at the output at the maximum input voltage. figure 18. constant current protection triggering hiccup mode during pulse skipping, high side off, low side keeps on till 26clks finish (13clks for LED2001) or ipk decreases to be zero value. 7.5 application circuit figure 19. demonstration board application circuit 5 5 n n ' ( / 9 ' ( / 9 & & ) x ) x 6 5 6 5 5 5 & & x x 5 5 0 1 0 1 8 8 ' ( / ' ( / 9,1b$ ',0 )% $*1' 3*1' 6: 9,1b6: 1& (3 & & 9 x 9 x + x + x / / 1 , 9 1 , 9 ' 1 * ' 1 * 0 , ' 0 , ' ' ( / 9 ' ( / 9 3 - 3 - $0y
docid024346 rev 1 31/42 LED2001 application information figure 20. pcb layout (component side) dfn package table 8. component list reference part number description manufacturer c1 1 f 25 v (size 0805) c2 grm31cr61e226ke15l 22 f 25 v (size 1206) murata c3 grm21br71e475ka73l 4.7 f 25 v (size 0805) murata r1 4.7 k 5% (size 0603) r2 not mounted rs erj14bsfr15u 0.15 1% (size 1206) panasonic l1 xal6030-332meb 3.3 h i sat = 8.4 a (20% drop) i rms = 7.3 a (40 c rise) (size 6.36 x 6.56 x 6.1 mm) coilcraft
application information LED2001 32/42 docid024346 rev 1 figure 21. pcb layout (bottom side) dfn package figure 22. pcb layout (component side) hsop8 package it is strongly recommended that the input capacitors are to be put as close as possible to the pins, see c1 and c2.
docid024346 rev 1 33/42 LED2001 application information figure 23. pcb layout (bottom side) hsop8 package
typical characteristics LED2001 34/42 docid024346 rev 1 8 typical characteristics figure 24. soft-start figure 25. load regulation figure 26. dimming operation figure 27. led current rising edge a figure 28. led current falling edge figure 29. hiccup current protection am12913v1 vin 12v vled 7v am12914v1 am12915v1 am12916v1 to maximize the dimming performance the embedded ls discharges c out when dim goes low. (d im = 0 & & v fb > 60mv): the low side is enabled as long as i l > -1a (implements negative current limitation) am12917v1 am12918v1
docid024346 rev 1 35/42 LED2001 typical characteristics figure 30. ocp blanking time figure 31. thermal shutdown protection 130 ns typ. am12919v1 am12920v1
ordering information LED2001 36/42 docid024346 rev 1 9 ordering information table 9. ordering information order code package packaging LED2001pur vfqfpn 4x4 8l tape and reel LED2001phr hsop8
docid024346 rev 1 37/42 LED2001 package mechanical data 10 package mechanical data 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. table 10. vfqfpn8 (4x4x1.08 mm) mechanical data dim. mm min. typ. max. a 0.80 0.90 1.00 a1 0.02 0.05 a3 0.20 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 e0.80 l 0.40 0.50 0.60
package mechanical data LED2001 38/42 docid024346 rev 1 figure 32. vfqfpn8 (4x4x1.08 mm) package dimensions
docid024346 rev 1 39/42 LED2001 package mechanical data table 11. hsop8 mechanical data dim mm min. typ. max. a 1.70 a1 0.00 0.150 a2 1.25 b 0.31 0.51 c 0.17 0.25 d 4.80 4.90 5.00 e 5.80 6.00 6.20 e1 3.80 3.90 4.00 e1.27 h 0.25 0.50 l 0.40 1.27 k 0.00 8.00 ccc 0.10
package mechanical data LED2001 40/42 docid024346 rev 1 figure 33. hsop8 package dimensions ' pp7\s ( pp7\s $0y
docid024346 rev 1 41/42 LED2001 revision history 11 revision history table 12. document revision history date revision changes 20-may-2013 1 initial release.
LED2001 42/42 docid024346 rev 1 please read carefully: information in this document is provided solely in connection with st products. stmicroelectronics nv and its subsidiaries (?st ?) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described he rein at any time, without notice. all st products are sold pursuant to st?s terms and conditions of sale. purchasers are solely responsible for the choice, selection and use of the st products and services described herein, and st as sumes no liability whatsoever relating to the choice, selection or use of the st products and services described herein. no license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. i f any part of this document refers to any third party products or services it shall not be deemed a license grant by st for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoev er of such third party products or services or any intellectual property contained therein. unless otherwise set forth in st?s terms and conditions of sale st disclaims any express or implied warranty with respect to the use and/or sale of st products including without limitation implied warranties of merchantability, fitness for a particular purpose (and their equivalents under the laws of any jurisdiction), or infringement of any patent, copyright or other intellectual property right. st products are not authorized for use in weapons. nor are st products designed or authorized for use in: (a) safety critical applications such as life supporting, active implanted devices or systems with product functional safety requirements; (b) aeronautic applications; (c) automotive applications or environments, and/or (d) aerospace applications or environments. where st products are not designed for such use, the purchaser shall use products at purchaser?s sole risk, even if st has been informed in writing of such usage, unless a product is expressly designated by st as being intended for ?automotive, automotive safety or medical? industry domains according to st product design specifications. products formally escc, qml or jan qualified are deemed suitable for use in aerospace by the corresponding governmental agency. resale of st products with provisions different from the statem ents and/or technical features set forth in this document shall immediately void any warranty granted by st for the st product or service described herein and shall not create or extend in any manner whatsoev er, any liability of st. st and the st logo are trademarks or register ed trademarks of st in various countries. information in this document supersedes and replaces all information previously supplied. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners. ? 2013 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - philippines - singapore - spain - swed en - switzerland - united kingdom - united states of america www.st.com

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