LT Channel LED Ballaster with PWM DESCRIPTION FEATURES APPLICATIONS TYPICAL APPLICATION

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1 FEATURES % LED Current Matching Up to ma Continuous Current per LED String Up to : True Color TM Dimming Range Input Disconnects LED Strings Can Operate in Buck, Boost and Buck-Boost Modes Wide Input Range: V to 4V Overtemperature Outputs Works with LT, LT4, LT44, LT4, LT4, LTC Thermally Enhanced -Pin MSOP Package APPLICATIONS High Power LED Ballaster Automotive Lighting Video Cameras TFT LCD Backlighting DESCRIPTION -Channel LED Ballaster with The LT drives three separate strings of LEDs up to ma/channel with % accurate current matching, resulting in uniform LED brightness and intensity. This approach is superior to conventional methods of running three separate channels with external ballast resistors requiring expensive factory calibration. The operates in boost, buck and buck-boost mode. A True Color Dimming ratio of up to : is achievable using a logic-level signal at the pin for all modes without the need for external level-shifting circuitry. For applications with input supply above and below the output voltage of the LED strings, the allows the LEDs to be returned to the input supply (buck-boost mode) instead of being limited to only SEPIC solutions. The is ideal for high power LED driver applications such as TFT LCD backlighting and heads-up displays. Additional overtemperature outputs allow appropriate system management for increased reliability. The is available in a small -pin MSE package., LT, LTC and LTM are registered trademarks of Linear Technology Corporation. True Color is a trademark of Linear Technolology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION P V Buck Mode with the LT4 µf.ω = 4 LEDs ma PER STRING 9 Effi ciency V TO V µf CAP LED DIMMING SIMPLIFIED LT4 LED OT V C SW OT LED nf µh µf.µf TAa EFFICIENCY (%) P = V NUMBER OF LEDS = = 4 4 TOTAL LED CURRENT (ma) TAb

2 ABSOLUTE MAXIMUM RATINGS (Note )...4V, LED, LED...4V,...4V...V...V...V OT, OT...V Operating Junction Temperature Range (Notes,, 4)... 4 C to C Storage Temperature Range... C to C Lead Temperature (Soldering, sec)... C PACKAGE/ORDER INFORMATION LED LED 4 TOP VIEW 9 OT OT MSE PACKAGE -LEAD PLASTIC MSOP T JMAX = C, θ JA = C/W EXPOSED PAD (PIN ) IS, MUST BE SOLDERED TO PCB ORDER PART NUMBER EMSE MSE PART MARKING LTCFF Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at T A = C. = V, = 4V, = V, = V, I LED = ma, OT = OT = Open, =. PARAMETER CONDITIONS MIN TYP MAX UNITS Operational Input Voltage = V, I,, = ma = 4V, I,, = ma 4 V V Minimum ( ) = V to V. V Quiescent Current = V, I,, = ma. = V, V = V LED = V LED 4 Shutdown Current = V, I,, = ma 4 µa Quiescent Current = V, I LED = ma = V, V = V LED = V LED 9 µa na Pin Threshold I LED = ma, V = V LED = V LED.. V LED Current Matching I LED = ma, V = V LED = V LED + % LED, LED LED LED Current Matching with LED Pin Voltage Mismatch I LED = ma, ( V LED V + V LED V LED ) = mv ma µa.. +. % LED Pin Voltage I LED = ma...9 V, LED, LED Maximum Current V,, <.V ma, LED, LED Maximum Leakage Current = V, V,, = 4V. µa Switching Threshold I,, = ma... V Turn-On Delay ( On to I LED On) = V to V, I LED > ma µs Pin Current in Buck Mode = V, = 4V, = 9V, = V. µa Overtemperature Sense Point (OT, OT) (Note 4) C Overtemperature Hysteresis Point (Note ) from Overtemp C Sense Point OT Pull-Down Current OT =.V (Note 4) µa

