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| ZXLD1321 Boost mode DC-DC converter for LED driving with 1A output and current control Description The ZXLD1321 is an inductive DC-DC converter, with an internal switch, designed for driving single or multiple LEDs in series up to a total of 1A output current. Applications cover commercial environments with input voltages ranging from 1.2V to 12V. The device employs a variable 'on' and 'off' time control scheme with adjustable peak switch current limiting and supports step-up (Boost) mode and self-powering Bootstrap operating modes, offering higher power efficiency and lower system cost than conventional PFM circuitry. The device includes the DC-DC converter, a high-side current monitor and an NPN switching transistor to provide an integrated solution offering small PCB size, competitive cost/performance, high power efficiency of DC-DC conversion and maximum LED brightness/reliability. More importantly, it retains design flexibility to add customer specific features. The feedback control circuitry inside the ZXLD1321 provides excellent load and current regulation, resulting in very stable LED current over the useful life of the battery and over the full operating temperature range. The LED current can be adjusted from 100% down to 10% of the set value by applying a dc voltage to the ADJ pin and down to 1% by applying a PWM signal to the ADJ pin. An onchip LED protection circuit also allows output current to be reduced linearly above a predetermined threshold temperature using an external thermistor at the TADJ pin. External resistors set nominal average LED current and coil peak current independently. The device can be shut down by applying a continuous low level dc voltage to the ADJ pin. Features * * * * * * * * * * 1.2V to 12V Input voltage range Up to 1A output current Typical efficiency# >85% Bootstrap operation enables input voltage down to 1V User-defined thermal control of LED output current using external thermistor High output current stability over input voltage and temperature 12A typical standby current LED current adjustable from 100% down to 2% Adjustable soft-start Drives up to 5 white LEDs in series Applications * * * * High power LED flashlights LED back-up lighting General LED lighting Emergency lighting Note# : Using standard external components as specified under electrical characteristics. Efficiency is dependent upon external component types and values. Higher efficiency is possible with alternative coils. Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 1 www.zetex.com ZXLD1321 Pin connections (top-view) Package view Package 14-Pin DFN with exposed pad 4mm x 3mm 0.50mm pitch 1.5W @TA=70C DFN14 package (bottom view). 45 chamfer denotes Pin 1 Block diagram Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 2 www.zetex.com ZXLD1321 Absolute maximum ratings (Voltages relative to GND unless otherwise stated) Operating temperature (top) Storage temperature (Tst) Junction temperature (Tj) Package power dissipation (Ptot) DFN-14 with exposed pad: 4mmx3mm, 0.5mm Pitch DC-DC converter Supply voltage (VIN) ADJ CFB ISENSE TADJ BIAS High-side current monitor Monitor supply voltage (M_VIN) Continuous sense voltage (M_VIN - M_LOAD) Switching NPN transistor Collector-Base voltage (VCBO) Collector-Emitter voltage (VCEO) Peak pulse current (ICM) Continuous Collector current (IC) 18V 18V 3A (Pulse width = 300s. Duty cycle<=2%) 2A -0.3V to +18V -0.3V to +5V -0.3V to +12V -0.3V to The lower of (+5.0V) or (VIN + 0.3V) -0.3V to The lower of (+5.0V) or (VIN + 0.3V) -0.3V to The lower of (+5.0V) or (VIN + 0.3V) -0.3V to The lower of (+5.0V) or (VIN + 0.3V) -0.3V to The lower of (+5.0V) or (VIN + 0.3V) -40 to 125C -55 to 150C -40 to 150C 1.5W at Tamb = 70C These are stress ratings only. Operation outside the absolute maximum ratings may cause device failure. Operation at the absolute maximum ratings for extended periods may reduce device reliability. Thermal resistance Junction to case (R JC) DFN-14 Nominal value 26.3C/W Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 3 www.zetex.com ZXLD1321 Pin description Name ADJ Pin # Description 1 Adjust input * Leave floating, or connect to VREF to set 100% output current. * Drive with dc voltage. (50mV 2 CFB 3 N/C ISENSE 4 5 EMITTER COLLECTOR M_LOAD M_VIN 6, 7 8, 9 10 11 VIN TADJ 12 13 VREF Exposed pad 14 15 Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 4 www.zetex.com ZXLD1321 Electrical characteristics (Test conditions: VIN = 3V, TAMB= 25C unless otherwise stated(a)) DC-DC converter supply parameters Symbol VIN VIN(Start) VUVVUV+ Iq Parameter Supply voltage Supply voltage for start-up(b) Conditions Normal operation Start-up mode Min 2.0 1.2 1.8 2.2 1.5 Typ Max 12 2.4 Units V V V V mA Under-voltage detection threshold VIN falling Normal operation to start-up mode Under-voltage detection threshold VIN rising Start-up mode to normal operation Quiescent current Measured into VIN ADJ pin floating. (Excluding switch base current). Measured into VIN. ADJ pin grounded ADJ pin floating 2.0V Standby current Internal reference voltage Internal reference temperature coefficient. 