STLD20D White LED power supply General features Typical guaranteed efficiency: 80% Drives up to 4 LEDs in series from a 2.8V up to 4.2V supply voltage ) s ( t c u d o ) r s ( P t c e t u Description e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r Application s P ( t c e t u e l d o o r s P b O e t e l o s b O Constant current regulation over the whole operating voltage range PWM control mode Integrated load disconnect switch that opens the LEDs path in shutdown mode Integrated soft start peak inductor current Programmable peak inductor current (STLD20D-C8 only) Shutdown pin allows digital dimming control up to 10kHz Over voltage and over temperature protection with automatic restart Low shutdown current (< 1A) Small external inductor (10H) Tiny external ceramic capacitor (1F) White Led supply for LCD backlight Mobile phone PDA and organizers, MP3 players, Toys QFN 3x3 8L SOT23-8L The STLD20D is a constant switching frequency boost regulator mainly dedicated to supply up to 4 white LEDs connected in series. A constant LED current is achieved by sensing the LED current through a sensing resistor RLED (see Figure 3.). The device also includes a supply voltage rejection circuit that prevents from any possible flickering effect on the display that might happen during input supply voltage variation. An integrated Load Disconnect Switch open the LED path to eliminate the current consumption in shutdown mode. The maximum peak inductor current can be programmed (STLD20D-C8 only). Order code Part number Package Marking Packing STLD20D-C8 SOT23-8L L2D Tape and reel STLD20D-DEF QFN 3x3 8L L2D Tape and reel October 2006 Rev 4 1/31 www.st.com 31 Contents STLD20D Contents 1 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O 5.1 Boost converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.2 Peak inductor current limitation and soft start function . . . . . . . . . . . . . . . . 9 5.3 Peak inductor current programmability (STLD20D-C8 only) . . . . . . . . . . . 9 5.4 Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.5 Brightness control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.6 Over temperature protection (OTP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.7 Over voltage protection (OVP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.8 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6 Typical performance characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 7 Components selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.1 L, Boost inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.1.1 Calculation of the inductor value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.1.2 Calculation of the saturation current I(sat) . . . . . . . . . . . . . . . . . . . . . . . 15 7.1.3 Choice of the RSET resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 7.1.4 Reference selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 7.2 CIN and COUT capacitors selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.3 1.3. D, Boost diode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.3.1 8 2/31 Electrical characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.4 RLED feedback resistance selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 7.5 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 PWM dimming control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 STLD20D 9 Contents Analog dimming control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 9.1 Minimum dimming current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 9.2 Rd1 Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 9.3 Rdim calculation for dimming mode control . . . . . . . . . . . . . . . . . . . . . . . 