Power supply, UVLO and voltage reference

AN937

14/29

Doc ID 5655 Rev 15

2.4

Feedback disconnection

In case of feedback disconnection, the duty cycle increases versus the max allowed value

bringing the output voltage close to the input supply. This condition could destroy the load.

To avoid this dangerous condition, the device is forcing a little current (1.4

μA typical) out of

the pin 8 (E/A Feedback).

If the feedback is disconnected, open loop, and the impedance at pin 8 is higher than 3.5

M

Ω, the voltage at this pin goes higher than the internal reference voltage located on the

non-inverting error amplifier input, and turns-off the power device.

2.5

Zero load

In normal operation, the output regulation is also guaranteed because the bootstrap

capacitor is recharged, cycle by cycle, by means of the energy flowing into the choke.

Under light load conditions, this topology tends to operate in burst mode, with random

repetition rate of the bursts. An internal new function makes this device capable of keeping

the output voltage in full regulation with 1mA of load current only.

Between 1 mA and 500

μA, the output is kept in regulation up to 8% above the nominal

value.

Here the circuitry providing the control:

1.

A comparator located on the bootstrap section is sensing the bootstrap voltage; when

this is lower than 5 V, the internal power VDMOS is forced ON for one cycle and OFF

for the next.

2.

During this operation mode, i.e. 500

μA of load current, the E/A control is lost. To avoid

output over voltage, a comparator with one input connected to pin 8, and the second

input connected to a threshold 8% higher that nominal output, turns OFF the internal

power device the output is reaching that threshold.

When the output current, or rather, the current flowing into the choke, is lower than 500

μA, that is also the consumption of the bootstrap section, the output voltage starts to

increase, approaching the supply voltage.

2.6

Output overvoltage protection (OVP)

The output overvoltage protection, OVP, is realized by using an internal comparator, which

input is connected to pin 8, the feedback, that turns-off the power stage when the OVP

threshold is reached. This threshold is typically 8% higher than the feedback voltage.

When a voltage divider is requested for adjusting the output voltage, the OVP intervention

will be set at:

where Ra is the resistor connected to the output.

V

OVP

1.08 V

fB

Ra

Rb

+

()

Rb

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