TDA8380A Datasheet, PDF (6/20 Pages) NXP Semiconductors – Control circuit for switched mode power supplies

Philips Semiconductors
Control circuit for switched mode power supplies
Product specification
TDA8380A
Reference block
A bandgap based reference generates a stabilized voltage of 7 V to supply most of the device’s internal circuits, this
decreases chip size and increases reliability. The only circuits connected to V_CC are:
• The initialization circuit
• The output circuitry
• The series transistor of the stabilized voltage
By means of a resistor (R₆) connected to the I_ref input a reference current is defined which determines six other device
settings.
Part of the reference current is used to charge the oscillator capacitor (C₁₀), therefore, the charging time is proportional
to R₆ × C₁₀. The maximum duty factor (D_max) is set by the resistor connected to pin 12 (R₁₂) and is defined by the ratio
R₆/R₁₂. The minimum supply voltage (pin 5) set by the resistor (R₄) at input V_CCmin is defined by: 4/6 × V₆ × R₄/R₆.
Oscillator
The oscillator capacitor is charged and discharged between the high and low voltage levels as defined by the bandgap
reference (high voltage typically 5 V and low voltage typically 1.4 V). The charge current is 1/6 of the reference current,
the discharge current having the same value as the reference current. The period is therefore defined by 10 × R₆ × C₁₀.
The oscillator flyback pulse is used to set the bistable in the output logic, however the output remains low until the positive
ramp starts (see Fig.4). The oscillator can be synchronized by means of the SYNC pin. When this pin is connected to
V_CC, the oscillator is free running. When it is between 0.85 and 5.6 V, the oscillator stops at the low voltage level prior to
the next positive ramp.
November 1993
Fig.4 Oscillator levels.
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