22 10 Layout recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 11 Evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O 12 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 13 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3/31 Pin description STLD20D Pin description Figure 1. Pin configuration (top view) SW GND VIN GND VOUT SHDN VIN LDS SW EXPOSED PAD 1 SHDN VOUT LDS ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O RSET 4/31 1 2 GND VIN N/C FB 3 4 SHDN RSET FB 5 6 7 8 1 2 3 4 5 6 7 8 FB LDS VOUT SW GND VIN SHDN N/C FB LDS VOUT SW STLD20D Block diagram 2 Block diagram Figure 2. Block diagram SW ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O OTP OSCILLATOR VIN OVP RAMP OSCILLATOR COMPENSATION VOUT POWER FAULT ENABLE VIN S + - SW Q R PWN COMP. SHDN + - T VIN DRAIN CURRENT REFERENCE RSET (*) LOAD DISCONNECT LDS VIN SHDN LDS FB + LED CURRENT REFERENCE GND (*) STLD20D-C8 only 5/31 Block diagram Figure 3. STLD20D Basic connection ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O Table 1. External components proposal Value Symbol Parameter Test conditions Unit Min. RLED CIN COUT LED current resistance D Note: 6/31 Max. 15 Input filtering capacitor 2.2 Ceramic type F Output capacitance 1 Inductance L Typ. Boost inductor (height < 2mm) Boost diode (STMicroelectronics STPS1L40M type) 10 Resistance at 500kHz H 1 ISAT (RSET = 100k) 300 mA VRRM 40 Vdc IF (peak forward current) 1 VF @ IF = 1A Tj = 25C 0.40 A 0.46 V The external components proposal should be considered as a design reference guide.The performances mentioned in the electrical characteristics table are not guaranteed for all the possible electrical parameters of the components included in this list. On the other hand the operation of STLD20D is not limited with the use of components included in this list. STLD20D Maximum ratings 3 Maximum ratings Table 2. Absolute maximum ratings Value Symbol Parameter Test conditions Unit Min. VIN Supply voltage range Typ. 2.5 Max. 5 V VESD ESD ratings TOP Operating temperature - 40 + 85 C Tstg Storage temperature - 65 150 C HBM MIL STD 883C BVDS Breakdown voltage at pin SW and TSS and VOUT SHDN Maximum voltage applied on SHDN pin Table 3. RthJA u d o r P e t e l o o s b SOT23-8L O ) s ( t c s b O e t e l ) s ( ct QFN e t le Pr Typ. V VIN c u d Value o r P Min. ) s ( t uc od Parameter Mounted on epoxy board without copper heatsink kV 20 Thermal data Symbol 2 V ) s t( Unit Max. 300 C/W 350 o s b O - u d o r P e t e l o s b O 7/31 Electrical characteristics STLD20D 4 Electrical characteristics Table 4. Electrical characteristics (VIN = 2.8 to 4.2V and TJ = 25C) Value Symbol Parameter Test conditions Unit Min. VIN Operating Input voltage range ILED Average regulated current ILED = 20mA ISD Stand-by current SHDN = low VIN = 4.2V IQ Quiescent current consumption SHDN = high VIN = 4.2V TJ = 25C ISW = 250mA VIN = 2.8V 0.48 SOT23-8L VIN = 4.2V 0.38 TJ = 25C ISW = 250mA VIN = 2.8V Boost switch RDSON SW QFN SOT23-8L Load Disconnect Switch RDSON LDS QFN FB )- s ( t c du fSW e t e l DCMIN o s b OVP o r P Minimum duty cycle ) s ( ct VIN = 4.2V so OTP bs u d o VIN = 2.8V 0.43 Max. 4.2 V 21 mA 1 A 0.6 mA ) s ( t c u d o r P 0.57 0.42 c u d 5.0 o r P 5.1 0.285 0.302 0.315 V 0.9 mA/V 80 % 85 500 600 20 17.5 Overvoltage hysteresis 18.5 640 mA 20 VDC VDC 110 SHDN Shutdown signal logic C 5 Disable Low VIL Enable high VIH kHz % 0.7 Over temperature protection hysteresis 4.3 400 Over temperature protection ) s t( 4.2 VIN = 4.2V Overvoltage protection 1. Guaranteed by design. 