7 2 3 6 1 10 9 5 8 logic
input
interface gnd mode
control c t
n/c 4 control
logic
digital
to
analog linear
to log
converter current
controlled
resistance clock
oscillator up
control down
control r a
r b
logic supply
(memory) analog
supply features ? 1.0 to 5 v dc supply voltage ? 70 m a of analog current drain (typical) ? 6 m a of memory current drain (typical) ? single or dual switch control ? touch plate compatible, (30 m w typ. impedance) ? typical 42 db range ? adjustable clock frequency increases or decreases time required to change diode impedance digitally controlled transconductance block gt560 data sheet standard packaging ? 10 pin micropac ? 10 pin plid ? ? chip (79 x 60 mils) au bump gennum corporation p.o. box 489, stn a, burlington, ontario, canada l7r 3y3 tel. (905) 632-2996 fax: (905) 632-5946 japan branch: b-201, miyamae village, 2-10-42, miyamae, suginami-ku, tokyo 168, japan tel. (03) 3334-7700 fax (03) 3247-8839 document no. 510 - 64 - 02
circuit description the gt560 is a low voltage transconductance block which can be used as an electronic volume control. the transconductance element consists of two diodes back-to- back, whose impedance is varied by changing the amount of current through the diodes. the impedance of the diodes is controlled by digital circuitry which consists of: ? an oscillator with an external capacitor, c t , to set the ramp frequency ? c, ? ? 5 x 10 -7 c c t ? logic interface, which senses the volume up-down controls ? a digital / analog converter ? the control logic/debounce logic ? synchronous up-down counter to increase or decrease the gt560 impedance, the switching mechanism can be touch sensitive contacts or a mechanical switch. in either configuration, the switches are connected from pin 8 to v cc and pin 9 to v cc . any resistance (30 m w typ.), from pin 8 or 9 to v cc will activate the gt560 control circuitry and the device will change impedance levels. block diagram revision date: march 1994
2 510 - 64 - 02 operation the impedance can be controlled using one of two methods. the first method utilizes a single pole single throw (spst), momentary contact switch which combines the up/down function in one action (see fig. 2). the second method (see fig. 3) uses one single pole double throw (spdt), or two spst switches which provide a separate action for the up-down function. these switches can be mechanical or touch sensitive. to operate the gt560 using a single switch, the mode control (mc) is connected to the positive supply, the switch will then act in an alternating manner first increasing impedance when activated, then decreasing impedance when activated a second time, both at a rate determined by the external timing capacitor. if separate up or down switches are used, the mode control pin connects to ground. making contact to the down switch, the diode impedance will decrease until the switch is released or the diodes reach minimum impedance (4 k w ). similarly, contact to the up switch will increase the diodes impedance until the switch is released or the maximum value of diode impedance has been reached (500 k w ). once the minimum (4 k w ) or maximum (500 k w ) impedance is reached and the switch is still closed, the impedance will stop changing until the switch is released and the direction is reversed. should both switches be closed simultaneously, the output impedance will remain unchanged. in order for the gt560 to be activated and change impedance the switch must be pressed for at least one time constant, t c , where t c = 1 c normally the timing capacitor c t , is connected to ground, as shown in figure 1. when the circuit is turned on, c t is required to charge from 0 v dc to approximately 0.56 v dc . during this charge up condition, the clock is disabled and ramping of the impedance by the touch contacts or switches will be inactive until c t is charged. small values of c t eg. 0.1 m f, will cause a time delay of less than 2 seconds, however as the value of c t is increased, eg. 1.0 m f the charge time can be as high as 20 seconds. if the length of the charge time is undesirable, the capacitor can be referenced to a microphone decoupling point as shown in figure 3. in this configuration the capacitor charge time will be less than 2 seconds with a 1.0 m f capacitor. it is required that c t be connected to a positive reference voltage which has a high supply line rejection ratio, such as a voltage regulator, or a microphone decoupling point. if c t was referenced directly to the supply, any battery line signals greater than 10 mvrms will affect the clock rate on the gt560 this would cause the ramp speed to become erratic as the impedance is being changed. when the hearing aid is initially turned on, e.g. insertion of the battery , the diode impedance will be at mid gain setting (which is typically 7 steps from the minimum impedance setting). once the desired impedance is determined this value will be held constant by the memory circuitry on the gt560. connecting the memory supply (vm), to the battery positive bypassing the on-off switch, will allow the volume control to retain the selected impedance even with the hearing aid switched off. the memory current drain is extremely low at 6 m a. if however the battery is removed, memory will be lost. the gt560 may be used as the feedback impedance across an inverting gain block, such as a lc508 preamplifier. the gain of the amplifier would then be determined by the ratio of the diode impedance to the source impedance of the lc508 preamplifier. for example, if a microphone with a typical impedance of 4 k w were to be used as the source to the lc508 preamplifier, and the gt560 connected across the input and output pins with a minimum impedance of 4 k w , the minimum gain would be a ratio of 4:4 (or 0 db). with the maximum impedance at 500 k w , the maximum gain would be a ratio of 500:4 approximately (or 42 db). the volume control range is 42 db. it is important that either r a or r b on the gt560 be dc coupled to the lc/lp508 preamplifier stage to set up a reference voltage on the diodes, otherwise the volume control will not be biased properly. the gt560 pins r a and r b , will operate from 200 mv below supply to 500 mv above ground. note: a resistor of 1.2 m w is required between pin 2 and pin 3 to limit the step size to 3.0 db. if this resistor is not used at high gain settings the steps will be greater than 3.0 db, resulting in high distortion on the lc/lp508.