3 ELECTRICAL CHARACTERISTICS The denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at T A = C. = V, = 4V, = V, = V, I LED = ma, OT = OT = Open, =. PARAMETER CONDITIONS MIN TYP MAX UNITS OT Pull-Down Current OT =.V (Note 4) µa OT, OT Leakage Current OT = OT = V µa Note : Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note : The E is guaranteed to meet performance specifications from C to C junction temperature. Specifications over the 4 C to C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. Note : This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed C when overtemperature protection is active. Continuous operation above the specifi ed maximum operating junction temperature may impair device reliability. Note 4: Correlation to static test at T A = C. Note : Guaranteed by design. TYPICAL PERFORMANCE CHARACTERISTICS T A = C unless otherwise noted. I, vs I LED (%) I, to I LED Matching I, I LED I LED I LED = ma V = V LED = V LED INPUT QUIESCENT CURRENT (ma) Quiescent Current vs I LED T J = C PIN CURRENT (na) Pin Current (Out of the Pin) vs Temperature µs AFTER = 4 4 JUNCTION TEMPERATURE ( C) I LED (ma) JUNCTION TEMPERATURE ( C) G G G PIN CURRENT (na) 9 4 Pin Current vs Temperature µs AFTER = SHUTDOWN CURRENT (µa) 4 Shutdown Current vs Temperature = (V,, ) (mv) 9 9 (V,, ) vs I LED JUNCTION TEMPERATURE ( C) JUNCTION TEMPERATURE ( C) 4 I LED (ma) G4 G G

4 TYPICAL PERFORMANCE CHARACTERISTICS T A = C unless otherwise noted. Quiescent Current vs I LED Shutdown Current vs QUIESCENT CURRENT (ma) SHUTDOWN CURRENT (µa) 4 4 = I LED (ma) (V) G G PIN FUNCTIONS (Pin ): Controlled current input for a string of LEDs with a cathode lead connected to the pin. Connect the fi rst string of LEDs to this pin. LED (Pin ): Controlled current input for a string of LEDs with a cathode lead connected to the pin. Connect the second string of LEDs to this pin. LED (Pin ): Controlled current input for a string of LEDs with a cathode lead connected to the pin. Connect the third string of LEDs to this pin. (Pin 4): Boost: Connect to V OUT Buck Mode: Connect to Input Supply Buck-Boost Mode: Connect to V OUT (Pin ): Input Supply, Upper Rail. This pin must be locally bypassed with a capacitor to ground. powers the internal control circuitry. Boost: Connect to Input Supply Buck Mode: Connect to Input Supply Buck-Boost Mode: Connect to V OUT (Pin ): Input Pin for Dimming Control. A signal above.v (on threshold) turns the 4 channels on. A signal below.v completely disconnects each LED string. If the application does not require dimming, then the pin can be left either open (an internal µa source current pulls high) or it can be connected to a supply between.v to V. OT (Pin ): Overtemperature Output. OT pulls µa from the pin when the junction temperature exceeds C. The part has to cool down by C for the fl ag to reset; ideal for providing an overtemperature fl ag to the system microprocessor. OT (Pin ): Overtemperature Output. OT Pulls µa from the pin when the junction temperature exceeds C. The part has to cool down by C for the fl ag to reset; ideal for connecting to the switching regulator g m error amplifi er output to defeat switching. (Pin 9): Micropower Shutdown Pin. Below.V shuts down the IC. Typically I VIN = 4µA for = V. Boost: Connect to System Shutdown Signal or Buck Mode: Connect to System Shutdown Signal or Buck-Boost Mode: If dimming, connect to (Pin ); if no dimming, connect to system shutdown signal or

5 + + + PIN FUNCTIONS (Pin ): Lower Rail. Boost: Connect to System Ground Buck Mode: Connect to Inductor Buck-Boost Mode: Connect to Input Supply Exposed Pad (Pin ):. The ground for the IC should be soldered to a continuous copper ground plane under the die. Soldering the Exposed Pad to the copper ground plane under the device will reduce thermal resistance and increase the power capability of the. BLOCK DIAGRAM 4 LED LED B B + B B B B LOGIC V + 9.V.V OT BIAS THERMAL SHUTDOWN BG BG C C OT EXPOSED PAD F Figure. Block Diagram