20 L = 10H RSET = GND (STLD20D-C8) HystOT 8/31 e t le VIN = 4.2V o s b O - t e l o HystOV VIN = 2.8V Switching frequency Peak current boost switch (1) r P e e t e l VIN = 2.8V b O TJ = 25C ILDS = 20mA 19 VIN = 4.2V TJ = 25C ILDS = 20mA Efficiency with 4 LEDS, VOUT = 16V ILIM RLED = 15 Variation of the LED current versus the input voltage: RLED = 15 Eff O 2.8 Feedback voltage Line O Typ. C 0.3 V 1.2 STLD20D Functional description 5 Functional description 5.1 Boost converter The STLD20D is a PWM mode control boost converter operating at 500kHz. An automatic compensation of the oscillation ramp allows rejection of the battery voltage transient. The LED current regulation (see Figure 3.) is done by sensing the LED current through the resistance RLED. The voltage across RLED is used by the feedback loop of the controller (FB pin). ) s ( t c u d o ) r s ( P t c e(STLD20D-C8 t 5.3 Peak inductor current programmability u only) e d l o o r s P b e O t e l ) o s ( s t b c u O 5.4 Shutdown d o ) r s P ( t c e t u e l d o o r s P b O5.5 Brightness e t control e l o s b O 5.2 Peak inductor current limitation and soft start function An integrated current sensor limits the switching current at 640 mA maximum. Should the peak drain current exceed 640mA (if RSET = 0 for STLD20D-C8), the flip flop will turn off the switch SW. During start up, this peak drain current limitation acts like a soft start function. The converter peak current must be always below the inductor saturation current. For flexibility reasons, the maximum peak inductor current can be programmed by connecting a resistor at the pin RSET. The Figure 12. gives the value of the resistance RSET versus the peak inductor current limit ILMAX. The SHDN pin is a low logic input signal and allows turning off the controller without cutting the input voltage from the boost regulator circuit. An integrated Load Disconnect Switch LDS disconnects the LEDs branch in shutdown mode.This arrangement allows eliminating the DC current path that normally exists with traditional boost regulator in shutdown mode. The brightness of the LED is adjusted by pulsing the shutdown pin with a PWM signal as high as 10kHz. By using such a PWM signal the controller is alternatively ON and OFF and the LED current changes from full current to zero. The duty cycle allows regulating the average LED current. This scheme ensures that when the LEDs are ON, they are driven at the full current without risk of color change. 9/31 Functional description 5.6 STLD20D Over temperature protection (OTP) An integrated temperature sensor senses the temperature of the junction of the controller. As soon as this temperature exceeds 110C min fixed internally, the controller is automatically turned OFF. When the temperature is reduced of 5C the operation of the device automatically recovers. 5.7 Over voltage protection (OVP) In case of failure and if the LED branch is cut, then there is no signal at the feedback pin FB (Figure 3.), the PWM controller will then switches with a maximum duty cycle. This will generate a voltage at the pin SW and VOUT that can exceed the maximum rating of the device. ) s ( t c u d o ) r s ( P t 5.8 Efficiency c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O The overvoltage protection block senses the output voltage at the pin VOUT (Figure 3.). If the voltage exceeds 18.5VDC typical the controller is automatically turned OFF. When the voltage is reduced of 0.7V, the operation of the device automatically recovers. (Figure 4. & Figure 9.) The efficiency takes into account these following losses: 10/31 RLED ohmic losses Boost switch SW losses Load Disconnect Switch LDS Boost inductor losses Boost diode losses STLD20D Typical performance characteristics 6 Typical performance characteristics Figure 4. 4 LEDs efficiency measurement Figure 5. Efficiency (%) LED current vs input voltage ILED(mA) 90.0 Ta = 25C 21.00 ILED = 20mA, n.VLED= 16V, DC = 100% Ta = 25C 20.80 20.60 LQH32CN100K33 20.40 20.20 LPO04815-103 ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O 80.0 20.00 Shielded TDK VLF3010AT-100MR49 19.80 19.60 CRDH2D14-100 19.40 19.20 70.0 19.00 2.5 3.0 3.5 4.0 4.5 2.5 3 3.5 VIN(V) Figure 6. 