3 510 - 64 - 02 7 0.1 2 4 3 6 s1 v in 1 0.1 10 ic 1 9 5 50pf -15vdc +15
vdc +15vdc 10 + 8 gt560 1k v b =1.3vdc n/c 15 22k v out +15vdc 0.1 v a 1.2m w m c c u r b r a c d g v l c t s2 3.9k 10 + -15vdc parameter value & units supply voltage 5 v dc power dissipation at t a 70 c 25 mw operating temperature -10 c to + 40 c storage temperature -20 c to + 70 c absolute maximum ratings pin 1 5 6 10 top view pin connection vreg vcc rect i/p rect cap a1 o/p a1 i/p gnd cb ca a2 o/p up control pin connection 5 6 1c t logic supply down control mode control analog supply gnd r b r a n/c 10 caution class 1 esd sensitivity electrical characteristics c onditions: temperature = 25 o c, frequency = 1 khz, supply voltage = 1.3 vdc parameter symbol conditions min typ max units total current i tot minimum impedance 10 70 130 m a logic current i logic - 6 10 m a distortion thd v out = 10 mv rms - 1 2.9 % step size 1.0 2.5 4.0 db impedance : low z low minimum impedance, v out = 15 mv rms , see note 1 2.5 4 6 k w high z high maximum impedance, v out = 15 mv rms , see note 1 250 500 700 k w volume control range 36 42 48 db all parameters and switches remain as shown in test circuit unless otherwise stated in conditions column notes: 1 measure v in for v out = 15 mv rms z = v out x 3.9k v in all resistors in ohms, all capacitors in microfarads unless otherwise stated ic1 = ca 3140 operational amplifier fig.1 gt560 test circuit
4 510 - 64 - 02 7 2 3 6 1 10 1 / 3 9 5 lc508 0.1 0.1 8 gt560 v b =1.3vdc s1 r e 22k 0.1 1.2m s3 3.9k 1 / 3 lc508 1 / 3 lc508 + 1.0 + r s 3.9k s2 2 4 5 10 3 7 9 8 1 6 r cvr all resistors in ohms, all capacitors in microfarads unless otherwise stated s1, s2 are momentary switches or contacts closing s1 increases gain closing s2 decreases gain fig. 3 typical hearing aid application all resistors in ohms, all capacitors in microfarads unless otherwise stated s1 can be a momentary spst switch or a pair of contacts. on initial power-up (insertion of the battery) the gain will be at a random setting fig. 2 typical hearing aid circuit 7 2 3 6 1 10 + 5 lc549 0.068 0.1 8 gt560 v b =1.3vdc s1 7 0.1 6.8 1.2m s2 6.8 1 / 2 lp508 1.0 r s ?3.9k r cvr
2 4 5 5 4 6 1 8 3 1 6.8 2
5 510 - 64 - 02 7 2 3 6 1 10 9 5 0.1 8 gt560 v b =1.3 vdc s1 1.2m 47n ld502 on-off 39k lc551 0.1 r cvr
4 4 5 0.1 3 7 9 8 1 6 3 10 1.0 100k 2 0.1 7 2 6 1 10 gain
set 5 8 s2 47n all resistors in ohms, all capacitors in microfarads unless otherwise stated gennum corporation assumes no responsibility for the use of any circuits described herein and makes no representations that they are free from patent infringement. ? copyright april 1989 gennum corporation. all rights reserved. printed in canada. document identification product proposal this data has been compiled for market investigation purposes only, and does not constitute an offer for sale. advance information note this product is in development phase and specifications are subject to change without notice. gennum reserves the right to remove the product at any time. listing the product does not constitute an offer for sale. preliminary data sheet the product is in a preproduction phase and specifications are subject to change without notice. data sheet the product is in production. gennum reserves the right to make changes at any time to improve reliability, function or design, in order to provide the best product possible. revision notes: pb/sn bump omitted fig. 4 ld502 / gt560 / lc551 typical application a - i bo = 50 na r ext = 1.2m w b - i bo = 25 na r ext = 1.2m w c - i bo = 0 na r ext = 1.2m w d - i bo = 25 na r ext = 390 k w steps note: i bo is the input bias current of the lp/lc508 preamplifiers steps a b c a - i bo = 50 na b - i bo = 25 na c - i bo = 0 na 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 fig. 6 output impedance characteristics with r ext = 1.2 m w a b c d 70 60 50 40 30 20 10 0 impedance range (db) fig. 5 output impedance characteristics without external shunt resistor 70 60 50 40 30 20 10 0 impedance range (db)
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