AN937 Datasheet, PDF (14/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

Power supply, UVLO and voltage reference
AN937
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
MΩ, 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:
V_OVP
=
1.08
⋅
V_fB
⋅
(_---R_-----a_----+-----R_-----b_----)
Rb
where Ra is the resistor connected to the output.
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Doc ID 5655 Rev 15