AN50 Feed forward compensation for ZXSC300 LED driver Yong Ang, Application Engineer, Zetex Semiconductors Input voltage feed forward compensation for ZXSC300 to improve control of the LED current Introduction The ZXSC300 LED drivers do not directly control the LED current. As a consequence the LED current is dependent of the input voltage. This application note describes a way of reducing the supply voltage dependency by a method of supply voltage feed forward compensation. The method can also be used to provide temperature compensation of the LED. The ZXSC300 works on the PFM control scheme where the LED current is simply regulated by controlling the peak current through transistor Q1. The internal voltage threshold of current sense pin is around 19mV and transistor Q1 is switched off when its current reaches the preset threshold, thereby necessitating fewer external components required. However, this threshold value is invariant to the supply voltage level. In the event where input voltage increases, peak Q1 current will stay the same and current delivered to the LED creeps up which could potentially damage the LED if it exceeds the maximum rated current of the device. The circuit diagram in Figure 1 shows how to apply input voltage and thermal correction to a typical LED. A simple design guide for a single LED driver has also been put forward. The equations can generate a design capable of sourcing up to 200mA LED current, when used with the Zetex high current gain NPN transistor-ZXTN25012EFH. Input voltage feed forward compensation Normally, IPK is set by the output current threshold voltage VISENSE divided by RSENSE. As the input voltage increases, the inductor ripple current level ⌬I decreases because the transistor off time, TOFF is fixed by the ZXSC300 ⌬I = (VOUT - VIN) • TOFF ÷ L L discharges at a flatter slope to a higher minimum choke current IMIN = IPK - ⌬I, before transistor Q1 is turned on again. Figure 1 Circuit diagram of ZXSC300 with feed forward and thermal compensation Issue 1 - October 2007 © Zetex Semiconductors plc 2007 1 www.zetex.com AN50 Consequently, the average current IAV flowing through L increases and a shorter transistor on time, TON is required to charge boost inductor to the preset threshold current level IPK TON = ⌬I • L ÷ VIN By making the aforementioned assumptions for turn-on period and average coil current, the output power delivered to the LED is now determined from POUT = VLED • IAV • TOFF ÷ (TON + TOFF) Therefore, a higher power and LED current is delivered to the LED at high VIN for a fixed RSENSE and this elevated current could potentially damage the LED if it exceeds the maximum rated current of the device. Ignoring the effect of thermistor RT for the moment, a 100⍀ resistor ROFF can be inserted in series with RSENSE and feed forward resistor Rfb (see Figure 1) to inject a slight voltage offset across resistor RSENSE. This enables a lower Q1's current to build up the required VISENSE to turn the driver off, which regulates the LED current. The Rfb value has to be sufficiently big to lower dissipation and to prevent circuit from stalling. The circuit could stall at high input voltage if Rfb drops 19mV or more across 100⍀ resistor forcing the driver off all the time. It must be noted that ISENSE pin threshold on ZXSC300 has a positive temperature coefficient of 0.4%/°C. If a circuit nominal operating temperature is higher than 65°C, it could give approximately 20% increase in average LED current from that in 25°C ambient. When a feed forward network is used, this injects an offset voltage to the threshold pin. For instance, if an offset voltage of 9.5mV is used, the effective VISENSE temperature coefficient becomes double. Therefore, it is essential that thermal compensation is used with a feed forward approach. Feed forward components calculation For initial estimation, the associated IAV(VMAX) that delivers the required LED current can be determined from IAV(VMAX) = POUT ÷ (F • TOFF • VOUT) Where the transistor switching frequency F is given by F = VIN(MAX) ÷ VOUT ÷ TOFF Figure 2 www.zetex.com Example of current and voltage waveforms for circuit using ZXSC300 with feed forward network 2 Issue 1 - October 2007 © Zetex Semiconductors plc 2006 AN50 IAV(VMAX) is used to establish the required DC current rating, IDC for boost inductor L. The minimum inductor current is given by, IMIN(VMAX) = IAVE – 0.5 • (VOUT – VIN(MAX)) • TOFF /L A high L value is recommended to minimize errors due to propagation delays at high input voltage, which results in increased ripple and lower efficiency. And the maximum inductor current which relates to the Q1 peak current