tk75020 Toko Inc., tk75020 Datasheet - Page 8

• Manufacturer Part Number: tk75020
• Description: Zvs Resonant Controller
• Manufacturer: Toko Inc.

TK75020
mirrored by the transistors Q
timing capacitor. The incoming mirror current is diverted
from the mirror with transistor Q
set.
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 G
latch inhibits gate G
output of the ODP latch also turns on transistors Q
Q
inverting input of amplifiers A
EA
SS pin begins to discharge through the soft-start resistor
(see application circuit) and the on-time setting resistor.
When the voltage at the T
ODP discharge comparator turns on Q
the ODP pin voltage and discharges the capacitor
connected to that pin. When the voltage at the T
drops below a V
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
12
OUT
removes the 2 V reference signal from the non-
pin. The Soft-Start capacitor connected to the T
BE
3
, voltage the Soft-Start comparator
and forces the drive output low. The
ON
4
. The output signal of the ODP
/SS pin drops below 2 V
9
1
and A
and Q
11
when the drive latch is
2
10
. Q
15
PIN DESCRIPTION (CONT.)
and it charges the
, which pulls down
13
pulls down the
ON
/SS pin
12
BE
-Q
, the
ON
14
.
/
winding of the transformer in the CCFL inverter. When the
OVP comparator detects an overvoltage, it initiates a
shutdown via G
4
and a Soft-Start cycle begins.
January 1999 TOKO, Inc.