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 Applications • • • • • High power LED flashlights • LED back-up lighting • General LED lighting • Emergency lighting • • • • • • 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 12µA typical standby current LED current adjustable from 100% down to 2% Adjustable soft-start Drives up to 5 white LEDs in series 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 © 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=70°C DFN14 package (bottom view). 45° chamfer denotes Pin 1 Block diagram Issue 1 - January 2008 © Zetex Semiconductors plc 2008 2 www.zetex.com ZXLD1321 Absolute maximum ratings (Voltages relative to GND unless otherwise stated) Operating temperature (top) -40 to 125°C Storage temperature (Tst) -55 to 150°C Junction temperature (Tj) -40 to 150°C Package power dissipation (Ptot) DFN-14 with exposed pad: 4mmx3mm, 0.5mm Pitch 1.5W at Tamb = 70°C DC-DC converter Supply voltage (VIN) -0.3V to +12V ADJ -0.3V to The lower of (+5.0V) or (VIN + 0.3V) CFB -0.3V to The lower of (+5.0V) or (VIN + 0.3V) ISENSE -0.3V to The lower of (+5.0V) or (VIN + 0.3V) TADJ -0.3V to The lower of (+5.0V) or (VIN + 0.3V) BIAS -0.3V to The lower of (+5.0V) or (VIN + 0.3V) High-side current monitor Monitor supply voltage (M_VIN) -0.3V to +18V Continuous sense voltage (M_VIN – M_LOAD) -0.3V to +5V Switching NPN transistor Collector-Base voltage (VCBO) 18V Collector-Emitter voltage (VCEO) 18V Peak pulse current (ICM) 3A (Pulse width = 300µs. Duty cycle<=2%) Continuous Collector current (IC) 2A 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 (RUJC) Nominal value DFN-14 26.3°C/W Issue 1 - January 2008 © Zetex Semiconductors plc 2008 3 www.zetex.com ZXLD1321 Pin description Name Pin # Description ADJ 1 Adjust input • Leave floating, or connect to VREF to set 100% output current. • Drive with dc voltage. (50mV<VADJ< VREF) to adjust output current from 10% to 100% of set value. (DC brightness control mode) • Drive with low frequency (200Hz) PWM control signal to gate output ‘on’ and ‘off’ at the PWM frequency. (PWM brightness control mode) • Drive with low level dc voltage (VADJ<28mV) to turn off device (Standby mode) BIAS 2 Bias pin for setting base current of internal switch transistor • Short pin to ground to define maximum base drive current for output switch (Maximum output current condition) • Connect resistor (RBIAS) from this pin to ground to reduce base drive current (Reduced output current condition) CFB 3 Compensation point • Connect 10nF capacitor from this pin to ground to provide loop compensation N/C 4 No connection to this pin (open circuit) ISENSE 5 Switch peak current sense pin • Connect resistor (RSENSE) from this pin to ground to define peak switch current (ISWPEAK)=VSENSE/RSENSE EMITTER 6, 7 Switch emitters (connect both pins to top of RSENSE to sense emitter current) COLLECTOR 8, 9 Switch collectors (connect both pins to lower side of coil) M_LOAD 10 Load side input of high side current monitor M_VIN 11 Input supply to high side current monitor • Connect to output voltage (cathode of Schottky) • Connect resistor (RM) between M_LOAD and M_VIN to define nominal average output (LED) current of 0.1/RM VIN 12 Positive supply to device (1.2-12V) • Decouple to ground with capacitor close to device TADJ 13 Temperature adjust input for LED thermal compensation • Connect thermistor/resistor network to this pin to reduce output current above a preset temperature threshold. • Connect to VREF to disable thermal compensation function (see section on temperature control for details) VREF 14 Internal 0.5V reference voltage output Exposed pad 15 Connect to ground (0V) Issue 1 - January 2008 © Zetex Semiconductors plc 2008 4 www.zetex.com ZXLD1321 Electrical characteristics (Test conditions: VIN = 3V, TAMB= 25°C unless otherwise stated(a)) DC-DC converter supply parameters Symbol Parameter VIN Supply voltage start-up(b) Conditions Min Normal operation Start-up mode Typ Max Units 2.0 12 V 1.2 2.4 V VIN(Start) Supply voltage for VUV- Under-voltage detection threshold VIN falling Normal operation to start-up mode 1.8 V VUV+ Under-voltage detection threshold VIN rising Start-up mode to normal operation 2.2 V Iq Quiescent current Measured into VIN ADJ pin floating. (Excluding switch base current). 