Oscillator
VIPER16
9
Oscillator
The switching frequency is internally fixed at 60 kHz (part number VIPER16LN or LD) or 115
kHz (part number VIPER16HN or HD).
In both cases the switching frequency is modulated by approximately 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
11
Soft start-up
During the converters' start-up phase, the soft-start function progressively increases the
cycle-by-cycle drain current limit, up to the default value I
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 , see typical value
on Table 8 on page 7, and the function is activated for any attempt of converter start-up and
after a fault event.
. By this way the drain current is
Dlim
SS
This function helps prevent transformers' saturation during start-up and short-circuit.
Adjustable current limit set point
The VIPer16 includes a current mode PWM controller: cycle by cycle the drain current is
sensed through the integrated resistor R
and the voltage is applied to the non
SENSE
inverting input of the PWM comparator, see Figure 2 on page 3. 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 3, checks
the level of the drain current and switch OFF the power MOSFET in case the current is
higher than the threshold I
, see Table 8 on page 7.
Dlim
The level of the drain current limit, I
, can be reduced depending from the sunk current
Dlim
from the pin LIM. The resistor R , between LIM and GND pins, fixes the current sunk and
LIM
than the level of the current limit, I
, see Figure 13 on page 9.
Dlim
When the LIM pin is left open or if the R
has an high value (i.e. > 80 kΩ) the current limit
LIM
is fixed to its default value, I
, as reported on Table 8 on page 7.
Dlim
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Doc ID 15232 Rev 5
STMICROELECTRONICS [ ST ]