12 500 50 20 520 A mV ppm /K NOTES: (a) Production testing of the device is performed at 25C. Functional operation of the device and parameters specified from -40C to +125C are guaranteed by design, characterisation and process control. (b) Between 1.2V and 2.2V the device will run in the Low Voltage Startup Mode (for details refer to section "Low Voltage Operation") DC-DC converter input parameters Symbol VSENSE VSENSE (SU) Parameter Peak switch current sense voltage Peak switch current sense voltage in start-up mode Conditions Measured on ISENSE pin CFB pin at 0V Measured on ISENSE pin. Start-up mode VIN=1.2V Measured into ISENSE with pin at 0V. CFB pin at 0V -15 Min 45 Typ 55 10.5 Max 65 Units mV mV ISENSE Sense input current -7 -1 A CFB VADJ Control loop compensation capacitor External dc control voltage applied to ADJ pin to adjust output current Switching threshold of ADJ pin Temperature coefficient of VADJ(th) Internal resistor between VREF and ADJ VADJ<500mV 100A injected into ADJ pin 5 DC brightness control mode Standby state to normal operation 50 10 500 nF mV VADJ(th) TCO(VADJ) RADJ 26 28 +0.3 100 575 30 mV %/K k mV VADJ(clmp) Clamp voltage on ADJ pin Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 www.zetex.com ZXLD1321 DC-DC converter output parameters Symbol Toff(100) Toff(10) fLXmax fSU Parameter Discharge pulse width Discharge pulse width Maximum operating frequency Switching frequency in startup mode VIN=1.2V 50 Conditions 100% output current 10% output current Min 0.7 4 Typ 1.2 8 Max 1.7 12 600 Units s s KHz KHz Switching NPN transistor Symbol ISW IBON(max) Parameter Average continuous switch current(c) Maximum base current into 2V 10 mA V(BR)CEO VCE(sat) V mV mV NOTES: (c) Measured under pulse conditions. (d) This current is measured via the collectors and emitters of the switch with these connected to ground (0V) (e) Measured under pulse conditions. Peak Current = Ic Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 6 www.zetex.com ZXLD1321 High-side current monitor Symbol VM_VIN VMON IM_LOAD TCO (MON) Parameter Supply voltage Sense voltage Input current Temperature coefficient Bandwidth Transconductance Iout/ VMON Accuracy Conditions Min 3 Typ Max 18 Units V mV A ppm/ K KHz MHz mA/V =V(M_VIN) - V(M_LOAD) Measured into M_LOAD pin VMON=10mV VMON=100mV VMON=10mV VMON=100mV 0 100 0.08 370 150 350 2.5 1 200 1 BW Gm Acc RM = 0.1 VMON = 100mV -3 3 % Reference current monitor Symbol VADJ TCO (MON) BW Gm Acc Parameter Adjust Voltage Temperature Coefficient Bandwidth Transconductance Iout/ VADJ Accuracy VADJ=500mV -3 VADJ=50mV VADJ=500mV VADJ=50mV VADJ=500mV Conditions Min 0 160 200 275 3 200 3 Typ Max 500 Units mV ppm/ K KHz MHz A/V % LED thermal control circuit (TADJ) parameters Symbol VTADJH VTADJL Parameter Upper threshold voltage Lower threshold voltage Conditions Onset of output current reduction (VTADJ falling) Output current reduced to <10% of set value (VTADJ falling) Min Typ 75 50 Max Units mV mV Gm(TADJ) Transconductance Iout/ VTADJ 4 mA/V Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 7 www.zetex.com ZXLD1321 Output current regulation parameters Symbol IOUT IOUT Parameter Minimum output/ LED current(f) Output current accuracy(g) Conditions VIN>3V 3.0V Output current temperature drift Load current regulation Line voltage regulation of output current Efficiency(f) 100 200 ppm/ K %/A %/V % 2 0.5 350mA NOTES: (f) System parameter only. This value is dependent upon external components and circuit configuration. (g) This refers to the accuracy of output current regulation under normal operation when the feedback loop incorporating the current monitor is active. The tolerances of external components are not included in this figure. Ordering information Device ZXLD1321DCATC Reel size (mm) 33.02 Reel width (mm) 12 Quantity per reel 3,000 Device mark 1321 Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 8 www.zetex.com ZXLD1321 Device description The ZXLD1321 is a inductive boost DC-DC converter, with an internal switch, designed for driving single or multiple LEDs in series up to a total of 1A output current. Depending upon supply voltage (VIN), LED forward voltage drop (VLED) and circuit configuration, this can provide up to 8W of output power. Applications cover VIN ranging from 1.2V to 12V. The device employs a modified Pulse Frequency Modulation (PFM) control scheme, with variable "ON" and "OFF" time control and adjustable peak switch current limiting. General device operation (refer to block diagram) Normal operation