TK83854
APPLICATION INFORMATION (CONT.)
used to calculate these resistors:
R
IAC
= V
PK(MAX)
/ 400 E - 6
= (260 VAC x 2 ) / 400
µA
= 910 k
R
REF
= R
IAC
/ 4 = 220 k
where V
PK
is the peak line voltage.
Line Current (I
SENSE
/MULTOUT )
The voltage drop across the 0.25
Ω
current-sense resistor
is applied to Pins 4 and 5 as shown. The current-sense
amplifier also operates with high low-frequency gain, but
unlike the voltage amplifier, it is set up to give the current-
control loop a very wide bandwidth. This enables the line
current to follow the line voltage as closely as possible. In
the present example, this amplifier has a zero at about
500 Hz, and a gain of about 18 dB thereafter.
RMS Line Voltage (V
RMS
)
An important feature of the TK83854 preregulator is that it
can operate with a three-to-one range of input line voltages,
covering everything from low line in Japan (85 VAC) to
high line in Europe (255 VAC). This is done using line feed-
forward, which keeps the input power constant with varying
input voltage (assuming constant load power). To do this,
the multiplier divides the line current by the square of the
rms value of the line voltage. The voltage applied to Pin 8,
proportional to the average of the rectified line voltage (and
proportional to the RMS value), is squared in the TK83854,
and then used as a divisor by the multiplier block. The
multiplier output, at Pin 5, is a current that increases with
the current at Pin 6 and the voltage at Pin 7, and decreases
with the square of the voltage at pin 8.
PWM Frequency
The PWM oscillator frequency in Figure 1 is 100 kHz. This
value is determined by C
T
at Pin 14 and R
SET
at Pin 12.
R
SET
should be chosen first because it affects the maximum
value of I
OM
according to the equation:
I
OM(MAX)
= -3.75 V / R
SET
This effectively sets a maximum PWM-controlled current.
I
RCS(MAX)
= (-I
OM(MAX)
x 3.9 k) / 0.25
Ω
= -3.9 A
Having selected R
SET
, the current sense resistor, and the
resistor from MULT
OUT
to the current sense resistor,
calculate C
T
for the desired PWM oscillator frequency from
the equation:
C
T
= 1.25 / (f
OSC
x R
SET
)
It is also important to note that the multiplier output current
will never exceed twice IAC.
With the 3.9 k resistor from MULT
OUT
to the 0.25
Ω
current
sense resistor, the maximum current in the current sense
resistor will be:
With R
SET
= 15 k:
I
OM(MAX)
= -3.75 V / 15 k = -250
µA
Page 10
January 1999 TOKO, Inc.
TOKO [ TOKO, INC ]