4 4.5 5 VIN(V) Feedback voltage Figure 7. Boost switch resistance (STLD20DC8) RDSon() VFB(mV) 310.0 0.8 0.7 305.0 Ta = 85C Ta = 25C 300.0 0.6 0.5 Ta = -40C Ta = 85C 0.4 Ta = 25C 295.0 Ta = -40C 0.3 290.0 0.2 2.5 3.0 3.5 4.0 4.5 2.5 3.0 3.5 VIN(V) Figure 8. 4.0 4.5 VIN(V) Boost switch resistance (STLD20D- Figure 9. DEF) RDSon() Efficiency vs input voltage (ILED=20mA; TA=25C) Efficiency (%) 0.8 88 87 0.7 86 85 0.6 Ta = 85C 0.5 84 83 Ta = 25C 82 0.4 81 Ta = -40C 0.3 80 79 0.2 2.5 3.0 3.5 VIN(V) 4.0 4.5 78 2.5 3 3.5 4 4.5 5 Input voltage (VDC) 11/31 Typical performance characteristics STLD20D Figure 10. Load disconnect switch resistance Figure 11. Quiescent Current consumption RDSon() IQ(A) 500.0 7.0 Ta = 85C 450.0 6.0 Ta = 25C Ta = -40C 400.0 5.0 Ta = 85C 350.0 Ta = 25C 300.0 Ta = -40C 250.0 4.0 3.0 ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O 200.0 2.0 2.5 3.0 3.5 4.0 2.5 4.5 3.0 3.5 4.0 4.5 VIN(V) VIN(V) Figure 12. Max peak inductor current IL versus Figure 13. Max peak inductor current IL versus L and RSET RSET ILIM(mA) ILIM(mA) 600 600.0 ILIM = f(RSET) (L = 10H) 550 550.0 RSET = GND 500.0 500 450.0 450 400.0 400 ILIM = 450 mA RSET = 56 k 350 350.0 RSET = 100K 300.0 300 250 250.0 200 200.0 4.0 8.0 12.0 16.0 20.0 24.0 30 40 50 60 70 80 90 100 RSET(k) L(H) Figure 14. ILED versus duty cycle Figure 15. Typical waveform ILED(mA) 20 18 ILED = F(Duty), Ta= 25C VOUT 16 14 12 VSW 10 ILED 8 6 Theoretical 4 Real Values 300Hz 2 Real Values 10kHz IL 0 0 10 20 30 40 50 60 Duty(%) 12/31 70 80 90 100 STLD20D Figure 16. Typical performance characteristics Supply voltage rejection Figure 17. Overvoltage protection ILED VIN VSW VOUT IL ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O 13/31 Components selection STLD20D 7 Components selection 7.1 L, Boost inductor selection To get a good trade-off thickness/efficiency, an attention must be given on the inductor choice. The inductance value must be selected to remain in the discontinuous conduction mode. Its saturation current (Isat) must be equal or higher than the programmed current (ILIM). An attention must be taken on the dynamic inductor parameters. Actually, some power losses can occur in the boost inductor when it works at several hundred KHz and can reduce the efficiency. ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O 7.1.1 Calculation of the inductor value The inductor must be dimensioned so that the STLD20D stays running in discontinuous conduction mode operation in the worst operation condition (VIN = VIN_min= 2.8V). The limit between continuous and discontinuous mode is called critical mode and characterized by an uninterrupted current through the inductor (see figure 18). Figure 18. 3 different conduction modes IL Continuous Discontinuous Critical t The formula [1] gives the maximum typical value of the inductor for a discontinuous mode operation in the worst case condition (critical mode). Figure 19 shows the typical L value versus the voltage across the LED branch N.VLED. Note that this curve includes the STLD20D and inductance dispersions (20%). N.VLED = 4x4V = 16V and ILOAD = 20mA 2 V in ( min ) ( N VLED - Vin ( min ) + V FB + ILED R LDS ) L typ -------------------------------------------------------------------------------------------------------------------------------------------------------------- [ 1 ] 2.4 I LED N VLED Fmax ( N V LED + I LED R LDS ) 14/31 STLD20D Components selection Where: is the efficiency (80%) N is the number of the white LEDs in series VLED is the forward voltage of the LED for the ILED current (VLED = 4V in our example) VIN(min) is the minimum input voltage (2.8V) VFB is the error amplifier reference (0.3V) RLDS is the