6 OPERATION The is an easy-to-use -channel LED ballaster. It allows three strings of LEDs to be driven in parallel with accurate LED current matching. A pin is also provided allowing LED currents to be turned on and off at very low duty cycles for very wide LED dimming ranges. For reliable system thermal management, two output flags, OT and OT, are provided to indicate when the junction temperature exceeds C. The Block Diagram in Figure best illustrates the features of. The internal bias circuitry is turned on when > V and >.V. For LED current to be active in each channel, the pin must exceed.v. should be connected to the highest supply in the application (see various application modes of boost, buck, buck-boost in the Typical Applications section). The ballasting feature is achieved by using the current monitored in the LED channel to control the current in channels and LED. A servo loop exists for each channel and LED that compares the current in each channel to the LED reference channel. The current in LED channel is determined by the LED current programmed by the partner LED driver. Details of how LED current can be programmed by the partner IC are covered in the Typical Applications section using various ICs as LED drivers. Maximum fault current in each of the LED pins is internally limited to ma. If any LED string experiences an open-circuit fault, all LED string currents are turned off. The pin has unique level-shifting circuitry to allow a simple logic-level signal to turn each LED pin current on and off regardless of the pin voltage. This allows very simple dimming control of LED current without any need for external level-shifting components in buck mode and buck-boost mode applications. Care is taken to ensure low current (nano amps) in the and pins µs after low edge. This feature minimizes leakage currents in each application to maximize dimming ratio. Details of dimming and critical parameters are given in the Applications Information section Dimming. The incorporates internal junction temperature sensing and provides two open-collector outputs, OT and OT, which become active low when junction temperature exceeds C. OT is sized to pull µa and can be used as an input to the microprocessor for system thermal management. OT is sized to pull µa to defeat switching for most of LTC s LED drivers by pulling down the g m error amplifi er output. is forced to a zero LED current state by a special internal protection circuit when junction temperature reaches C. To calculate the junction temperature, see Thermal Calculations in the Applications Information section.

7 APPLICATIONS INFORMATION Input Capacitor Selection The is typically driven from the same input voltage used for the partner LED driver IC. The LED driver and inductor govern the requirements for the input capacitor of the application. A ceramic input capacitor in the range of µf to µf works for most applications. In cases where the input voltage is derived separately from the LED driver, a µf input capacitor works well. LED Current Matching An LED driver programs the LED current and actively ballasts three separate strings of LEDs. The currents in all three channels will be matched to better than ±%. To achieve best current matching, ( ) should be between V and V. A low ( ) also minimizes internal power dissipation (see Thermal Calculation section for more information). I, vs I LED (%) I LED = ma V LED =.4V (V) F Figure. I LED Matching vs ( ) LED Pin Current Range The steady-state operational current range for each LED pin is between ma and ma. Internal protection circuitry limits absolute maximum pin current to ma. LED Open-Circuit Protection If any LED string is open, then currents in all three channels reduce to zero. The driver chip, which supplies LED current, should have an overvoltage clamp to protect the from high LED pin voltages. Dimming The has a wide dimming range for constant color LED dimming. dimming is superior to analog dimming as it preserves true color quality. dimming control with the is achieved using a simple ground referenced signal with a.v on/off threshold. The LEDs operate at either programmed or zero current but their brightness changes with the signal duty cycle. When is low, LED strings are completely disconnected. In addition, the switches to low power standby mode ~µs after low edge, resulting in higher system power effi ciency. For the widest dimming range, the signal should be Hz. The human eye is typically sensitive to fl ickering below ~Hz. Operating the higher than Hz results in a reduced dimming ratio. Achieving high dimming ratios require attention to circuit leakages, such as reverse bias leakage currents through the external Schottky. Hence, for high dimming ratios, components should be chosen to minimize leakage currents. If an application does not require dimming, the pin can be left open (unconnected) and an internal µa source current pulls high. Boost Dimming The supports up to a : dimming ratio with a Hz dimming frequency. To achieve such high ratios, leakages of the LED driver and other external components should be minimal. Buck Mode: Dimming The supports up to a : dimming ratio in buck mode. The dimming in buck mode is achieved by an architecture that allows the power ground ( ) to move with output capacitor voltage. dimming control is achieved by a simple ground referenced signal, eliminating the need for any external levelshift component. High dimming ratios require very low and pin currents during the off state. The