Control is achieved by sensing the LED current in a series resistor (RM), connected between the two inputs of the LED Current Monitor. This generates a proportional current (IMON) that charges the external integrator capacitor CFB. IMON is balanced against a reference discharge current (IADJ) generated at the output of a second voltage to current converter driven from the demand voltage (VADJ) on the ADJ pin. The difference between IMON and IADJ is integrated by CFB to produce an error voltage. A comparator takes a summed version of the voltage at the ISENSE pin and a fraction of this CFB voltage and resets the latch driving the switch when the sum is greater than 50mV. The switch transistor is turned on by the output of the SR latch, which remains set until the emitter current in the switch transistor produces a voltage drop Vsense (=50mV nominal) in external resistor Rsense, defining a preset maximum switch current of 50mV/Rsense. Operation is such that a rising error voltage on CFB will effectively lower the voltage required on the ISENSE pin and therefore reset the latch earlier in the switching cycle. This will reduce the 'ON' time of the switch and reduce the peak current in the switch from its preset maximum value. Similarly, a falling error voltage will reset the latch later and the peak switch current will be increased. The control loop therefore reduces or increases the energy stored in the coil during each switching cycle, as necessary, to force the LED current to the set value. This results in high accuracy, as no error is needed in the LED current to drive the servo to the required region. The time taken for the coil current to reach the peak value depends on several factors: the supply voltage, the peak coil current required at that particular LED power and whether the system operates in "continuous" or "discontinuous" mode. The time allowed for the coil current to discharge into the LED is fixed by the 'Variable Off Delay' monostable, whose period is modified by the power demand signal on the ADJ pin. This monostable determines the time for which the latch remains reset (switch off) and provides a longer "OFF" period at lower power settings, helping to keep the parameters within an acceptable range. Note that the "ON" period and the "OFF" period are set by the supply voltage, LED power and external components chosen. The frequency is therefore determined by these parameters and is NOT fixed. In this modified PFM scheme, the external components can be chosen to keep the frequency well above the audio range for all extremes of parameters, so no audible whistling should ever occur. The 500mV reference voltage defines the nominal VADJ voltage and this defines the 100% output current. For lower LED currents, the ADJ pin can be-driven from an external dc voltage (50mV (c) Zetex Semiconductors plc 2008 9 www.zetex.com ZXLD1321 Low voltage operation (start-up mode) For supply voltages below 2V, the normal control loop will have insufficient headroom to operate reliably. This condition is detected by the 'under-voltage comparator', which compares a fraction of the internal supply voltage (Vcc) against VREF. When the comparator output is active (Vcc<1.8V), the output of the normal switch drive circuit is disabled and an alternative 'Start-up oscillator and driver' enabled. The start-up oscillator provides a nominal 50kHz fixed frequency drive signal to the base of the switch transistor, which is independent of VADJ and the voltage on CFB. Under low voltage conditions, the peak current in the coil ramps to approximately 25% of the normal value and the "OFF" time is fixed. The low voltage start-up mode allows the device to operate down to 1.2V nominal. This allows the chip to work from a single cell in boost mode and by configuring the device in Bootstrap mode, normal operation of the control loop will occur once the output has risen above 2.2V. Details of Bootstrap-Boost mode are given in the application notes. ADJ pin The ADJ pin is connected to the internal 500mV reference (VREF) via a 100k resistor. This biases the ADJ pin to the reference voltage and defines nominal 100% LED current. The ADJ pin can be overdriven with an external dc voltage between 50mV and 500mV to reduce the LED current proportionally between 10% and 100% of the nominal value. LED current can also be adjusted by applying a low frequency PWM signal to the ADJ pin to turn the device On and Off. This will produce an average output current proportional to the duty cycle of the control signal. The device can be shut down by shorting the ADJ pin to ground, or pulling it to a voltage below 28mV with a suitable open collector NPN or open drain NMOS transistor. In the shutdown state, most of the circuitry inside the device is switched off and residual quiescent current will be typically 12A. Thermal