1.5 mA ISTBY Standby current Measured into VIN. ADJ pin grounded 12 20 µA VREF Internal reference voltage ADJ pin floating 2.0V<VIN<18V 500 520 mV TCO(REF) Internal reference temperature coefficient. 480 50 ppm /K NOTES: (a) Production testing of the device is performed at 25°C. Functional operation of the device and parameters specified from -40°C to +125°C 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 Parameter Conditions VSENSE Peak switch current sense voltage Measured on ISENSE pin CFB pin at 0V VSENSE (SU) Peak switch current sense voltage in start-up mode Measured on ISENSE pin. Start-up mode VIN=1.2V ISENSE Sense input current Measured into ISENSE with pin at 0V. CFB pin at 0V CFB Control loop compensation capacitor VADJ External dc control voltage applied to ADJ pin to adjust output current DC brightness control mode 50 VADJ(th) Switching threshold of ADJ pin Standby state to normal operation 26 TCO(VADJ) Temperature coefficient of VADJ(th) RADJ Internal resistor between VREF and ADJ VADJ(clmp) Clamp voltage on ADJ pin Issue 1 - January 2008 © Zetex Semiconductors plc 2008 Min Typ Max Units 45 55 65 mV 10.5 -15 -7 mV -1 10 28 µA nF 500 mV 30 mV +0.3 %/K VADJ<500mV 100 kΩ 100µA injected into ADJ pin 575 mV 5 www.zetex.com ZXLD1321 DC-DC converter output parameters Symbol Parameter Conditions Min Typ Max Units Toff(100) Discharge pulse width 100% output current 0.7 1.2 1.7 µs Toff(10) Discharge pulse width 10% output current 4 8 12 µs fLXmax Maximum operating frequency 600 KHz fSU Switching frequency in startup mode VIN=1.2V 50 KHz Switching NPN transistor Symbol Parameter Conditions Min ISW Average continuous switch current(c) IBON(max) Maximum base current into 2V<VIN<18V switch transistor from internal BIAS pin at 0V drive circuit(d) IBON Base current into switch RBIAS = 1680Ω transistor using external resistor (RBASE) from BIAS pin to ground V(BR)CEO Collector-Emitter breakdown voltage IC=10µA VCE(sat) Collector-Emitter saturation voltage IC=0.1A, IB=10mA 50 mV IC=2A, IB=50mA(e) 120 mV 30 Typ 50 Max Units 2 A 70 mA 10 mA 20 V hFE Static forward current transfer IC=200mA, VCE=2V ratio IC=2A, VCE=2V 209 116 COBO Output capacitance VCB=10V,f=1MHz 64 pF t(on) Turn-on time Ic=0 to IC=2A VIN=10V 30 ns t(off) Turn-off time IC=2A to Ic<100µA 28 ns 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 © Zetex Semiconductors plc 2008 6 www.zetex.com ZXLD1321 High-side current monitor Symbol Parameter Max Units VM_VIN Supply voltage 18 V VMON Sense voltage =V(M_VIN) – V(M_LOAD) 100 200 mV IM_LOAD Input current Measured into M_LOAD pin 0.08 1 µA TCO Temperature coefficient VMON=10mV VMON=100mV 370 150 ppm/ K BW Bandwidth VMON=10mV VMON=100mV 350 2.5 KHz MHz Gm Transconductance DIout/DVMON 1 mA/V Acc Accuracy (MON) Conditions Min Typ 3 RM = 0.1Ω VMON = 100mV 0 -3 3 % Max Units 500 mV Reference current monitor Symbol Parameter Conditions Min Typ VADJ Adjust Voltage TCO (MON) Temperature Coefficient VADJ=50mV VADJ=500mV 160 200 ppm/ K BW Bandwidth VADJ=50mV VADJ=500mV 275 3 KHz MHz Gm Transconductance DIout/DVADJ 200 µA/V Acc Accuracy 0 VADJ=500mV -3 3 % Max Units LED thermal control circuit (TADJ) parameters Symbol Parameter Conditions VTADJH Upper threshold voltage Onset of output current reduction (VTADJ falling) 75 mV VTADJL Lower threshold voltage Output current reduced to <10% of set value (VTADJ falling) 50 mV Gm(TADJ) Transconductance DIout/DVTADJ 4 mA/V Issue 1 - January 2008 © Zetex Semiconductors plc 2008 7 Min Typ www.zetex.com ZXLD1321 Output current regulation parameters Symbol Parameter Conditions IOUT Minimum output/ LED current(f) VIN>3V DIOUT Output current accuracy(g) 3.0V<VIN<12V, Iout=1A, VADJ=100mV TCO I(OUT) Output current temperature drift 3.0V<VIN<12V, Iout=1A, VADJ=100mV DIOUT/ IOUT Load current regulation 350mA<I(LED)<1A DIOUT/ DVIN Line voltage regulation of output current Eff Efficiency(f) Min Typ Max 2 A -5 100 350mA<I(LED)<1A Units +5 % 200 ppm/ K 2 %/A 0.5 %/V 85 % 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 Issue 1 - January 2008 © Zetex Semiconductors plc 2008 Reel size (mm) Reel width (mm) Quantity per reel 33.02 12 3,000 8 Device mark 1321 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<VADJ<500mV) or a low frequency Pulse Width Modulated (PWM) waveform. Issue 1 - January 2008 © 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 12µA. 