8 APPLICATIONS INFORMATION L D C R SENSE C uses novel circuit techniques to reduce and pin currents to nano amp range ~µs after low edge. This preserves the output capacitor voltage and results in higher dimming ratios. + SW EA DRV LED DRIVER DA DA DA DB DB DB DC DC DC LED LED Buck-Boost Mode: Dimming The can also perform dimming in buck-boost mode. The buck-boost confi guration requires the and pins to be tied together. This confi guration can support up to a : dimming ratio. OT OT F OT OT LED DC DB DA C Figure. Boost Mode R SENSE LED L DC DB DA DC DB DA R SENSE D DA DA DA DB DB DB C + EA SW C DC DC DC DRV LED LED LED DRIVER F OT OT Figure. Buck-Boost Mode + EA LED DRIVER L SW DRV Figure 4. Buck Mode C D F4 Overtemperature Protection The incorporates internal junction temperature sensing and provides two open-collector outputs, OT and OT, which become active low when junction temperature exceeds C. The active OT output can sink µa of current and can be connected to system microprocessor. The active low OT output can sink µa of current and can be connected to the switching regulator s g m error amplifi er output to defeat switching. The has to cool down by C (9 C) for OT and OT outputs to reset (collector outputs high).

9 APPLICATIONS INFORMATION In addition, has an internal C overtemperature protection circuitry that resets the chip to zero LED current mode. This prevents the chip from continuous operation at high temperature. Thermal Calculations To maximize output power capability in an application without exceeding the C maximum operational junction temperature, it is useful to be able to calculate power dissipation within the IC. The power dissipation within the comes from four main sources: switch DC loss, switch loss due to LED V LED mismatch and input quiescent current.. Switch DC Loss: P SW(DC) = I LED V LED (See (V,, ) vs I LED Typical Performance Characteristics graph.). Switch Loss due to V LED Mismatch: P SW(ΔVLED) = Total V LED mismatch I LED. Input Quiescent Loss: P Q = ( ) (I Q ma) + ma I I LED Q = + ma 4 4. Total Power Dissipation: P TOT = P SW(DC) + P SW(ΔVLED) + P Q. Junction Temperature: T J () = T A + θ JA (P TOT ); θ JA (P TOT ) = P TOT C/W Example = V; = V; I LED = ma/string; ma IQ = + ma = 9. ma 4 Total V LED mismatch = V: e.g., LED string voltage drop = V; LED string voltage drop =.V; LED string voltage drop =.V Total V LED mismatch = (V.V) + (V.V) = V V LED =.V at I LED = ma (see (V LED ) vs I LED Typical Performance Characteristics graph).. P SW(DC) = ma (.V) =.W. P SW(LED) = mv ma = mw. P Q = ( ) (9. )ma + ma = mw 4. P TOT =.W + mw + mw.w The uses a thermally enhanced -lead MSE package. With proper soldering of the Exposed Pad to the underside of the package, combined with a full copper plane underneath the device, the thermal resistance (θ JA ) is about C/W. For an ambient temperature of T A = C, the junction temperature of the, for the example application described above, can be calculated as:. T J = T A + θ JA P TOT = C + C/W.W = C Minimizing Internal Power Dissipation The requires at least V headroom between and. Hence, for systems with high system input voltage and low (such as running multiple series LEDs in a Buck Mode), it is benefi cial to lower the level of pin voltage ( upper rail) with an external zener to reduce power dissipation in the chip. Therefore, it is recommended to limit ( ) to less than V. To achieve best performance, ( ) should equal V. 9

10 TYPICAL APPLICATIONS.A Buck Mode with the LT4 P V D V TO V 4.µF V µf D V C µf REF CAP V ADJ R.Ω LED LED LED LED LED 4 9 C.µF L µh R k LT4 R T SW 9 SW V C NC 9 9-, - nf LED LED LEDs PER STRING; ma PER STRING LED C: TDK CXRCK C: TAIYO YUDEN GMKBJ4MG-T D: DIODES INC. DFLS4 L: TOKO A9CY-M-P LED: LUMILEDS LUXEON OT OT TAa Effi ciency 9 EFFICIENCY (%) P = V NUMBER OF LEDS = = 4 4 TOTAL LED CURRENT (ma) TAb