is IPK(VMAX) = 2 • IAVE - IMIN(VMAX) In practice, a higher IPK(VMAX) value can be used to account for the VCE saturation and switching edge loss in the transistor. The value of feed forward resistor Rfb is selected to give IPK(VMIN) at worse case input voltage and IPK(VMAX) at maximum input voltage. The internal VISENSE threshold on the ZXSC300 is typically 19mV with ±25% tolerance at 25°C. RSENSE has to drop less voltage than that demanded by VISENSE as Rfb will make a contribution to satisfy the threshold, which lowers IPK value with increasing input. Allowing for the positive temperature coefficient on ISENSE pin, effective threshold voltage level at operating temperature TAMB is; VISENSE@TAMB = 19mV ± 25% • 0.4%/°C • (TAMB - 25°C). At low supply voltage VIN(MIN) VISENSE@TAMB = IPK(VMIN) • RSENSE + VIN(MIN) • 100⍀ ÷ (Rfb + 100⍀) Whilst at VIN(MAX), VISENSE@TAMB = IPK(VMAX) • RSENSE + VIN(MAX) • 100⍀ ÷ (Rfb + 100⍀) Solving the above simultaneous equations gives the required RSENSE and Rfb resistor values. These design equations are also available as a spreadsheet calculator from Zetex website at www.zetex.com/zxsc300feedforward Figure 3 shows the measured LED current against variation in the input voltage with feed forward compensation. For comparison purpose, the same measurement is repeated with feed forward network removed, in which case the LED current at low supply is 3 times lower than that at nominal input voltage level. 70 LED Current (A) 60 50 Feed forward compensation No feed forward network 40 30 20 10 0 1.6 1.8 2 2.2 2.4 2.6 2.8 3 Input Voltage (V) Figure 3 LED current discrepancy for ZXSC300 with feed forward compensation Issue 1 - October 2007 © Zetex Semiconductors plc 2007 3 www.zetex.com AN50 The improvement in LED current regulation through feed forward compensation is self-evident. Although some discrepancy in LED current persists at low supply, this is predominantly due to the dependency of internal VISENSE threshold level on the input voltage level. To incorporate thermal compensation into the design, Rfb can be made up from a series combination of normal resistor R1 and NTC RT. During start-up condition, the printed circuit board’s and LED’s temperatures are low, hence RT has high resistance. As circuit temperature rises to its design operating value, the effective feed forward resistance drops, increasing the offset voltage on ISENSE pin, which in turn matches the elevated VISENSE value and hence regulates the actual output current fed to the LED. For instance, the required effective feed forward resistor value (R1+ RT) for 25°C ambient start-up can be determined from Rfb = VIN(MAX) • 100⍀ • (19mV ± 25% - IPK(VMAX) • RSENSE) And the required normal resistor R1 is equivalent to Rfb - RT. For this design, three NTC values (3.3K⍀, 4.7K⍀ and 6.8K⍀) are recommended. These resistors with MURATA 0603 or 0805 size NTC thermistors with beta-constant value of 3950K are chosen to give good current control response at both normal operating temperature and start-up conditions. The NTC works to reduce the peak transistor current, facilitating thermal feedback control to ensure that LED current and lumen maintenance expectation are achieved. Note that it is sometimes difficult to achieve perfect LED current matching between start-up and normal operating temperature. In extreme cases of large temperature gradients, the average LED current should be lower at start-up giving less lumen output, and then ramps up to the rated current once it reaches the normal operating temperature. Furthermore, the thermistor can be thermally coupled to the LED to provide response tracking and prevent overheating. Conclusion Two or three additional external components can be used to provide input voltage feed forward for ZXSC300. This serves to ensure that the LED current is closely regulated. The LED current regulation improves significantly when feed forward compensation is employed. The LED current at the worse case input voltage increases from 33% to 64% of the nominal LED current with a feed forward network. The remaining discrepancy is predominantly due to the dependency of the VISENSE threshold level on the input voltage level. In applications where the circuit is designed to operate in elevated ambient temperature, a NTC thermistor can be incorporated to facilitate thermal feedback control and prevent over heating. www.zetex.com 4 Issue 1 - October 2007 © Zetex Semiconductors plc 2006 AN50 Intentionally left blank Issue 1 - October 2007 © Zetex Semiconductors plc 2007 5 www.zetex.com AN50 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. 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