internal resistance of the Load Disconnect Switch power MosFET (6) Fmax is the maximum frequency of the STLD20D (600kHz) ILED_MAX is the current through the LED ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O For example, the case with 4 white LEDs can be considered in order to evaluate L value in the worst case conduction. Figure 19. Typical inductance value versus the white LED voltage for three IOUT L typ (H) Ltyp=f(nVLED) 2.1E-05 15mA 1.9E-05 20mA 1.7E-05 25mA 1.5E-05 1.3E-05 11H 1.1E-05 9.0E-06 7.0E-06 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 N.VLED (V) From figure 19, typical inductance must be lower than 11H. By minimizing the inductance to ensure the discontinuous mode operation, the standard coil value is equal to 10H. Then: L=10H 7.1.2 Calculation of the saturation current I(sat) The maximum peak current (Ip(max)) during steady state can be estimated by the formula [2]: Ip ( max ) = 2 I LED N VLED ( N VLED - V IN ( min ) + V FB + I LED R LDS ) -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- [2] Fmin 0.8 L typ ( N V LED + V FB + I LED R LDS ) Where: Ltyp is the typical inductance value Fmin is the minimum frequency due to the STLD20D spread-off (400kHz) 15/31 Components selection STLD20D Figure 20. Maximum peak current (Ip(max)) versus the white LEDs voltages for 3 outputs current - VIN > 2.8V IP (max) (A) 0.5 10H 0.45 0.4 0.35 0.3 15mA ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O 20mA 0.25 25mA 0.2 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 N.VLED (V) Figure 20 shows the maximum peak current Ip(max) through the coil for L=10H versus the voltage across the LED branch, N.VLED and the LED current ILED for VIN > 2.8V. As N.VLED = 16V and ILED = 20mA, then Ip(max) = 0.45A. The curve below ends when the converter reaches the critical mode operation. Therefore, the saturation current (Isat) of the inductor must be higher than 0.45A. To conclude: I sat I p ( max ) Isat 0.4A 7.1.3 Choice of the RSET resistor The resistor RSET fixes the maximum peak current flowing through the inductor whatever the operating conditions. Thus, current saturation (Isat) is never reached. If the height constraint is important, this function allows using low profile inductor with a small saturation current. The Figure 12. on page 12 gives the corresponding typical value of the external resistor RSET versus the ILIM value. This curve is slightly dependent of the temperature and the input voltage. To prevent the coil saturation RSET must be equal to 56k, see Figure 12. Thus: I sat I LIM I p ( max ) 7.1.4 Reference selection The table below gives some coil references suitable for the STLD20D versus L, DCR, Isat value and sizing requirements. 16/31 STLD20D Table 5. Components selection Reference selection Name Ref Height (mm) L typ (H) DCR () ISAT (A) LQH32CN4R7M33 2 4.7 0.15 0.65 LQH32CN100K33 2 10 0.3 0.45 LQH32CN4R7M53 1.55 4.7 0.15 0.65 LQH32CN100K53 1.55 10 0.3 0.45 LP04815-472MXC 1.5 4.7 0.15 0.77 LP04815-103MXC 1.5 10 0.23 0.55 Murata Coilcraft ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b 7.2 C and C capacitors selection e O t e l ) o s ( s t b c u O d o ) r s P ( t c selection eD, Boost diode t u 7.3 1.3. e l d o o r s P b O e t e l o s b O Wurth Elektronik (WE) 744031100 1.65 10 0.205 0.74 744031150 1.65 15 0.285 0.62 744042100 1.8 10 0.15 1.3 1.855 10 0.294 0.7 CLS4D14 1.5 6.8 0.13 0.8 CLS4D14 1.5 10 0.18 0.65 VLF3010AT 100HR49 shielded 10 10 0.67 0.49 CDRH2014-100 Sumida TDK IN OUT The capacitance values and its intrinsic resistance (ESR) must be selected in order to reduce the output ripple. The ceramic capacitor technology offers the best compromise between the space and the performance (low ESR, value, voltage rating). Nevertheless, their values changes with the time