AN103 The FET Constant-Current Source/Limiter Introduction The combination of low associated operating voltage and high output impedance makes the FET attractive as a constant-current source. An adjustable-current source (Figure 1) may be built with a FET, a variable resistor, and a small battery. For optimum thermal stability, the FET should be biased near the zero temperature coefficient point. D S RL RS – A change in supply voltage or a change in load impedance, will change ID by only a small factor because of the low output conductance goss. ID = (VDS)(goss) (3) The value of goss is an important consideration in the accuracy of a constant-current source where the supply voltage may vary. As goss may range from less than 1 S to more than 50 S according to the FET type, the dynamic impedance can be greater than 1 M to less than 20 k. This corresponds to a current stability range of 1 A to 50 A per volt. The value of goss also depends on the operating point. Output conductance goss decrease approximately linearly with ID. The relationship is + Figure 1. Field-Effect Transistor Current Source NO TAG Whenever the FET is operated in the current saturated region, its output conductance is very low. This occurs whenever the drain-source voltage VDS is at least 50% greater than the cut-off voltage VGS(off). The FET may be biased to operate as a constant-current source at any current below its saturation current IDSS. ID g oss IDSS g oss (4) where goss = goss (5) when VGS = 0 (6) So as VGS → VGS(off), goss → Zero. For best regulation, ID must be considerably less than IDSS. Cascading for Low goss Basic Source Biasing For a given device where IDSS and VGS(off) are known, the approximate VGS required for a given ID is V GS V GS(off) 1 – I ID DSS 1k It is possible to achieve much lower goss per unit ID by cascading two FETs, as shown in Figure 2. S D (1) D Q1 Q2 S RL where k can vary from 1.8 to 2.0, depending on device geometry. If K = 2.0, the series resistor RS required between source and gate is RS V GS ID or RS V GS(off) ID 1– ID IDSS RS – + VDD (2) Figure 2. Cascade FET Current Source Updates to this app note may be obtained via facsimile by calling Siliconix FaxBack, 1-408-970-5600. Please request FaxBack document #70596. Siliconix 10-Mar-97 1 AN103 JFET may have a typical goss = 4 mS at VDS = 20 V and VGS = 0. At VDS –VGS(off) = 2 V, goss 100 mS. D Q2 S The best FETs for current sources are those having long gates and consequently very low goss. The Siliconix 2N4340, J202, and SST202 exhibit typical goss = 2 mS at VDS = 20 V. These devices in the circuit of Figure 4 will provide a current source adjustable from 5 mA to 0.8 mA with internal impedance greater than 2 MW at 0.2 mA. Other Siliconix part types such as the 2N4392, J112, and SST112 can provide 10 mA or higher current. D Q1 S (a) IO VGS2gfs2 goss2 + VDS2 – I2 + goss1 + + VO – D – 30 V Q1 S VDS1 = –VGS2 – = IO/goss1 200 W (Optional) RS = 1 MW RS (b) Figure 3. Cascade FET VGS1 = 0 Now, ID is regulated by Q1 and VDS1 = –VGS2. The dc value of ID is controlled by RS and Q1. However, Q1 and Q2 both affect current stability. The circuit output conductance is derived as follows: If goss1 = goss2 go g oss 2 g fs g oss (7) (8) Figure 4. Adjustable Current Source RS = 1 MW Instead of the adjustable resistor, the JFETs can be put in IDSS range groupings with an appropriate RS resistor selected for each group. This method is common in high volume applications. The cascade circuit of Figure 5 provides a current adjustable from 2 mA to 0.8 mA with internal resistance greater than 10 MW. when RS 0 as in Figure 2 + g oss 2 go g fs1 R Sg fs (9) Q2 In either case (RS = 0 or RS 0), the circuit output conductance is considerably lower than the goss of a single FET. In designing any cascaded FET current source, both FETs must be operated with adequate drain-gate voltage, VDG. That is, VDG VGS(off), preferably VDG 2VGS(off) 30 V S D Q1 S 100 W (Optional) Q1 = 2N4340, J202, SST202 Q2 = 2N4341, J304, SST304 RS = 1 MW RS (10) If VDG < 2 VGS(off), the goss will be significantly increased, and circuit go will deteriorate. For example: A 2 D – Figure 5. Cascade FET Current Source Siliconix 10-Mar-97 AN103 CR160 CR180 CR200 CR220 CR240 CR270 TO-18 2-Lead CR300 Package CR330 CR360 CR390 CR430 CR470 IF Part Type VF J500 J501 SST/J502 SST/J503 SST/J504 SST/J505 SST/J506 SST/J507 SST/J508 SST/J509 J = TO-226AA 2-Lead Package SST = TO-236 (SOT-23) Package SST/J510 SST/J511 0.1 0.2 0.5 1 2 5 10 IF – Regulator