Operation descriptions

VIPER27

14/31

Doc ID 15133 Rev 5

7

Operation descriptions

VIPER27

is a high-performance low-voltage PWM controller chip with an 800 V, avalanche

rugged Power section.

The controller includes: the oscillator with jittering feature, the start up circuits with soft-start

feature, the PWM logic, the current limit circuit with adjustable set point, the second over

current circuit, the burst mode management, the brown-out circuit, the UVLO circuit, the

auto-restart circuit and the thermal protection circuit.

The current limit set-point is set by the CONT pin. The burst mode operation guaranties high

performance in the stand-by mode and helps in the energy saving norm accomplishment.

All the fault protections are built in auto restart mode with very low repetition rate to prevent

IC's over heating.

7.1

Power section and gate driver

The power section is implemented with an avalanche ruggedness N-channel MOSFET,

which guarantees safe operation within the specified energy rating as well as high dv/dt

at 25

°C.

The integrated SenseFET structure allows a virtually loss-less current sensing.

The gate driver is designed to supply a controlled gate current during both turn-on and turn-

off in order to minimize common mode EMI. Under UVLO conditions an internal pull-down

circuit holds the gate low in order to ensure that the power section cannot be turned on

accidentally.

7.2

High voltage startup generator

The HV current generator is supplied through the DRAIN pin and it is enabled only if the

reduced to 0.6 mA, in order to have a slow duty cycle during the restart phase.