TK75002DIMG データシートの表示（PDF） - Toko America Inc
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TK75002DIMG Datasheet PDF : 10 Pages
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APPLICATION INFORMATION (CONT.)
TK75002
ZFB
ZIN
+
OUTPUT OF ISOLATED
POWER SUPPLY
V2
V1
2.5 V
VCOMP
RB
Vref
TO PRIMARY SIDE
CONTROLLER
LM
VCC
n2
n1
COMP
DRV
INV
GND
FIGURE 4: ELIMINATING THE Feed-forward OF THE
SUPPLY VOLTAGE IN THE TK75002
ISOLATION WITH PULSE TRANSFORMER
Figure 5 shows an application where the error signal is
isolated with a pulse transformer. The signal is recovered
at the primary side with a peak detector. The circuit is self-
oscillating. The frequency of operation is:
f = VF / [[(IDRV(PK) x LM) / kVCOMP + tOFF(D)] x (kVCOMP + VF)]
(5)
where the various quantities are:
VF
IDRV(PK)
LM
k
tOFF,D
VCOMP
forward voltage drop of the internal clamp diode
peak drive current threshold
magnetizing inductance of the pulse transformer
gain of level shifter stage
turnoff delay from peak detection to DRV pin
error signal (voltage at the COMP pin)
A 20 µH magnetizing inductance, with the typical parameter
values of the TK75002 and an error signal of 1.4 V yields
about 500 kHz oscillation frequency. Such an inductance
can be obtained with three to five turns on a small ferrite
toroid core of 5 to 10 mm external diameter.
FIGURE 5: ISOLATING THE ERROR SIGNAL WITH A
PULSE TRANSFORMER
ISOLATION WITH OPTO-COUPLER
Figure 6 shows how the TK75002 can be used together
with an opto-coupler. The oscillation is automatically
disabled because the peak current will never reach the
nominal 50 mA peak drive current threshold. A resistor
must be connected in series with the photodiode of the
opto-coupler to limit the maximum current. The main
advantage of the TK75002 over the TL431 is the left-half-
plane zero is avoided. Another advantage is that the error
amplifier in the TK75002 has higher bandwidth than the
TL431.
+
OUTPUT OF ISOLATED
POWER SUPPLY
VCC
COMP
DRV
INV
GND
FIGURE 6: ISOLATING THE ERROR SIGNAL WITH
AN OPTO-COUPLER
January 1999 TOKO, Inc.
Page 7
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