AN4026 datasheet(8/40 Pages) STMICROELECTRONICS

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Main characteristics and circuit description

AN4026

8/40

Doc ID 022603 Rev 1

The L6699 enables the user to set the operating frequency range of the converter by means

of a high-accuracy externally programmable oscillator.

At startup, in addition to the traditional frequency-shift soft-start (the switching frequency

starts from a preset maximum value and then decays as far as the steady-state value

determined by the control loop), a proprietary circuit controls the half bridge to prevent hard-

switching from occurring in the initial cycles because of the imbalance of the VOS applied to

the transformer. At light load, the L6699 is forced to enter a controlled burst mode operation

that keeps the converter input consumption as low as possible.

IC protection functions include a current sense input for OCP with frequency shift and

delayed shutdown with automatic restart. Fast shutdown with automatic restart occurs if this

first-level protection cannot control the primary current.

Additionally, the IC prevents the converter from working in, or too close to, capacitive mode,

to guarantee soft-switching. A latched disable input (DIS) is used to implement the OVP.

Other functions include a not-latched active-low disable input with current hysteresis, useful

for power sequencing or for brownout protection, and an interface with the PFC controller

that enables the pre-regulator to be switched off during fault conditions or during burst mode

operation.

The transformer uses the integrated magnetic approach, incorporating the resonant series

inductance. Therefore no external additional coil is needed for the resonance.

The transformer secondary winding configuration is centre tap and makes use of a couple of

power Schottky rectifiers p/n STPS30H60CFP. A small LC filter has been added on the

output to reduce the high frequency ripple and noise.

D15, R56, R62, R65, R66, Q5 and Q6 implement an output voltage “fast discharge” circuit

discharging quickly the output capacitors when the converter is turned off. It has been

implemented to quickly decrease the residual output voltage once the converter is turned off

at no load.

1.5

Output voltage feedback loop

The feedback loop is implemented by means of a typical circuit using a TL431 modulating

the current in the optocoupler diode.

On the primary side, R34 - connecting pin RFMIN (#4) to the optocoupler's phototransistor -

closes the feedback loop and its value sets the maximum switching frequency at about 105

kHz. This value has been chosen to limit the switching losses at light load operation. R31,

connecting the same pin to ground, sets the minimum switching frequency. The R-C series

R44 and C18 sets both soft-start maximum frequency and duration.

1.6

L6699 overload and short-circuit protection

The current into the primary winding is sensed by the lossless circuit R41, C27, D11, D10,

R39, and C25 and it is fed into the ISEN pin (#6). In case of overcurrent, the voltage on the

pin surpasses an internal comparator threshold (0.8 V), triggering a protection sequence.

The capacitor (C45) connected to the DELAY pin (#2) is charged by an internal 150 µA

current generator and is slowly discharged by the external resistor (R24). 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

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