AN3106 datasheet(7/34 Pages) STMICROELECTRONICS

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AN3106

Main characteristics and circuit description

Doc ID 16775 Rev 2

7/34

All demonstration boards implement the voltage loop circuitry previously described but in

case a current loop is also required, it can be achieved by implementing the following

modifications:

●

Replace R30 and R31 0R0

Ω resistors with sensing resistors, 0R033 and 0R039

respectively, both 0805

●

Populate on PCB U4 and the relevant components shown on the schematic as N.M:

C36 = 1N0-0805; C37 = 100NF-0805; R51 = 15R-0805; R56 = 1K0-0805;

R6 = 22K-1206; C41 = 2N2-0805; U5 = SEA05TR

●

Remove TS2431AILT

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

to an output voltage value 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. If the output current is lower, the voltage loop will take over the

operation, regulating the output voltage at its nominal value as when using the TS2431AILT.

1.5

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 case of overcurrent, the voltage on the pin will exceed 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 will decay because of the external resistor connected between the pin and

GND. The L6599AT will be soft-restarted as the voltage drops below 0.3 V. In this way, under

short-circuit conditions, the converter will work intermittently with low input average power

and thus 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 PFC controller L6562AT implements an overvoltage protection against the output

voltage variation occurring in case of transients, due to the poor bandwidth of the error

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

disconnection or deviation from the nominal value of the feedback loop divider. If 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) connected to the L6562AT

pin INV (#1). The converter operation will be latched until the VCC capacitors are

discharged, then a new startup sequence will automatically take place and the converter will

resume operation if the failure is removed or a new sequence is triggered. The same

sequence occurs also in case of input voltage transients that 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 case a loop

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

latching off the device. The L6562AT operation will be stopped too by the PFC_STOP pin,

like in the previous case and then after some time the circuit will restart.

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