VIPer01

General description

DocID028751 Rev 1

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4.8

Direct feedback

The IC embeds a transconductance type error amplifier (E/A) whose inverting input, ground

reference and output are FB and COMP, respectively. The internal reference voltage of the

regulates positive output voltages through a simple voltage divider applied to the output

voltage terminal, FB and GND.

The E/A output is scaled down and fed into the PWM comparator, where it is compared to

the voltage across the sense resistor in series to the sense-FET, thus setting the cycle-by-

cycle drain current limitation.

An R-C network connected on the output of the E/A (COMP) is usually used to stabilize the

overall control loop.

The FB is provided with an internal pull-up to prevent a wrong IC behavior when the pin is

accidentally left floating.

4.9

Secondary feedback

When a secondary feedback is required, the internal E/A has to be disabled shorting FB to

, (60 kΩ, typ.) and the voltage across COMP

is set by the current sunk.

This allows the output voltage value to be set through an external error amplifier (TL431 or

similar) placed on the secondary side, whose error signal is used to set the DRAIN peak

current setpoint corresponding to the output power demand. If isolation is required, the

error signal must be transferred through an optocoupler, with the phototransistor collector

connected across COMP and GND.

4.10

Pulse frequency modulation

and some PWM cycles are skipped. This kind of operation is referred to as “pulse

frequency modulation” (PFM), the number of the skipped cycles depends on the balance

between the output power demand and the power transferred from the input. The result is

an equivalent switching frequency which can go down to some hundreds Hz, thus reducing

all the frequency-related losses.

This kind of operation, together with the extremely low IC quiescent current, allows very low

input power consumption in no-load and light load, while the low DRAIN peak current

reached.