Application Note 49 PIN Diode Drivers AN 0049
User Manual: AN-0049
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National Semiconductor Application Note 49 March 1986 INTRODUCTION The DH0035/DH0035C is a TTL/DTL compatible, DC coupled, high speed PIN diode driver. It is capable of delivering peak currents in excess of one ampere at speeds up to 10 MHz. This article demonstrates how the DH0035 may be applied to driving PIN diodes and comparable loads which require high peak currents at high repetition rates. The salient characteristics of the device are summarized in Table 1. The charge control model of a diode1,2 leads to the charge continuity equation given in Equation (1). Parameter Conditions Q = charge due excess minority carriers τ = mean lifetime of the minority carriers Value 30V Max. Voltage (V+ − V−) (2) where: I = steady state “ON” current. Output Current 1000 mA Maximum Power 1.5W trise PRF = 5.0 MHz V+ − V− = 20V tfall 10% to 90% V+ − V− = 20V tdelay (1) where: Equation (1) implies a circuit model shown in Figure 2. Under steady conditions hence: TABLE 1. DH0035 Characteristics Differential Supply PIN Diode Drivers PIN Diode Drivers 10 ns 15 ns 10 ns 90% to 10% PIN DIODE SWITCHING REQUIREMENTS Figure 1 shows a simplified schematic of a PIN diode switch. Typically, the PIN diode is used in RF through microwave frequency modulators and switches. Since the diode is in shunt with the RF path, the RF signal is attenuated when the diode is forward biased (“ON”), and is passed unattenuated when the diode is reversed biased (“OFF”). There are essentially two considerations of interest in the “ON” condition. First, the amount of “ON” control current must be sufficient such that RF signal current will not significantly modulate the “ON” impedance of the diode. Secondly, the time required to achieve the “ON” condition must be minimized. AN008750-1 AN008750-2 I = Total Current IDC = SS Control Current iRF = RF Signal Current FIGURE 2. Circuit Model for PIN Switch The conductance is proportional to the current, I; hence, in order to minimize modulation due to the RF signal, IDC @ iRF. Typical values for IDC range from 50 mA to 200 mA depending on PIN diode type, and the amount of modulation that can be tolerated. The time response of the excess charge, Q, may be evaluated by taking the Laplace transform of Equation (1) and solving for Q: (3) Solving Equation (3) for Q(t) yields: (4) Q(t) = L−1 [Q(s)] = Iτ (1 − e−t/τ) The time response of Q is shown in Figure 3. As can be seen, several carrier lifetimes are required to achieve the steady state “ON” condition (Q = IDC • τ). FIGURE 1. Simplified PIN Diode Switch AN-49 © 1999 National Semiconductor Corporation AN008750 www.national.com be removed. Again, in order to remove the charge rapidly, a large peak current (in the opposite direction) must be applied to the PIN diode: The time response of the charge, hence the time for the diode to achieve the “ON” state could be shortened by applying a current spike, Ipk, to the diode and then dropping the current to the steady state value, IDC, as shown in Figure 4. The optimum response would be dictated by: (Ipk) (t) = τ • IDC (5) (6) It is interesting to note an implication of Equation (5). If the peak turn on current were maintained for a period of time, say equal to τ, then the diode would acquire an excess charge equal to Ipk • T. This same charge must be removed at turn off, instead of a charge IDC • τ, resulting in a considerably slower turn off. Accordingly, control of the width of turn on current peak is critical in achieving rapid turn off. APPLICATION OF THE DH0035 AS A PIN DIODE DRIVER The DH0035 is specifically designed to provide both the current levels and timing intervals required to optimally drive PIN diode switches. Its schematic is shown in Figure 5. The device utilizes a complementary TTL input buffer such as the DM7830/DM8830 or DM5440/DM7440 for its input signals. Two configurations of PIN diode switch are possible: cathode grounded and anode grounded. The design procedures for the two configurations will be considered separately. ANODE GROUND DESIGN Selection of power supply voltages is the first consideration. Table I reveals that the DH0035 can withstand a total of 30V differentially. The supply voltage may be divided symmetrically at ± 15V, for example. Or asymmetrically at +20V and −10V. The PIN diode driver shown in Figure 6, uses ± 10V supplies. When the Q output of the DM8830 goes high a transient current of approximately 50 mA is applied to the emitter of Q1 and in turn to the base of Q5. Q5 has an hfe = 20, and the collector current is hfe x 50 or 1000 mA. This peak current, for the most part, is delivered to the PIN diode turning it “ON” (RF is “OFF”). Ipk flows until C2 is nearly charged. This time is given by: AN008750-3 FIGURE 3. (7) where: ∆V = the change in voltage across C2. Prior to Q5’s turn on, C2 was charged to the minus supply voltage of −10V. C2’s voltage will rise to within two diode drops plus a Vsat of ground: (8) V = |V−| − Vf(PIN Diode) − VfCR1 − VsatQ5 for V− = −10V, ∆V = 8V. Once C2 is charged, the current will drop to the steady state value, IDC, which is given by: AN008750-4 FIGURE 4. The turn off requirements for the PIN diode are quite similar to the turn on, except that in the “OFF” condition, the steady current drops to the diode’s reverse leakage current. A charge, IDC • τ, was stored in the diode in the “ON” condition and in order