control of LED current The 'Thermal compensation current' circuit produces a sourcing current (Itc) which is zero for voltages above 75mV on TADJ and increases to 100A when TADJ falls to 50mV. This current is summed into the control node and subtracted from the demand current, causing LED current to reduce from 100% down to zero over this input range. The potential divider, consisting of a fixed resistor Rt and an NTC Thermistor Rth between VREF and ground, defines the voltage on TADJ and sets the threshold temperature. Further details are given in the application notes. The Thermal Control feature can be disabled by leaving the TADJ pin floating, or by connecting it to VREF. Over-temperature shutdown The ZXLD1321 incorporates an over-temperature shutdown circuit to protect the device against damage caused by excess die temperature, resulting from excessive power dissipation in the switch. The output of the 'Over-temp Shutdown' circuit will go high when the die temperature exceeds 150C (nominal). This will turn off the drive to the switch during normal operation. Operation will resume when the device has cooled to a safe level. Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 10 www.zetex.com ZXLD1321 Application notes Setting peak coil current The peak current in the coil is set by the resistor (RSENSE) between the switch emitter and ground according to I SWpeak = 50mV Rsense The minimum peak current will depend on operating mode, coil inductance and supply voltage range. The maximum peak current must not exceed the specified value for the switch. (See Application circuits for details). Setting LED current The nominal average LED current is given by I LED (nom) = 100mV RM Where RM is the external resistor connected between pins M_VIN and M_LOAD. This current can be adjusted to a lower value by applying a dc control voltage or PWM control signal to the ADJ pin. DC control The LED current can be adjusted over a 10% to 100% range by connecting a variable resistor RADJ from the ADJ pin to ground to vary the dc voltage at the ADJ pin. RADJ forms the lower part of a resistive divider and the internal 100k resistor between the ADJ and VREF pins forms the upper part. A value of 1M for RADJ will therefore give a maximum current of 91% of ILED (nom) and the device will be turned off when the voltage on the ADJ pin falls below 28mV, corresponding to an RADJ value of approximately 5k. If required, an end-stop resistor in series with RADJ can be used to maintain the voltage on the ADJ pin above the turn-on threshold. Using a logarithmic potentiometer for RADJ will give an approximately linear variation of output current with shaft rotation. (Fig 1) If required, the maximum output current can be restored to 100% by adjusting the value of the LED current monitor resistor (RM). The tolerance of the internal 100k resistor and RADJ should be taken into account when calculating output current. The ADJ pin is clamped internally to a voltage of 575mV (nom), to limit maximum average output current to approximately 115% of ILED(nom). Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 11 www.zetex.com ZXLD1321 Fig 1 PWM control A wider dimming range can be achieved by applying a PWM control signal to the ADJ pin to turn the device on and off, giving an average output current proportional to the duty cycle of the control signal. The ADJ pin can be driven directly from the open drain NMOS output of a microcontroller, or indirectly with a low saturation voltage NPN transistor such as the Zetex ZXTN25015DFH. (Fig 2). ADJ ZXTN25015DFH ZXLD1321 Fig 2 In the circuit of Fig 4, the average LED output current will be I LED (avg ) = I LED (nom) * D Where duty cycle D= T2 (T 1 + T 2) A PWM frequency of 200Hz, or lower is recommended, to minimize errors due to the rise and fall times of the converter output. Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 12 www.zetex.com ZXLD1321 Thermal compensation of LED current High-luminance LEDs often need to be supplied with a temperature compensated current in order to maintain stable and reliable operation at high temperatures. This is usually achieved by reducing the LED current proportionally from its nominal set value when the LED temperature rises above a predefined threshold. (Fig.3) ILED LED temperature Fig 3 The 'Thermal compensation current' generator inside the ZXLD1321 provides the necessary thermal compensation current to meet this requirement, using an NTC thermistor and resistor. (Fig 4) Fig 4 The TADJ pin of the device has a voltage threshold of 75mV nominal, which is derived from the reference voltage VREF. If the voltage (VTADJ) on the TADJ pin is held above the threshold, the thermal compensation current will be zero and no thermal compensation is applied. However, if VTADJ falls below the threshold, a thermal compensation current (ITC) is produced that is proportional to VTADJ. ITC is injected into the control loop in such a way as to reduce the demand current IADJ, causing the control loop to decrease the LED current. The LED current will be reduced to less than 10% of the set value when VTADJ falls below 50mV. The threshold voltage has been chosen to set a nominal threshold of 105C and the device has been optimized to operate with a standard 103KT1608 thermistor and 5k resistor in the potential divider. Circuit details are given in the application notes. Alternative thermistor/resistor networks can be used providing the input resistance presented to the device at the TADJ pin is similar at the threshold temperature. If no LED thermal compensation is required, the TADJ pin should be connected to VREF to disable this function. Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 13 www.zetex.com ZXLD1321 Typical operating conditions Inductive converters can operate in either CONTINUOUS mode, where current always flows in the inductor, but rises during the ON period and falls during the OFF period, or DISCONTINUOUS mode, where the current falls to zero during the OFF period. The mode depends on several factors, including supply voltage, output (LED) voltage and the choice of peak current and inductor value. Calculations need to be done to determine which mode the converter will be in. The circuit should be designed to give slightly more LED current than required under the lowest supply voltage, so the control loop can regulate the current accurately. If the theoretical LED current is less than that required, the control loop will not be able to reach the required value. The calculations will give an idea of the ON and OFF times and hence the operating frequency, but bear in mind that the control loop will reduce the peak current to achieve the exact programmed LED current and this will raise the operating frequency. In general, values in the discontinuous mode are simpler to calculate because the current can go from zero to the theoretical maximum during the ON period and fall to zero during the OFF period. In continuous mode the current will start from some value, so the ON time will be lower to reach the theoretical maximum and lower still when the control loop reduces the peak current below the maximum. Circuit operation Operation of boost LED driver The input voltage must always be lower than the LED voltage. This circuit has an ON phase, where the coil is connected from the supply to ground and an OFF phase, where the coil current flows through the LED via a Schottky diode. The current therefore only flows into the LED circuit during the OFF phase, although the reservoir capacitor C3 should keep current flowing in the LED(s) continuously. ADJ is set between 50mV and 500mV to give between 10% and 100% power respectively. Making R2 = zero gives a base current to the output transistor of 50mA nominal and making R2 = 1.68k gives 10mA nominal. The reduced base current will lower supply current and hence improve efficiency in lower power applications. Making R1 = 25m gives a peak coil current of 2 Amps. The internal power transistor turns on until the coil current builds up to the peak value. At this point the transistor switches off and the coil current continues to flow in the LED(s) via Schottky diode D1. Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 14 www.zetex.com ZXLD1321 The LED current is sensed by R3 and the controller varies this until the drop in R3 equals 20% of VADJ. Hence making R3 = 100m and VADJ = 500mV gives a LED current of 1 Amp because the 500mV VADJ results in 100mV across R3 which equals 1 Amp. Making VADJ = 10mV gives a LED current of 100mA because the 50mV VADJ results in 10mV drop across R3 which equals 100mA. The power is controlled by the chip backing off the peak coil current, so it is necessary to calculate the coil inductance and current to guarantee slightly more than 100% LED power, so the circuit can control it effectively. The internal control loop is compensated by C1, which is normally 10nF. If the thermistor (R5) is used, the power will be backed off progressively as the TADJ pin goes low. With the TADJ pin above 75mV, power is 100% and this is reduced to zero when the TADJ pin reaches 50mV. Making R4 = 5k and using a 103KT1608 thermistor, the thermistor will reach 869 at 105C giving VTADJ = 74mV which will start to reduce the LED power above 105C. By 125C the thermistor will reach 547 giving VTADJ = 50mV which gives zero power. This will protect the LED from damage. These temperature values can be set by the customer by using a different thermistor or a different value of R4. If protection is not required, leaving the TADJ pin open circuit will make it float to a high voltage and always give 100% power. Reference U1 D1 L1 L1 L1 C1 C2 C3 R1 R2 R3 R4 