11 TYPICAL APPLICATIONS ma Boost with the LT4 V TO V C µf V 9 L 4.µH 4 I SP I SN SW SW FBN NC I ADJ NC I ADJ NC 9 D R.M % R 4.k % V OUT C4 4.µF V D N44W FBP V REF V C Q N C nf LT4 C, C: TAIYO YUDEN TMKBJMG-BR C4: MURATA GRMERH4KAL D: DIODES INC. DFLS4 L: SUMIDA CDRHD-4R SS 4 I SP I SN R T C.µF R.k LED LED OT OT R4.Ω % I LED ma PER STRING to LEDs PER STRING 4 9 V OUT 4V C µf V TAa TOTAL I LED (ma). Dimming I LED = ma PER STRING NUMBER OF LEDS = = 4 DIMMING RATIO TAb EFFICIENCY (%) 9 9 Effi ciency I LED = ma PER STRING NUMBER OF LEDS = = (V) TAb

12 TYPICAL APPLICATIONS ma Buck-Boost Mode with the LT4 D 4 µf LED9 LED LED LED LED LED LED V TO V nf CER µf CER V V LED 9 Hz D OT OT LED LED NMOS nf µh I LED = ma I SP I SN SW FBN I ADJ FBP I ADJ SS V C LT4 V REF I SP I SN R T D LED4 k k.k TA4a D: ZETEX ZLLS D: DIODES INC N44 D: PHILIPS PDZ9.B NMOS: ZETEX N R SENSE.Ω.µF V CER Effi ciency NUMBER OF LEDS = 4 = EFFICIENCY (%) 4 I LED = ma I LED = ma 4 (V) TA4b

13 TYPICAL APPLICATIONS V C 4.µF V CER A Buck Mode with the LT R SENSE.Ω %, A I LED = ma PER STRING LED LED OT D 4V OT R M C µf V ISP ISN NC SW L µh.a C OUT µf V CER ZLLS V µf V CER LT V C I ADJ FB Q4 N N44W k pf nf Hz, V : MAX DIMMING RATIO TAa Effi ciency NUMBER OF LEDS = = EFFICIENCY (%) TOTAL LED CURRENT (A). TAb

14 TYPICAL APPLICATIONS A Buck with the LT4 4V LED LED4 LED LED LED9 LED C OUT.µF V D B4A D N444W L µh SUMIDA C IN 4.µF V C.µF XR 4 R k BOOST OUT SW LT4 BOOST OUT SW LED ONE OF TWO CHANNELS SHOWN 9 SECOND CHANNEL REF LED LED R R4.k.k 9 VADJ V ADJ VC V C ma PER STRING S P LUXEON I WHITE C pf R k C.µF R T 4 V C µf V LED LED OT OT Hz : MAX DIMMING RATIO R.M C pf Q N R.k % TAa f SW = MHz Effi ciency 9 9 EFFICIENCY (%)..4.. TOTAL LED CURRENT (A).. TAb 4

15 PACKAGE DESCRIPTION MSE Package -Lead Plastic MSOP (Reference LTC DWG # --4).94 ±. (. ±.4).9 ±. (. ±.) BOTTOM VIEW OF EXPOSED PAD OPTION. ±. (. ±.4). ±. (. ±.4). (.) MIN. ±. (. ±.4)..4 (..). ±. (. ±.) TYP. (.9) BSC RECOMMENDED SOLDER PAD LAYOUT. ±. (. ±.4) (NOTE ) 9.49 ±. (.9 ±.) REF.4 (.) DETAIL A TYP 4.9 ±. (.9 ±.). ±. (. ±.4) (NOTE 4) GAUGE PLANE 4. (.) DETAIL A NOTE:. DIMENSIONS IN MILLIMETER/(INCH). DRAWING NOT TO SCALE. ±. (. ±.) SEATING PLANE. (.4) MAX.. (..) TYP. (.9) BSC. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED.mm (.") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED.mm (.") PER SIDE. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE.mm (.4") MAX. (.4) REF. ±. (. ±.) MSOP (MSE) Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

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