to achieve the “OFF” state this charge must (9) where: VCC = 5.0V R1 = 250Ω R3 = 500Ω (10) www.national.com 2 AN008750-5 FIGURE 5. DH0035 Schematic Diagram AN008750-6 FIGURE 6. Cathode Grounded Design 3 www.national.com For the driver of Figure 6, and IDC = 100 mA, RM is 56Ω (nearest standard value). Q3 absorbs the stored base charge of Q5 facilitating its rapid turn-off. As Q5’s collector begins to rise, Q4 turns “ON”. At this instant, the PIN diode is still in conduction and the emitter of Q4 is held at approximately −0.7V. The instantaneous current available to clear stored charge out of the PIN diode is: Returning to Equation (7) and combining it with Equation (5) we obtain: (11) Solving Equation (11) for C2 gives: (12) (15) For τ = 10 ns, C2 = 120 pF. where: One last consideration should be made with the diode in the “ON” state. The power dissipated by the DH0035 is limited to 1.5W (see Table I). The DH0035 dissipates the maximum power with Q5 “ON”. With Q5 “OFF”, negligible power is dissipated by the device. Power dissipation is given by: hfe + 1 = current gain of Q4 = 20 VBE Q4 = base-emitter drop of Q4 = 0.7V Vf(PIN) = forward drop of the PIN diode = 0.7V For typical values given, Ipk = 400 mA. Increasing V+ above 10V will improve turn-off time of the diode, but at the expense of power dissipation in the DH0035. Once turn-off of the diode has been achieved, the DH0035 output current drops to the reverse leakage of the PIN diode. The attendant power dissipation is reduced to about 35 mW. (13) where: D.C. = Duty Cycle = CATHODE GROUND DESIGN Figure 7 shows the DH0035 driving a cathode grounded PIN diode switch. The peak turn-on current is given by: In terms of IDC: (16) = 800 mA for the values shown. The steady state current, IDC, is set by Rp and is given by: (14) For the circuit of Figure 6 and a 50% duty cycle, P diss = 0.5W. Turn-off of the PIN diode begins when the Q output of the DM8830 returns to logic “0” and the Q output goes to logic “1”. Q2 turns “ON”, and in turn, causes Q3 to saturate. Simultaneously, Q1 is turned “OFF” stopping the base drive to Q5. www.national.com (17) where: 2VBE = forward drop of Q4 base emitter junction plus Vf of the PIN diode = 1.4V. 4 AN008750-7 FIGURE 7. Anode Grounded Driver For a diode with τ = 10 ns and IDC = 100 mA, C1 = 250 pF. Again the power dissipated by the DH0035 must be considered. In the “OFF” state, the power dissipation is given by: In terms of Rp, Equation (17) becomes: (18) For the circuit of Figure 7, and IDC = 100 mA, Rp is 62Ω (nearest standard value). It now remains to select the value of C1. To do this, the change in voltage across C1 must be evaluated. In the “ON” state, the voltage across C1, Vc, is given by: (23) where: D.C. = duty cycle = The “ON” power dissipation is given by: (19) For the values indicated above, (Vc)ON = 3.8V. In the “OFF” state, Vc is given by: (24) where: (Vc)ON is defined by Equation (19). Total power dissipated by the DH0035 is simply PON + POFF. For a 50% duty cycle and the circuit of Figure 7, P diss = 616 mW. The peak turn-off current is, as indicated earlier, equal to 50 mA x hfe which is about 1000 mA. Once the excess stored charge is removed, the current through Q5 drops to the diodes leakage current. Reverse bias across the diode = V− − Vsat ≅ −10V for the circuit of Figure 7. (20) = 8.0V for the circuit of Figure 7. Hence, the change in voltage across C1 is: (21) The value of C4 is given, as before, by Equation (12): (22) 5 www.national.com PIN Diode Drivers SUMMARY A unique circuit utilized in the driving of PIN diodes has been presented. Further a technique has been demonstrated which enables the designer to tailor the DH0035 driver to the PIN diode application. REPETITION RATE CONSIDERATIONS Although ignored until now, the PRF, in particular, the “OFF” time of the PIN diode is important in selection of C2, RM, and C1, Rp. The capacitors must recharge completely during the diode “OFF” time. In short: 4 RMC2 ≤ tOFF 4 RpC1 ≤ tOFF (25) (26) REFERENCES 1. “Pulse, Digital, & Switching Waveforms”, Jacob Millman & Herbert Taub, McGraw-Hill Book Company, Inc., New York, N.Y. CONCLUSION The circuit of Figure 7 was breadboarded and tested in conjunction with a Hewlett-Packard 33622A PIN diode. IDC was set at 100 mA, V+ = 10V, V− = 10V. Input signal to the DM8830 was a 5V peak, 100 kHz, 5 µs wide pulse train. RF turn-on was accomplished in 10–12 ns while turn-off took approximately 5 ns, as shown in Figure *NO TARGET FOR fig NS0292* and Figure *NO TARGET FOR fig NS0292*. In practice, adjustment C2 (C1) may be required to accommodate the particular PIN diode minority carrier lifetime. 2. “Models of Transistors and Diodes”, John G. Linvill, McGraw-Hill Book Company, Inc., New York, N.Y. 3. National Semiconductor AN-18, Bert Mitchell, March 1969. 4. Hewlett-Packard Application Note 314, January 1967. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: AN-49 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: support@nsc.com www.national.com 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Europe Fax: +49 (0) 1 80-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 1 80-530 85 85 English Tel: +49 (0) 1 80-532 78 32 Français Tel: +49 (0) 1 80-532 93 58 Italiano Tel: +49 (0) 1 80-534 16 80 National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: sea.support@nsc.com National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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