R5 Part No ZXLD1321 ZXCS2000 MSS7341-103ML NPIS64D100MTRF 744 77810 Generic GRM31CR71H475K GRM31MR71E225K Generic Generic Generic Generic Thermistor NTC Value LED Driver Schottky diode 10H 2A 10H 2A 10H 2A 10nF 10V 4.7F 50V 2.2F 25V 25m Manufacturer Zetex Zetex Coilcraft NIC Wurth Generic Murata 1206 Murata 1206 Generic 0805 Generic 0603 Contact Details www.zetex.com www.coilcraft.co m www.niccomp www.wurth.co.uk www.murata.com www.murata.com 100m 5.1k 10k Generic 0805 Generic 0603 103kt1608 Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 15 www.zetex.com ZXLD1321 Operation in bootstrap mode Diagram B : Bootstrap mode Operation of bootstrap LED driver This is used when the input voltage is less than 2 volts. Note that the chip VIN now goes to the cathode of the Schottky diode D1. The control loop can not operate at this low voltage, so the chip goes into a start-up mode, where the output transistor is switched on and off at nominally 50kHz with a 50:50 duty cycle with about 10mA of base current into the power transistor (20% of nominal). The emitter current is still sensed by R1 and the "ON" part of the duty cycle will be terminated either when the emitter sense voltage reaches 10mV (corresponding to 20% of the set peak current) or the ON part of the duty cycle finishes after 10s. There is no control of the LED current yet, the circuit just operates in Boost mode. Eventually, the reservoir capacitor C3 charges up to 2V and the chip goes into "Normal" mode, where it delivers 50mA to the base of the power transistor and the control loop works normally. It will continue to charge C3 until the LED current is correctly established, with the chip now running from a voltage equal to the LED forward drop (around 3.6V for one LED) even though the supply is still below 2 volts. Once the circuit has reached this condition, the rest of the description of the operation is the same as for the Boost operation. Like the Boost circuit, this circuit has an ON phase, where the coil is connected from the supply to ground and an OFF phase, where the coil current flows through the LED via the Schottky diode D1. The current therefore only flows into the LED circuit during the OFF phase, although the reservoir capacitor C2 should keep current flowing in the LED(s) continuously. ADJ is set between 50mV and 500mV to give between 10% and 100% power respectively. Making R2 = zero gives a base current to the output transistor of 50mA nominal and making R2 = 1.68k gives 10mA nominal. The reduced base current will lower supply current and hence improve efficiency in lower power applications. Making R1 = 50m gives a peak coil current of 1 Amps. The internal power transistor turns on until the coil current builds up to the peak. At this point the transistor switches off and the coil current continues to flow in the LED(s) via Schottky diode D1. Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 16 www.zetex.com ZXLD1321 The LED current is sensed by R3 and the controller varies this until the drop in R3 equals 20% of VADJ. Hence making R3 = 100m and VADJ = 500mV gives a LED current of 1 Amp because the 500mV VADJ results in 100mV across R3 which equals 1 Amp. Making VADJ = 10mV gives a LED current of 100mA because the 50mV VADJ results in 10mV drop across R3 which equals 100mA. The power is controlled by the chip backing off the peak coil current, so it is necessary to calculate the coil inductance and current to guarantee slightly more than 100% LED power, so the circuit can control it effectively. The internal control loop is compensated by C1, which is normally 10nF. Note that in Bootstrap mode, the input current will be 2 or 3 times larger than the LED current and the duty cycle will be such that TON is larger than TOFF, due to the fact that the supply voltage charging the coil is low. Because of this, large LED currents can not be programmed at very low supply voltages, as the transistor current would need to exceed 2 Amps. If the thermistor (R5) is used, the power will be backed off progressively as the TADJ pin goes low. With the TADJ pin above 75mV, power is 100% and this is reduced to zero when the TADJ pin reaches 50mV. Making R4 = 5k and using a 103KT1608 thermistor, the thermistor will reach 869 at 105C giving TADJ = 74mV which will start to reduce the LED power above 105C. By 125C the thermistor will reach 547 giving TADJ = 50mV which gives zero power. This will protect the LED from damage. These temperature values can be set by the customer by using a different thermistor or a different value of R4. If protection is not required, leaving the TADJ pin open circuit will make it float to a high voltage and always give 100% power. Reference U1 D1 L1 L1 L1 C1 C2 C3 R1 R2 R3 R4 R5 Part No ZXLD1321 ZXCS2000 MSS7341-103ML NPIS64D100MTRF 744 777910 Generic GRM31CR71H475K GRM31MR71E225K