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TEA1504 Datasheet(PDF) 7 Page - NXP Semiconductors |
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TEA1504 Datasheet(HTML) 7 Page - NXP Semiconductors |
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7 / 20 page  1999 Dec 07 7 Philips Semiconductors Preliminary specification GreenChip ™ SMPS control IC TEA1504 Pulse width modulator The PWM comprises an inverting error amplifier and a comparator (see Fig.1) which drives the power MOSFET with a duty cycle that is inversely proportional to the voltage at CTRL (pin 9). A signal from the oscillator sets a latch that turns on the power MOSFET. The latch is then reset either by the signal from the PWM or by a duty cycle limiting signal from the oscillator. The latch stops the power MOSFET from being switched incorrectly if the PWM output signal becomes unstable. The maximum duty cycle is internally set to 80%. The IC switching signals during normal operation are shown in Fig.7. Oscillator The oscillator determines the switching duty cycle. Its ramp signal voltage is compared to the output of the error amplifier by the PWM. The fully integrated oscillator circuit works by charging and discharging an internal capacitor between two voltage levels to create a sawtooth waveform with a rising edge that is 80% of the oscillator period (high frequency mode). This ratio sets a maximum switching duty cycle of 80% for the IC. The accuracy of the oscillator frequency is internally set to 5%. Its frequency can be adjusted between 50 and 100 kHz by changing the value of RREF. This gives the power supply designer greater flexibility in the choice of system components. The relationship between frequency and the value of RREF is shown in Fig.6. The range of RREF values and the frequencies of foscL and foscH are specified in Chapter “Characteristics”. Multi frequency control When the power supply operates at or below 1 ⁄9 of its peak power, the IC changes to low power operation mode. This lowers the frequency of the oscillator to reduce the power supply switching losses. The ratio between the high and the low oscillator frequency is maintained at 1 : 2.5 (see foscL in Chapter “Characteristics”). An innovative design ensures that the transfer from high-to-low frequency and vice versa does not effect output voltage regulation. Gate driver The driver circuit to the Gate of the power MOSFET has a totem-pole output stage that has current sourcing capability of 120 mA and a current sink capability of 550 mA. This permits fast turn-on and turn-off of the power MOSFET for efficient operation. This circuit design allows the power supply designer to control the source and sink currents of the Gate driver circuit with a minimum number of external components. A low driver source current has been chosen in order to limit the ∆V/∆t at switch-on. This reduces Electro Magnetic Interference (EMI) and also the current spike across Rsense. Demagnetization protection The demagnetization protection feature ensures discontinuous conduction of the power supply, simplifying the design of feedback control and giving a faster transient response. It protects against saturation of the transformer/inductor and also protects the power supply components against excessive stresses at start-up, when all energy storage components are completely discharged. During a system output short-circuit fault condition, it provides cycle-by-cycle protection of the converter configuration. The demagnetization resistor (RDEM) value can be calculated using the formula given in Section “Sample-and-hold”. Negative clamp The negative clamp circuit ensures correct operation of the IC by preventing the voltage at DEM (pin 13) dropping below −0.45 V, during the period when the power MOSFET turns on and the auxiliary winding voltage goes negative. handbook, halfpage 10 20 30 40 110 90 50 30 70 MGS573 RREF (kΩ) foscH (kHz) 55 45 25 15 35 foscL (kHz) (2) (1) Fig.6 Frequency as function of RREF value. (1) High frequency mode. (2) Low frequency mode. |
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