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 100µA 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 150°C (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 © 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 = 50 mV 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 100kΩ resistor between the ADJ and VREF pins forms the upper part. A value of 1MΩ 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 5kΩ. 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 © 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 © 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 105°C 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 © 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 = 1.68kΩ gives 10mA nominal. The reduced base current will lower supply current and hence improve efficiency in lower power applications. Making R1 = 25mΩ 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 © 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 = 100mΩ 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 = 5kΩ and using a 103KT1608 thermistor, the thermistor will reach 869Ω at 105°C giving VTADJ = 74mV which will start to reduce the LED power above 105°C. By 125°C the thermistor will reach 547Ω 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 Part No Value Manufacturer Contact Details U1 ZXLD1321 LED Driver Zetex www.zetex.com D1 ZXCS2000 Schottky diode Zetex L1 MSS7341-103ML 10µH 2A Coilcraft www.coilcraft.co m L1 NPIS64D100MTRF 10µH 2A NIC www.niccomp L1 744 77810 10µH 2A Wurth www.wurth.co.uk C1 Generic 10nF 10V Generic C2 GRM31CR71H475K 4.7µF 50V Murata 1206 www.murata.com C3 GRM31MR71E225K 2.2µF 25V Murata 1206 www.murata.com R1 Generic 25mΩ Generic 0805 R2 Generic R3 Generic 100mΩ Generic 0805 R4 Generic 5.1kΩ Generic 0603 R5 Thermistor NTC 10k 103kt1608 Issue 1 - January 2008 © Zetex Semiconductors plc 2008 Generic 0603 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 10µs. 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 = 1.68kΩ gives 10mA nominal. The reduced base current will lower supply current and hence improve efficiency in lower power applications. Making R1 = 50mΩ 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 © 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 = 100mΩ 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 = 5kΩ and using a 103KT1608 thermistor, the thermistor will reach 869Ω at 105°C giving TADJ = 74mV which will start to reduce the LED power above 105°C. By 125°C the thermistor will reach 547Ω 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 Part No Value Manufacturer Contact Details U1 ZXLD1321 LED Driver Zetex www.zetex.com D1 ZXCS2000 Schottky diode Zetex L1 MSS7341-103ML 10µH 2A Coilcraft www.coilcraft.co m L1 NPIS64D100MTRF 10µH 2A NIC www.niccomp L1 744 777910 10µH 2A Wurth www.wurth.co.uk C1 Generic 10nF 10V Generic C2 GRM31CR71H475K 4.7µF 50V Murata 1206 www.murata.com C3 GRM31MR71E225K 2.2µF 25V Murata 1206 www.murata.com R1 Generic 25mΩ Generic 0805 R2 Generic R3 Generic 100mΩ Generic 0805 R4 Generic 5.1kΩ Generic 0603 R5 Thermistor NTC 10k 103kt1608 Issue 1 - January 2008 © Zetex Semiconductors plc 2008 Generic 0603 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 1.2µs at 100% LED power (VADJ = 500mV), increasing to about 8µs 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 100mΩ 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 25mΩ 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 © Zetex Semiconductors plc 2008 18 www.zetex.com ZXLD1321 Package outline - DFN14 (4x3x0.75) 60.3938 E E2 A PIN #1 IDENTIFICATION CHAMFER 0.300 X 45° b A3 D D2 PIN 1 DOT BY MARKING 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 © Zetex Semiconductors plc 2008 19 www.zetex.com 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. 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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. 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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. 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