Generic Generic Generic Generic Thermistor NTC Value LED Driver Schottky diode 10H 2A 10H 2A 10H 2A 10nF 10V 4.7F 50V 2.2F 25V 25m Manufacturer Zetex Zetex Coilcraft NIC Wurth Generic Murata 1206 Murata 1206 Generic 0805 Generic 0603 Contact Details www.zetex.com www.coilcraft.co m www.niccomp www.wurth.co.uk www.murata.com www.murata.com 100m 5.1k 10k Generic 0805 Generic 0603 103kt1608 Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 17 www.zetex.com ZXLD1321 Additional notes which apply to all operational modes Note with all these circuits that the ON time is set by the time it takes the coil to reach the peak current. This peak value is reduced by the control loop to give the desired LED power, so the ON time can vary over a wide range. The minimum coil current can be zero (discontinuous operation) or finite (continuous operation) depending on the supply voltage, LED current and the LED voltage. The OFF time is set by an internal timer and is nominally 1.2s at 100% LED power (VADJ = 500mV), increasing to about 8s at 10% LED power (VADJ = 50mV). The longer OFF time and variable peak current enables the circuit to dim the LED whilst maintaining continuous switching, rather than "skipping" or stalling and continuous running is better for reducing electrical noise and also for eliminating audible noise from the coil core. Layout considerations As with all switching DC to DC converters, the currents can be large. Using small inductors with a reasonably high supply voltage will cause currents to change quickly. High dI/dt can cause inductively-coupled spikes into adjacent tracks. At the transition from of the ON phase to the OFF phase and back, where the power transistor switches, the voltage at the collector rises and falls quickly. High dV/dt can cause capacitively coupled spikes into adjacent tracks, especially if they have a high impedance. For this reason, all tracks on the PCB should be thick, to minimise drops, and short to keep all the components coupled tightly together. A double-sided board should be used with a ground plane to screen the tracks and provide a good ground return for the various functions and the rear exposed pad on the package should have an appropriately-sized land with good ground connections, both to reduce electrical noise due to ground drops and to improve thermal conductivity. The input decoupling capacitor C1 should be very close to the chip pins and the LED sense resistor R3 should have Kelvin tracks to M_VIN and M_LOAD to achieve LED current measurement accuracy, as the PCB tracks will have comparable resistance to the 100m resistor, so taking sense tracks to the current monitor which are not connected close to the ends of R3 will cause a measurement error. The peak current sense resistor R1 should have short tracks to the ground at the bottom end and Kelvin tracks to ISENSE at the top end. This resistor might need to be only 25m and PCB track resistance becomes comparable if the tracks are not very short. ISENSE is a high impedance input, so a thin track from this pin directly to the top of RSENSE resistor R1 will still give an accurate measurement. The ADJ pin should have short tracks, as this is a fairly low-level signal controlling the power of the system. As it needs to be less than 28mV for shutdown, a close ground connection is needed for the pull-down device, as any ground drops could raise the potential. In particular, if a bipolar transistor is used as a pull-down device, this will have an appreciable VSAT, which could perhaps be half the shutdown potential. The bottom of the thermistor must be coupled very closely to ground, as the TADJ pin varies the LED current from 100% to 0% for a voltage change of only 25mV, so any noise on the bottom of the thermistor will seriously affect the accuracy of the Thermal Protection circuit. Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 18 www.zetex.com ZXLD1321 Package outline - TDFN1443 60.3938 E A A3 E2 PIN #1 IDENTIFICATION CHAMFER 0.300 X 45 PIN 1 DOT BY MARKING D2 D L A1 DIM A A1 A3 b D Inches Min Max 0.0276 0.0315 0.00 0.002 0.008 REF. 0.0079 0.0118 0.1555 0.1594 Millimeters Min Max 0.70 0.80 0.00 0.05 0.203 REF. 0.20 0.30 3.95 4.05 DIM D2 e E E2 L Inches Min Max 0.1240 0.1279 0.0197 BSC 0.1161 0.1201 0.0650 0.0689 0.0138 0.0177 Millimeters Min Max 3.15 3.25 0.50 BSC 2.95 3.05 1.65 1.75 0.35 0.45 Note: Controlling dimensions are in millimeters. Approximate dimensions are provided in inches Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 19 www.zetex.com b ZXLD1321 Definitions Product change Zetex Semiconductors reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or service. Customers are solely responsible for obtaining the latest relevant information before placing orders. Applications disclaimer The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for the user's