as well as with temperature, DC bias voltage and switching frequency. Thus it might be necessary to use higher capacitor value if low ripple is an absolute need. The diode selection is based upon two major criteria: 7.3.1 Low losses to get the best converter efficiency Mechanical size Electrical characteristic VRRM (Repetitive peak reverse voltage) is the first parameter to consider in the selection of the boost diode. Its value must be always higher than the reverse voltage (VR) occurring during the steady state. Note that, some transient voltages occurs during the commutation period due to the leakage inductance of the PCB. Generally, a power diode with a maximum reverse voltage equal or just higher than 20V suits perfectly. Therefore a Schottky diode technology can be used. Schottky diode has a low forward voltage, nevertheless they have an additional reverse current which provides additional losses at high ambient temperature. 17/31 Components selection STLD20D In fact, in boost backlighting converter, the conduction losses (Pcond) lead by the forward characteristics can be negligible compared to the losses induced by the reverse current (Prev), especially at high temperature. 7.4 RLED feedback resistance selection The average output current is regulated by sensing a low external ohmic sensing resistor RLED. Thus, a constant current value is fixed for each LED whatever the ambient temperature conditions. RLED is given by: VFB 0.3V R LED = ----------- = ---------------- = 15 I LED 20mA [7] ) s ( t 7.5 Efficiency c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O Efficiency is a significant parameter for the application. The higher the efficiency, the longer the life time of the battery. The efficiency is given by: . 18/31 P output N VLED I LED Efficiency = ------------------- = -------------------------------------------V IN I input P input [8] STLD20D 8 PWM dimming control PWM dimming control By applying a PWM signal on the shutdown pin SHDN, the average current and the brightness of the LED can be adjusted. Figure 21. shows ILED current and the other typical waveform during this dimming control mode. Figure 21. Typical waveform when the PWM dimming is used at 300Hz Vshutdown ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O Vshutdown = 0.38 ILED ILED VOUT VOUT ILIM Ireg IL Note that the Load Disconnect Switch LDS turns ON/OFF at the same frequency and with the same duty cycle as the PWM signal. Thus, the LED current is a perfect square wave phased with the dimming signal. This leads to a good correlation between the real average current of the LED and the theoretical current given by: I LED - Theo = DC x I LED Where: ILED: is the nominal current programmed by the RLED resistance DC: is the duty cycle of the dimming signal. Figure 14. shows that the correlation between the real average current and the theoretical value is given for a minimum duty cycle of 5% when the dimming frequency is 300Hz and 20% for a 10kHz dimming signal. 19/31 Analog dimming control 9 STLD20D Analog dimming control Some application are sensitive to low frequency dimming signal; in this case an analog dimming control technic with a DC voltage Vdim to control the brightness of the LED can be used with the circuit shown Figure 22. The formula below gives the LED current versus the dimming voltage Vdim: V FB ( R dim + R d1 + R LED ) - Vdim ( R LED + R d1 ) I LED = -------------------------------------------------------------------------------------------------------------------------------------Rdim R LED [ 19 ] ) s ( t c u d o ) 9.1 Minimum dimming current r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O Where: Vdim: Analog Dimming Voltage Rdim, Rd1: Resistors of the dimming circuit (see figure 26) The PWM control of the