2252-DEMO THAT Corporation, 2252-DEMO Datasheet - Page 7

• 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

600032 Rev 02
The gain of the in put op amp (OA1) is suf fi cient to
pro vide ef fec tive log ging for R
with R
per for mance will suf fer due to the fi nite open-loop
gain of OA1. This is be cause OA1 is re quired to
swing across a “dead zone” be tween turn ing on Q1
and Q3. The dead zone is re duced by V1. But, V1
is small ( 0.5 V) to main tain low-level track ing, so
OA1 must still swing through sev eral tenths of a
volt at each re ver sal of the in put po lar ity. The
smaller the value of R
de manded from OA1 for ac cu rate rec ti fi ca tion.
There fore, for good high-frequency per for mance,
R
R
match, E
The neg a tive in put of OA1 typ i cally rests at +8 mV.
If dc cou pled, this will cause an in put cur rent to
flow which will ef fec tively set a low-level “floor” be -
low which read ings will be ob scured. There fore, ac
cou pling is re quired if low level sig nals are to be
ac cu rately ob served. Choose the value of the ac
cou pling ca pac i tor (C
value of R
C
point. (For dc cou pling, see the sec tion on page 9,
DC Mea sure ments.)
Sym me try Ad just ment
As noted ear lier, the rec ti fier re lies on the match -
ing of Q1 and Q2 for pre cise re pro duc tion of pos i -
tive-half in put cur rents. Q2’s base is brought out
to pin 4 to al low ad just ment of this match. Pin 4
should be con nected to a 20
ble of sup ply ing from –4 mV to +20 mV. The ap pli -
ca tion cir cuits in Fig ure 4 (page 4) and Fig ure 11
(page 6) are typ i cal.
To set the sym me try, ap ply a low-frequency sine
wave to the in put. Nei ther the fre quency nor the
level are crit i cal: 100 Hz at near level match is
usu ally a good choice. Ob serve the out put wave -
form with a ‘scope while ad just ing the sym me try
trim. With proper ad just ment, the rip ple in the
out put will be al most pure sec ond har monic of the
in put fre quency. No fun da men tal fre quency should
be pres ent in the out put. An other method would
be to sense rip ple in the out put via a nar row
bandpass fil ter cen tered at the fun da men tal feed -
ing an ac volt me ter: ad just the trim for min i mum
volt age read ing.
The ac tual volt age re quired for proper sym me try
de pends on the in put off set volt age of OA1 (typ i -
cally +8 mV), and the V
and Q2 (<
where pre cise rec ti fi ca tion is not re quired, pin 4
may be con nected to a volt age match ing the in put
off set volt age of OA1, pref er a bly through a 20
sis tor (to match the 20
plest cir cuit con nects pin 4 to pin 1 through a
in
in
in
=
should be 10 k
based on the de sired in put volt age at level
2p
in
R
in0
1
in
in
F
, and I
10 k ., low level and high fre quency
C
and the de sired low-frequency limit.
6 mV). For less crit i cal ap pli ca tions
, where F
in0
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
as fol lows: R
in
or larger. Oth er wise, choose
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
in
C
in Fig ure 4) based on the
, the higher the loop gain
is the de sired 3 dB-down
BE
in Q1’s base). The sim -
mis match be tween Q1
in
volt age source ca pa -
in
10 k . How ever,
=
E
I
in
in
0
0
re -
20
tion en sures that the V
Q2, but does not al low for ad just ment for any mis -
match in the two de vices. When us ing this con fig u -
ra tion,
pro gram ming cur rent be low 20 A, to en sure sta -
ble op er a tion.
Time Con stants
Both the ca pac i tor (C
con nected to pin 6 con trol the time con stant over
which the rms value of the in put cur rent is eval u -
ated. Ei ther may be var ied, but a few prac ti cal con -
sid er ations in flu ence the choice of val ues. First,
the in put bias cur rent of OA3 in Fig ure 3 will add
to the charg ing cur rent I
this will af fect the ac cu racy of the re sult ing time
con stant. The in put bias cur rent for OA3 is typ i -
cally 100nA, so I
At the other ex treme, I
through Q6 un der steady-state con di tions. Dy -
namically, as was men tioned on page 4 in Com -
puting the Mean, the cur rent which charges C
pro por tional to the square of the short-term in -
crease in I
causes a 60 dB in crease in charg ing cur rent. For
ex am ple, if I
will be 10 mA.
peak charg ing cur rent called for would be 100 mA.
The de vices within the 2252 will not sup port this
high a cur rent de mand, so tim ing will be less ac cu -
rate un der this con di tion. (The steady-state out -
put volt age will be ac cu rate, since over time the
er ror will be “for got ten” due to the time-weighting
e
rect.)
The best per for mance nor mally re sults when
5
con ve nient value which re sults in the de sired time
con stant , and an I
Con stants (page 4) for the re la tion ship be tween ,
I
Since C
is of ten a po lar ized elec tro lytic or tan ta lum. V6 is
nor mally pos i tive ap prox i mately 2V
which de ter mines the po lar ity of C
will add to I
tantalums are pre ferred. Tan ta lum, once pre ferred
for long-term sta bil ity and low leak age, has been
largely superceded by pre mium elec tro lytic types
which are avail able in low-leakage, high-stability
ver sions. When us ing an elec tro lytic, how ever, it is
good prac tice to se lect a volt age rat ing higher than
ab so lutely nec es sary (at least 15 V is rec om -
mended).
When a re sis tor to the neg a tive sup ply is used as
the I
here),
T
t -
t
and C
. How ever, the dy namic re sponse will be in cor -
T
re sis tor, as shown in Fig ure 9. This con nec -
cur rent source (as in the cir cuits shown
T
de ter mine
T
one
.
is com monly 1 f or larger, this ca pac i tor
< 50
in
T
. A sud den 30 dB in crease in in put
is 10
T
should
, so low-leakage electrolytics or
T
How ever, if I
should be kept above 1 A.
T
its
C
A, the peak charg ing cur rent
within these lim its. See Time
T
T
BE
) and cur rent source (I
try
value
is usu ally cho sen to be a
T
T
. For small val ues of I
of Q1 will equal that of
flows from OA2 and
to
T
by
were 100
keep
T
. Leak age in C
the
be
the
( 1.4 V),
equa tion:
A, the
Page 7
bias
T
T
T
is
T
)
,