TDA8380A Datasheet, PDF (9/20 Pages) NXP Semiconductors – Control circuit for switched mode power supplies

Philips Semiconductors
Control circuit for switched mode power supplies
Product specification
TDA8380A
Error amplifier
The error amplifier compares the feedback voltage of the SMPS with a reference voltage (nominally 2.5 V). The amplifier
output at pin 8 enables gain setting. The amplifier is stable for a gain greater than 20 dB.
The output of the error amplifier is not internally connected to the Pulse Width Modulator (PWM). One input to the PWM
is available at the DUTY input (pin 9) via the Control Slicing Level (CSL) circuit. Normally the STAB and DUTY pins are
connected together, but direct driving of pin 9 via an optocoupler from the secondary side is also possible. A type of
current mode control can be achieved by mixing the STAB signal with the primary current signal before applying it to the
DUTY input.
The feedback (FB) input (pin 7) is used as the input to the Transfer Characteristic Generator (TCG) circuit which ensures
well defined duty factors at low FB voltages; a voltage foldback is an inherent characteristic. In Fig.8, the duty factor is
shown as a function of the voltages at the FB, DUTY and SS inputs. The input which gives the lowest duty factor overrides
the others.
The left hand curve is passed through during a slow-start (via the slow-start input pin 12) when the duty cycle slowly
increases linearly with respect to V₁₂. The right-hand curve is passed through at start-up. The FB voltage slowly
increases from zero and the duty factor, starting at 12%, increases until the maximum duty factor (D_max) is reached. A
few hundred millivolts later, the FB voltage reaches the start of the regulation curve which is at approximately 2.5 V. The
plateau area between reaching D_max and starting the regulation curve is kept as small as possible (typically 200 mV).
Fig.8 The duty factor as a function of the FB, SS and DUTY voltages.
Due to the characteristics of the TCG, and the fact that an open FB input results in a low voltage at the FB input, open-
and short-circuit feedback loops will result in low duty factors. When DC feedback is used across the error amplifier, the
current capability of the error amplifier must be considered when determining the feedback resistor value.
When the input to the PWM (pin 9) is driven by an optocoupler, the TCG can be used when a rough primary voltage is
applied to the FB input. In this situation an open feedback loop will cause an increase in the FB voltage as the duty factor
rises to its maximum. As soon as the FB voltage exceeds the reference by 0.7 V, the slow-start is triggered.
November 1993
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