STLD20D has a minimum duty cycle DCMIN that limits the dimming current range. It exists a minimum dimming current ILEDC corresponding to the typical DCMIN of the control loop. Figure 22. Analogical dimming schematics LDS C1 Vdim C2 R2 Rdim - ILED R1 Vfd PWM Ve STLD20D + Vfd Vref Rd1 RLED GND This minimum dimming current depends on the maximum input voltage and the forward voltage of the LED and can be estimated by: 2 ( DC min V IN ( max ) ) ILEDC -------------------------------------------------------------------------------------------------------------------------- [ 20 ] 2 L typ F typ [ N VLED + V FB - V IN ( max ) ] 20/31 STLD20D Analog dimming control Where: VIN(max): is the maximum input voltage DCMIN: is the typical minimum duty cycle of the STLD20D (18%) Ltyp: is the typical vale of the inductance Ftyp: is the typical switching frequency Figure 23. gives the ILED versus the LED branch voltage N.VLED. This curve is calculated with: Ltyp = 10H, Ftyp = 500kHz, DCMIN = 18% and VIN(max) = 4.2VDC. Figure 23. ILEDC current vs N.VLED corresponding at the DCMIN ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l R Calculation d 9.2 o o r s P b O e t e l o s b O ILEDC (A) 0.014 ILEDC = f (VLED) 0.012 0.010 0.008 0.006 0.004 0.002 0.000 9 10 11 12 13 14 15 16 17 18 19 20 N.VLED (V) Higher the voltage across the branch LEDs, higher the range current control. After these considerations, it is described here the basics rule to help the designer to choose the external components such as Rd1, Rdim and RLED versus Vdim and brightness control current ILED. d1 To avoid significant shifting of the cross over frequency and to keep enough high the corrector network gain of the error amplifier, it is recommended to dimension the resistor Rd1 below 10k (10% of R1). Dimension RLED for full brightness operating mode RLED is dimensioned to get the nominal current ILED for the full brightness of the LED. It is recommended to fix Vdim = VFB during the full brightness operating mode so that the LED current correspond to the programmed value ILED. Thus: V FB R LED = ----------ILED [ 21 ] 21/31 Analog dimming control STLD20D Where: VFB is the feedback voltage ILED is the LED current for full brightness Note: If Vdim is equal to 0 the LED current can be higher the programmed value. 9.3 Rdim calculation for dimming mode control Rdim and Rd1 are dimensioned to get a current in the dimming circuit much smaller than the LED current. From the formula 19, Rdim can be calculated by: ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O [ R d1 + R LED ] [ VFB - V dim - max ] R dim = ------------------------------------------------------------------------------------------I LEDmin RLED - V FB [ 22 ] Where: 22/31 Vdimmax is the maximum dimming voltage ILEDmin is the expected minimum dimming current STLD20D 10 Layout recommendation Layout recommendation The package connection of the STLD20D has been realized in order to facilitate the layout of the PCB. The golden rule to obtain an optimized layout is to split the power and signal track as shown on the Figure 24. It is necessary to place the input capacitor as closed as possible between pin1 and pin2 of the STLD20D package. If the CIN capacitor is not closed to the device, high frequency noise due to gate driver dI/dt flows through the copper track of the board and can generate some line voltage drop due to the line inductance. For the same reason, in order to eliminate high frequency current loop, the connection of the diode (D) and the output capacitor (COUT) must be as close as possible to the internal power MosFET (SW) (close to pin 8 and 1). ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O Concerning the signal path, we recommend to create the PCB GND signal from the pin 1 ("A" point in the Figure 24.). Thus all signal references such as feedback and the voltage across Rset are not disturbed by the power stage. Figure 24. Layout suggested D L 8 STLD20D A RLED RSET COUT 1 2 GND CIN VIN SHDN 23/31 Evaluation board 11 STLD20D Evaluation board Figure 25. shows the top view of the evaluation board that show all the application features of the STLD20D. Figure 25. Evaluation board top view with its connections at the external equipment ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O Figure 26. Demo board layout top view 24/31 STLD20D 12 Package mechanical data Package mechanical data In order to meet environmental requirements, ST offers these devices in ECOPACK(R) packages. These packages have a Lead-free second level interconnect. The category of second level interconnect is marked on the package and on the inner box label, in compliance with JEDEC Standard JESD97. The maximum ratings related to soldering conditions are also marked on the inner box label. ECOPACK is an ST trademark. ECOPACK specifications are available at: www.st.com ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O 25/31 Package mechanical data STLD20D QFN8 (3x3) MECHANICAL DATA DIM. mm. inch MIN. TYP MAX. MIN. TYP. MAX. 0.80 0.90 1.00 0.032 0.035 0.039 0.03 0.05 0.001 0.002 A2 0.65 0.70 0.75 0.026 0.028 0.030 A3 0.15 0.20 0.25 0.006 0.008 0.010 b 0.29 0.31 0.39 0.011 0.012 0.015 b1 0.17 0.30 0.007 A A1 ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O D D2 3.00 1.92 E E2 2.02 0.118 2.12 0.076 3.00 1.11 e 1.21 0.012 0.080 0.084 0.118 1.31 0.044 0.65 0.048 0.052 0.026 K 0.20 0.008 L 0.20 0.29 0.45 0.008 0.011 0.018 L1 0.16 0.24 0.40 0.006 0.009 0.016 L2 0.13 0.005 r 0.15 0.006 r1 0.15 0.006 7517789 26/31 STLD20D Package mechanical data SOT23-8L MECHANICAL DATA mm. mils DIM. MIN. TYP MAX. MIN. TYP. MAX. A 0.90 1.45 35.4 57.1 A1 0.00 0.15 0.0 5.9 A2 0.90 1.30 35.4 51.2 ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O b 0.22 0.38 8.6 14.9 C 0.09 0.20 3.5 7.8 D 2.80 3.00 110.2 118.1 E 2.60 3.00 102.3 118.1 E1 1.50 1.75 59.0 68.8 e 0 e1 L 0.35 .65 25.6 1.95 76.7 0.55 13.7 21.6 27/31 Package mechanical data STLD20D Tape & Reel QFNxx/DFNxx (3x3) MECHANICAL DATA mm. inch DIM. MIN. TYP A MAX. MIN. TYP. 330 C 12.8 D 20.2 13.2 MAX. 12.992 0.504 0.519 0.795 ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O N 60 2.362 T 28/31 18.4 0.724 Ao 3.3 0.130 Bo 3.3 0.130 Ko 1.1 0.043 Po 4 0.157 P 8 0.315 STLD20D Package mechanical data Tape & Reel SOT23-xL MECHANICAL DATA mm. inch DIM. MIN. TYP A MAX. MIN. TYP. 180 13.0 13.2 MAX. 7.086 C 12.8 0.504 D 20.2 0.795 N 60 2.362 0.512 0.519 ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O T 14.4 0.567 Ao 3.13 3.23 3.33 0.123 0.127 0.131 Bo 3.07 3.17 3.27 0.120 0.124 0.128 Ko 1.27 1.37 1.47 0.050 0.054 0.0.58 Po 3.9 4.0 4.1 0.153 0.157 0.161 P 3.9 4.0 4.1 0.153 0.157 0.161 29/31 Revision history STLD20D 13 Revision history Table 6. Revision history Date Revision Changes 3-Aug-2004 1 Initial release. 12-Oct-2004 2 Table 4 on page 4 following parameters values updated: . ILED (min), IQ (min), SW (QFN max), LDS (QFN max), ILIM, Hyst OT . FB VAR symbol changed to Line and value changed from 0.7 to 0.9 mA/V 08-May-2006 3 Change figure 25, add figure 26 and new template. ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O 23-Oct-2006 30/31 4 The SW, LDS and DCMIN values on table 4 have been updated, add note in ILIM. STLD20D ) s ( t c u d o ) r s ( P t c e t u e d l o o r s P b e O t e l ) o s ( s t b c u O d o ) r s P ( t c e t u e l d o o r s P b O e t e l o s b O 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 herein at any time, without notice. All ST products are sold pursuant to ST's terms and conditions of sale. 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