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TK75002DIMG

ERROR SIGNAL ISOLATOR

Toko America Inc  

TK75002

THEORY OF OPERATION

The TK75002 can be used in conjunction with either an

opto-coupler or a pulse transformer to isolate an error

signal developed by its internal op-amp. The op-amp can

be externally compensated and features a precision

reference voltage at the non-inverting input. When

configured to drive a pulse transformer, the TK75002 will

automatically oscillate to drive the pulse transformer in a

pulse-amplitude-modulation (PAM) mode.

When VCC is below the UVLO threshold (~4.1V) the

TK75002 does not operate and the DRV pin remains in a

high-impedance state. When VCC is above the UVLO

threshold, the PAM switch turns on and forces VDRV with

respect to VCC to be equal to a gain constant times the op-

amp output voltage, VCOMP, with respect to 2.5 V.

Essentially, then, the inverse of the error voltage (referenced

to 2.5 V, a virtual ground) times a small gain constant is

what appears between the DRV and VCC pins. Note that

this is only valid when VCOMP is less than 2.5 V, which also

implies that VDRV is less than VCC.

If an opto-coupler and series resistor are hooked between

the DRV and VCC pins, an error current is transmitted

across the opto-coupler which is free from the characteristic

of having the zero that is a sort of parasitic effect of the

standard configuration of driving an opto-coupler with a

TL431.

The TK75002 has a saturation limiter in the feedback loop.

For either the opto-coupler or pulse transformer

configurations, the error voltage which is transmitted across

the isolation device by the TK75002 is limited to less than

5 V. This limiting occurs regardless of the applied VCC

(generally, it is VCC which is being regulated by the feedback

loop for which the TK75002 transmits the error signal).

Thus, when the TK75002 is used in a variable output

voltage power supply or in a standard line of various fixed

output power supplies, no supplemental signal-limiting

circuitry is required in the feedback loop.

The characteristic waveforms of the TK75002 driving a

pulse transformer are shown in Figure 1. The top trace

shows the error voltage (@ 1 V / div.) referenced two

divisions below the top; it is equal to approximately 1.3 V.

The middle trace shows the DRV Pin voltage with respect

to VCC (@ 1 V / div.) referenced three divisions below the

top. The bottom trace shows the DRV Pin current (@ 20

mA / div.) referenced one division above the bottom.

If a pulse transformer is connected between the DRV and

VCC pins, the magnetizing current will begin to increase

until it reaches a threshold of ~ 55 mA, as detected by the

Peak Current Detector. After an internal turnoff delay, the

PAM switch turns off and the magnetizing current forces

the DRV-pin voltage above VCC. An internal clamp diode

between the DRV pin and the VCC pin clamps the voltage

and then essentially compares it to a VCC-referenced

voltage which corresponds to ~ 1 mA of current flow in the

clamp diode (i.e., ~ 650 mV at room temperature). When

the magnetizing current has decayed to nearly zero (i.e.,

~ 1 mA) and after an internal turn-on delay, the Transformer

Reset Detector turns on the PAM switch to initiate the

process all over again. By using peak rectification on the

secondary side of the pulse transformer, the error signal

can be recovered. The time constant of the peak detection

circuit is chosen to yield negligible ripple but also an

acceptable response time. The magnetizing inductance of

the pulse transformer is chosen to yield an acceptable

peak current overshoot and/or power dissipation when the

switching frequency is at a maximum (maximum switching

frequency occurs when the op-amp output is low).

FIGURE 1: CHARACTERISTIC WAVEFORMS OF THE

TK75002

Page 4

January 1999 TOKO, Inc.

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