Main characteristics and circuit description

AN3105

6/30

Doc ID 16774 Rev 2

With these modifications the circuit is able to keep the output current constant at 2.7 A down

to an output voltage value of around 30 V. This function can be used to optimize the voltage

drop and power dissipation in case current linear regulators are used to regulate the current

flowing in each LED strip. In case the output current is lower than the current loop setpoint,

the voltage loop takes over the operation regulating the output voltage at its nominal value,

like using the TS2431AILT.

1.5

L6599AT overload and short-circuit protection

The current flowing into the primary winding, proportional to the output load, is sensed by

the lossless circuit C34, R53, D19, D18, R57, and C35 and it is fed into the ISEN pin (#6) of

L6599AT. In the case of overcurrent, the voltage on the pin overpasses an internal threshold

(0.8 V), triggering a protection sequence. The capacitor (C21) connected to the DELAY pin

(#2) is charged by an internal 150 µA current generator. If the voltage on the pin reaches

2 V, the soft-start capacitor is completely discharged so that the switching frequency is

pushed to its maximum value. As the voltage on the pin exceeds 3.5 V the IC stops

switching and the internal generator is turned off, so that the voltage on the DELAY pin

decays because of the external resistor connected between the pin and GND. The L6599AT

is soft-restarted as the voltage drops below 0.3 V. In this way, under short-circuit conditions,

the converter works intermittently with low input average power, limiting the stress of

components during shorts.

1.6

Overvoltage and open-loop protection

Both circuit stages, PFC and resonant, are equipped with their own overvoltage protections.

The L6562AT PFC controller implements an overvoltage protection against the output

voltage variation due to the poor bandwidth of the error amplifier, happening in the case of

transients. Unfortunately it cannot protect the circuit in the case of a feedback loop failure

such as disconnection or deviation from the nominal value of the feedback loop divider. In

the case where a similar failure condition is detected, the L6599AT pin DIS (#8) stops the

operation and also stops the PFC operation by means of the L6599AT pin PFC_STOP (#9)

capacitors are discharged, then a new startup sequence takes place automatically and the

converter resumes operation if the failure is removed or a new sequence is triggered. The

same sequence occurs also in the case of input voltage transients which may damage the

converter.

The DIS pin is also used to protect the resonant stage against loop failures. The Zener

diode D17 detects the auxiliary voltage generated by the LLC transformer. In the case of

loop failure, it conducts, and voltage on the DIS pin exceeds the internal threshold, latching

off the device. The L6562AT operation is also stopped by the PFC_STOP pin as in the

previous case, and after some time has elapsed the circuit restarts.