
Rev. A.2 - J uly., 2004
HPW 7095/ A
18
Copyright
HIPAC Semiconductor, Inc.
www.hipacsemi.com
Application Information (Cont.)
Dead-Time Setting
The dead-time control pin (DTC) is designed to set
the maximum ON duty of the main-side MOSFET.
When the device is set for step-up inverted output based
on the step-up or step-up/down Zeta method or flyback
method, the FB pin voltage may reach and exceed
the triangular wave voltage due to load fluctuation. If
this is the case, the output MOSFET is fixed to a ON
duty of 100 %. To prevent this, set the maximum duty
of the output MOSFET. Connecting a resistor- divider
between VREF, DTC and GND pins provides a volt-
age V
DTC
to DTC pin. When the the voltage at the
DTC pin is higher than the triangular wave voltage (CT1/
2), the output transistor is turned on. The maximum
duty is calculated as the following equation:
DTC1
VREF
R1
R2
V
DTC
(%)
100
V
0.7
V
1.1
-
V
Duty
ON
DTC
(max)
=
REF
DTC
V
R2
R1
R2
+
(V)
V
=
where V
REF
is the output of the reference voltage (2.
49V typical) at VREF pin. The amplitude of the trian-
gular waves CT1 and CT2 are typically 0.7V from 1.
1V to 1.8V.