TK75020 データシートの表示（PDF） - Toko America Inc

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TK75020

ZVS RESONANT CONTROLLER

Toko America Inc  

TK75020

PIN DESCRIPTION (CONT.)

mirrored by the transistors Q9 and Q10 and it charges the

timing capacitor. The incoming mirror current is diverted

from the mirror with transistor Q11 when the drive latch is

set.

winding of the transformer in the CCFL inverter. When the

OVP comparator detects an overvoltage, it initiates a

shutdown via G4 and a Soft-Start cycle begins.

OVERDISSIPATION PROTECTION PIN (ODP)

The ODP pin is used to realize a protection against

overdissipation of the power MOSFET due to the loss or

absence of zero-voltage switching (ZVS). (ZVS can be lost

if the load or the input voltage changes too much. ZVS is

absent if the component values of the load network are far

from optimal, or if the ZVD function is not implemented and

the set off time is either too short or too long.) If ZVS is not

present in a converter or inverter that was originally meant

to operate with it, the MOSFET is turned on with a substantial

voltage across it and the capacitor in parallel with it. Due

to the periodic discharge of the parallel capacitor, a

significant dissipation appears in the MOSFET. That

dissipation is proportional to the switching frequency, the

capacitance value, and the square of the voltage across

the MOSFET at the instant of turn-on. The overdissipation

protection works as follows: a current source is enabled

when the MOSFET drain voltage is above the ZVD

comparator threshold when the DRV pin voltage goes

high. A short current pulse flows into the parallel combination

of a resistor and capacitor connected between the ODP

pin and ground and gradually begins to raise the pin

voltage. When the pin voltage reaches about 0.7 V, the

ODP latch is set via gate G4. The output signal of the ODP

latch inhibits gate G3 and forces the drive output low. The

output of the ODP latch also turns on transistors Q12-Q14.

Q12 removes the 2 V reference signal from the non-

inverting input of amplifiers A1 and A2 . Q13 pulls down the

EA OUT pin. The Soft-Start capacitor connected to the TON/

SS pin begins to discharge through the soft-start resistor

(see application circuit) and the on-time setting resistor.

When the voltage at the TON/SS pin drops below 2 VBE, the

ODP discharge comparator turns on Q15, which pulls down

the ODP pin voltage and discharges the capacitor

connected to that pin. When the voltage at the TON/SS pin

drops below a VBE, voltage the Soft-Start comparator

resets the ODP latch. At that time the 2 V reference is

enabled and a new Soft-Start cycle begins. The turnoff/soft

restart cycle repeats until zero-voltage switching is

reestablished.

OVERVOLTAGE PROTECTION PIN (OVP)

The OVP pin is used to monitor the voltage across a

Page 8

January 1999 TOKO, Inc.

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