Datasheet
Oscillator VIPER06
16/28 Doc ID 022794 Rev 1
9 Oscillator
The switching frequency is internally fixed at 30 kHz or 60 kHz or 115 kHz (respectively part
numbers VIPER06Xx, VIPER06Lx and VIPER06Hx).
The switching frequency is modulated by approximately ±3 kHz (30 kHz version) or ±4 kHz
(60 kHz version) or ±8 kHz (115 kHz version) at 230 Hz (typical) rate, so that the resulting
spread spectrum action distributes the energy of each harmonic of the switching frequency
over a number of sideband harmonics having the same energy on the whole, but smaller
amplitudes.
10 Soft startup
During the converter’s startup phase, the soft-start function progressively increases the
cycle-by-cycle drain current limit, up to the default value I
Dlim
. In this way the drain current is
further limited and the output voltage is progressively increased, reducing the stress on the
secondary diode. The soft-start time is internally fixed to t
SS
, see typical value
in Table 7 on page 8, and the function is activated for any attempt of converter startup and
after a fault event.
This function helps prevent saturation of the transformer during startup and short-circuit.
11 Adjustable current limit set point
The VIPER06 includes a current-mode PWM controller. The drain current is sensed cycle-
by-cycle through the integrated resistor R
SENSE
and the voltage is applied to the non-
inverting input of the PWM comparator, see Figure 2 on page 4. As soon as the sensed
voltage is equal to the voltage derived from the COMP pin, the power MOSFET is switched
OFF.
In parallel with the PWM operations, the comparator OCP, see Figure 2 on page 4, checks
the level of the drain current and switches OFF the power MOSFET in case the current is
higher than the threshold I
Dlim
, see Table 7 on page 8.
The level of the drain current limit I
Dlim
can be reduced using a resistor R
LIM
connected
between the LIM and GND pins. Current is sunk from the LIM pin through the resistor R
LIM
and the setup of I
Dlim
depends on the level of this current. The relation between I
Dlim
and
R
LIM
is shown in Figure 13 on page 11.
When the LIM pin is left open or if R
LIM
has a high value (i.e. > 80 kΩ), the current limit is
fixed to its default value, I
Dlim
, as given in Table 7 on page 8.