2252-DEMO THAT Corporation, 2252-DEMO Datasheet - Page 8

• Manufacturer Part Number: 2252-DEMO
• Description: Power Management Modules & Development Tools RMS Detector Demo Board
• Manufacturer: THAT Corporation
• Type: AC/DC Switching Convertersr

Specifications of 2252-DEMO

• Input Voltage: 12 V
• Maximum Operating Temperature: + 75 C
• Minimum Operating Temperature: - 20 C
• Product: Power Management Modules
• Supply Current: 1 mA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Page 8
R
volt age.
Bias Pro gramming
As shown in Fig ure 1 (page 1), pin 2 is con nected
to the neg a tive sup ply through three di odes in se -
ries. Its typ i cal volt age is V
in these di odes, I
through out the 2252, and there fore af fects the
band width of the de vice, as well as I
is re spon si ble for pro gram ming so many cur rents,
I
range about the nom i nal 24 A.
Normally, I
ref er ence point. I
rives from the ge om e try of the de vices in the 2252.
As with the value of R
sired I
lows: R
Power Sup ply By passing
The 2252 is not par tic u larly sen si tive to power
sup ply im ped ances, but the high charg ing cur rents
which are pos si ble in C
PC board ground and volt age sup ply sys tem for
good re sults. The ap pli ca tion cir cuits show a 1 k
re sis tor in se ries with the pos i tive sup ply pin
(pin 8), and a 10 f ca pac i tor by pass ing this pin to
ground. The ground end of this ca pac i tor is in -
tended to be con nected di rectly to the ground end
of C
should be con nected to the cir cuit ground sys tem,
since high cur rents flow in this trace dur ing in put
sig nal tran sients.
Fig ure 8 il lus trates the flow of cur rent, I
dur ing an in put tran sient. Note that I
sys tem, while C
po ten tial through the sin gle con nec tion to ground.
An other by pass ca pac i tor is re quired at pin 2 (the
I
ance at pin 2 is ap prox i mately 3 k
put sig nal ca pac i tively cou ples into pin 2, I
BIAS
BIAS
T
PART OF
De cou pling Sup ply Transients
OA2
WITHIN
=
2252
T
pin). Typically, with I
should be re stricted to a rel a tively nar row
. One (and only one) point on this trace
V
BIAS
EE
b
I
=
T
+
8
BIAS
Fig ure 10.
and the neg a tive sup ply volt age, as fol -
V
1.4
EE
Q6
I
, where V
BIAS
I
1k
CHARGE
-
is cho sen to fine-tune I
T
2.1
BIAS
BIAS
main tains its ref er ence to ground
6
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
.
, con trols t he cur rent sources
T
=
C
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
, R
V+
T
8.4
EE
T
b
must be kept out of the
BIAS
is de ter mined by the de -
EE
I
is the neg a tive sup ply
in0
I
C
T
B
+ 2.1 V. The cur rent
2
. The fac tor 8.4 de -
con fined to a lo cal
loop, flow ing from
C
and Q6 (within the
2252) into C
re turn ing di rectly
to C
the
short,
nect ing it to sys -
tem
only
high cur rents are
kept out of the
over all
24 A, the im ped -
B
to V–. If the in -
in0
B
through OA2
re turn
. By keep ing
in0
ground
one
. Be cause it
CHARGE
and
, the 0 dB
BIAS
CHARGE
ground
T
point,
, and
trace
con -
will
is
at
,
be mod u lated at the in put sig nal fre quency. This
mod u la tion adds sig nal volt age (as rip ple) to the
out put at pin 7. (Re call that, ide ally, the 2252 pro -
duces rip ple only at the sec ond har monic of the in -
put sig nal.) When the 2252 is used to con trol the
gain of a volt age-controlled am pli fier (VCA) such
as the THAT 2180 or 2181, this fun da men -
tal-frequency rip ple will cause sec ond har monic
dis tor tion in the VCA. By passing pin 2 to the neg a -
tive sup ply shunts any sig nal-related cur rents
around the 2252 bias sys tem, pre vent ing this rip -
ple from occuring.
Out put Con sid er ations
The out put of the 2252 (pin 7) pres ents a low
source im ped ance, but it is some what lim ited in
avail able cur rent drive. Re ferring to Fig ure 1
(page 1), the neg a tive cur rent source at the out put
pin is ap prox i mately 10 I
typ i cal I
at pin 7 is 240 A. (The 2252 will source con sid er -
ably more than this cur rent if re quired.) Since the
out put volt age nor mally swings be tween 300 mV,
the 2252 will di rectly drive loads of greater than
1.5 k
In the sec tion on In put Bias Cur rents and Volt -
ages (page 6), it was noted that the 2252 out put is
gen er ally pro por tional to the log of the ac rms in -
put cur rent. But, at low lev els (<20 nA in put cur -
rent), the in put bias com pen sa tion in in ter nal
opamp OA2 in Fig ure 3 (page 3) can cause a sharp
in crease in the out put change with in put, as shown
in Fig ure 8, (page 6). Since this ef fect is de pend ent
on the ac cu racy of the bias com pen sa tion, some
2252s will ex hibit this be hav ior, but most will not.
For most ap pli ca tions, where con sis tent low-level
re sponse is desireable, the 2252 should be con fig -
ured to sup ply its own “cor rec tion” sig nal to pre -
vent this dis con ti nu ity from occuring, as shown in
the cir cuit of Fig ure 4, (page 4). In this cir cuit, a
22 M
of the 2252 back to the in put.
When the out put volt age is near 0 V, Rf con trib utes
neg li gi ble cur rent to the in put of the de tec tor. For
pos i tive out put volt ages, the cur rent in Rf is very
small com pared to the ac in put sig nal which
causes a pos i tive out put, so its ef fect is again neg li -
gi ble. How ever, as the ac in put sig nal drops in
level, the dc out put goes neg a tive, sup ply ing more
and more in put cur rent through Rf, which tends to
drive the dc out put pos i tive. The point at which
the two ef fects will bal ance is de pend ent on I
I
cuit of Fig ure 4, the out put will “bot tom out” at ap -
prox i mately -300 mV, where Rf sup plies ~14 nA of
dc cur rent to the in put.
Rf es tab lishes a sta ble “floor” in the 2252’s re -
sponse. How ever, this self-supplied lower limit will
ob scure very low-level in put sig nals which could
be mea sured by those 2252s with good in put-bias
cor rec tion at OA2 of Fig ure 3. If ac cu rate low-level
T
, and the size of the feed back re sis tor. In the cir -
with out dif fi culty.
re sis tor (Rf) is con nected from the out put
BIAS
of 24
A, the max i mum cur rent sink
THAT 2252 RMS-Level De tec tor
BIAS
. There fore, for the
BIAS
,