Current (mA) Figure 6. Standard Series Current Regulator Range Standard Two-Leaded Devices Siliconix offers a special series of two-leaded JFETs with a resistor fabricated on the device, thus creating a 10% current range. Devices are available in ranges from 1.6 mA (CR160) to 4.7 mA (CR470). For designs requiring a 20% current range, Siliconix offers devices rated from 0.24 mA typical (J500) through 4.7 mA typical (J511) in a two-leaded TO-226A (TO-92) package. The SST502 series is available in surface mount TO-236 (SOT-23). Bias Resistor Selection All industry JFET part types exhibit a significant variation in IDSS and VGS(off) on min/max specifications and device-to-device variations. Using the simple source biasing current source as illustrated in Figure 1, the designer can graphically calculate the RS which best fits the desired drain current ID. Figure 7 plotting ID versus VGS over the military temperature range shows the resulting ID for different values of RS. The RS lines are constructed by drawing the slope of the RS desired value starting at the origin, eg. RS = 2 k slope. Find a convenient point on the X – Y axis to mark a Each of these two-leaded devices can be used to replace several typical components. V GS of 2 k such as VGS = –1.5 V and ID = 0.75 mA. ID Figure 6 shows the current ranges of these two device series. Further information is contained in the individual data sheets appearing elsewhere in this data book or from Siliconix FaxBack. Then, draw a straight line from this point to the origin. The intersection of this RS line and the device ID versus VGS will be the operating ID. In this example, the resulting ID = 0.35 mA at TJ = 25C. The intercepts of the TJ = –55C and 125C show the minimal variation with temperature. The CR160 series features guaranteed peak operating voltage minimum of 100 V with a typical of 180 V. The J500 series features 50 V minimum with a typical of 100 V. The lower current devices in both series provide excellent current regulation down to as little as 1 V. Siliconix 10-Mar-97 Also note that JFETs have a ID current where there is no change with temperature variation. To achieve this TC, the –VGS voltage (ID x RS) is approximately: VGS(0TC) VGS(off) – 0.65 V (11) 3 AN103 2.00 TJ = –55C 1.75 I D – Drain Current (mA) RS = 0.2 k 1.50 VDS = 4 to 20 V 2N4339 max SST/J202 (low end) 0.5 k 1.25 1k 25C 1.00 2k 0.75 125C 0.50 5k 0.25 10 k 20 k 0 0 –0.4 –0.8 –1.2 –1.6 –2 VGS – Gate-Source Voltage (V) Figure 7. JFET Typical Transfer Characteristic 1000 I D – Drain Current ( A) VDD = 5 to 30 V TJ = 25C except as noted TJ = –55C 25C 2N/PN4119A SST4119 Max 2N/PN4118A SST4118 Max 125C 100 2N/PN4118A SST4118 Min 2N/PN4117A SST4117 Max 2N/PN4119A SST4119 Min 2N/PN4117A SST4117 Min 10 0.5 0.1 1 5 10 50 100 RS – Source Resistance (k) Figure 8. Source Biased Drain-Current vs. Source Resistance 2 I D – Drain Current (mA) SST/J202 Max 25C TJ = –55C 1 2N4339 Max 125C VDD = 4 to 20 V TJ = 25C except as noted SST/J201 Max 2N4338 Max SST/J202 Min 2N4339 Min SST/J201 2N4338 Min 0.1 VDD RS 0.01 0.1 0.5 1 5 10 20 RS – Source Resistance (k) Figure 9. JFET Source Biased Drain-Current vs. Source Resistance 4 Siliconix 10-Mar-97 AN103 Table 1: Source Biasing Device Recommendations Choosing the Correct JFET for Source Biasing Each of the Siliconix device data sheets include typical transfer curves that can be used as illustrated in Figure 7. Several popular devices are ideal for source biased current sources covering a few As to 20 mA. To aid the designer, the devices in Table 1 have been plotted to show the drain current, ID, versus the source resistance, RS, in Figures 8, 9, and 10. Most plots include the likely worst case ID variations for a particular RS. For tighter current control, the JFET production lot can be divided into ranges with an appropriate resistor selection for each range. Practical Current Range ID (mA) Through-Hole Plastic Device Surface Mount Device Metal Can Device 0.01 – 0.02 PN4117A SST4117 2N4117A 0.01 – 0.04 PN4118A SST4118 2N4118A 0.02 – 0.1 PN4119A SST4119 2N4119A 0.01 – 0.1 J201 SST201 2N4338 0.02 – 0.3 J202 SST202 2N4339 0.1 – 2 J113 SST113 2N4393 0.2 – 10 J112 SST112 2N4392 20 –55C I D – Drain Current (mA) 10 VDD = 5 to 30 V TJ = 25C except as noted TJ = 25C Mid 2N4392, SST/J112 125C Mid 2N4393, SST/J113 Max Min 2N4393 SST/J113 1 Min 2N4392, SST/J112 VDD RS 0.1 0.1 0.5 1 5 10 RS – Source Resistance (k) Figure 10. JFET Source Biased Drain-Current vs. Source Resistance Siliconix 10-Mar-97 5