AN938

Power supply, UVLO and voltage reference

Doc ID 5410 Rev 10

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2.4

Feedback disconnection

In case of feedback disconnection, the duty cycle increases to the maximum allowed value,

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

To avoid this dangerous condition, the device forces a little current (1.4 µA typical) out of

pin 12 (E/A feedback).

If the feedback is disconnected and the impedance at pin 12 is higher than 3.5 M

Ω, the

voltage at this pin goes higher than the reference voltage of the error amplifier, disabling the

embedded 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, 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 1 mA of load current only.

Embedded circuitry keeps the output voltage regulated up to 8% above the nominal value at

light load (0.5 mA < Iout < 1 mA):

●

an internal comparator senses the voltage across the bootstrap capacitor. When this

voltage drops below 5 V, it enables the embedded power element for about 300 nsec,

recharging the bootstrap capacitor.

●

the OVP protection disables the power element when the output voltage is 8% higher

than nominal output (see Section 2.6).

When the load current is lower than 500 µA, which is also the current consumption of the

boostrap section, the output voltage regulaton becomes critical.

A bleeding resistor is suggested to keep the output voltage voltage regulated regardless of

Figure 16.

Figure 17.

Output rising voltage with

Css 680 nF, 470 nF, 330 nF, 220 nF