application and meets with the user's requirements. No representation or warranty is given and no liability whatsoever is assumed by Zetex with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Zetex does not assume any legal responsibility or will not be held legally liable (whether in contract, tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business, contract, opportunity or consequential loss in the use of these circuit applications, under any circumstances. Life support Zetex products are specifically not authorized for use as critical components in life support devices or systems without the express written approval of the Chief Executive Officer of Zetex Semiconductors plc. As used herein: A. Life support devices or systems are devices or systems which: 1. are intended to implant into the body or 2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labelling can be reasonably expected to result in significant injury to the user. B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or to affect its safety or effectiveness. Reproduction The product specifications contained in this publication are issued to provide outline information only which (unless agreed by the company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products or services concerned. Terms and Conditions All products are sold subjects to Zetex' terms and conditions of sale, and this disclaimer (save in the event of a conflict between the two when the terms of the contract shall prevail) according to region, supplied at the time of order acknowledgement. For the latest information on technology, delivery terms and conditions and prices, please contact your nearest Zetex sales office. Quality of product Zetex is an ISO 9001 and TS16949 certified semiconductor manufacturer. To ensure quality of service and products we strongly advise the purchase of parts directly from Zetex Semiconductors or one of our regionally authorized distributors. For a complete listing of authorized distributors please visit: www.zetex.com/salesnetwork Zetex Semiconductors does not warrant or accept any liability whatsoever in respect of any parts purchased through unauthorized sales channels. ESD (Electrostatic discharge) Semiconductor devices are susceptible to damage by ESD. Suitable precautions should be taken when handling and transporting devices. The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extent of damage can vary from immediate functional or parametric malfunction to degradation of function or performance in use over time. Devices suspected of being affected should be replaced. Green compliance Zetex Semiconductors is committed to environmental excellence in all aspects of its operations which includes meeting or exceeding regulatory requirements with respect to the use of hazardous substances. Numerous successful programs have been implemented to reduce the use of hazardous substances and/or emissions. All Zetex components are compliant with the RoHS directive, and through this it is supporting its customers in their compliance with WEEE and ELV directives. Product status key: "Preview" Future device intended for production at some point. Samples may be available "Active" Product status recommended for new designs "Last time buy (LTB)" Device will be discontinued and last time buy period and delivery is in effect "Not recommended for new designs" Device is still in production to support existing designs and production "Obsolete" Production has been discontinued Datasheet status key: "Draft version" This term denotes a very early datasheet version and contains highly provisional information, which may change in any manner without notice. "Provisional version" This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance. However, changes to the test conditions and specifications may occur, at any time and without notice. "Issue" This term denotes an issued datasheet containing finalized specifications. However, changes to specifications may occur, at any time and without notice. Zetex sales offices Europe Zetex GmbH Kustermann-park Balanstrae 59 D-81541 Munchen Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 europe.sales@zetex.com Americas Zetex Inc 700 Veterans Memorial Highway Hauppauge, NY 11788 USA Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 usa.sales@zetex.com Asia Pacific Zetex (Asia Ltd) 3701-04 Metroplaza Tower 1 Hing Fong Road, Kwai Fong Hong Kong Telephone: (852) 26100 611 Fax: (852) 24250 494 asia.sales@zetex.com Corporate Headquarters Zetex Semiconductors plc Zetex Technology Park, Chadderton Oldham, OL9 9LL United Kingdom Telephone: (44) 161 622 4444 Fax: (44) 161 622 4446 hq@zetex.com (c) 2008 Published by Zetex Semiconductors plc Issue 1 - January 2008 (c) Zetex Semiconductors plc 2008 20 www.zetex.com |
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