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TEA1507 Datasheet(PDF) 6 Page - NXP Semiconductors |
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TEA1507 Datasheet(HTML) 6 Page - NXP Semiconductors |
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6 / 20 page  2000 Dec 05 6 Philips Semiconductors Preliminary specification GreenChip ™II SMPS control IC TEA1507 Current mode control Current mode control is used for its good line regulation behaviour. The ‘on-time’ is controlled by the internally inverted control pin voltage, which is compared with the primary current information. The primary current is sensed across an external resistor. The driver output is latched in the logic, preventing multiple switch-on. The internal control voltage is inversely proportional to the external control pin voltage, with an offset of 1.5 V. This means that a voltage range from 1 to 1.5 V on pin CTRL will result in an internal control voltage range from 0.5 to 0 V (the maximum external control voltage results in a minimum duty cycle). Oscillator The maximum fixed frequency of the oscillator is set by an internal current source and capacitor. The maximum frequency is reduced once the control voltage enters the VCO control window. Then, the maximum frequency changes linearly with the control voltage until the minimum frequency is reached (see Figs 5 and 6). Valley switching (see Fig.7) A new cycle starts when the power switch is switched on. After the ‘on-time’ (which is determined by the ‘sense’ voltage and the internal control voltage), the switch is opened and the secondary stroke starts. After the secondary stroke, the drain voltage shows an oscillation with a frequency of approximately where Lp is the primary self inductance of the transformer and Cd is the capacitance on the drain node. As soon as the oscillator voltage is high again and the secondary stroke has ended, the circuit waits for the lowest drain voltage before starting a new primary stroke. This method is called valley detection. Figure 7 shows the drain voltage together with the valley signal, the signal indicating the secondary stroke and the oscillator signal. In an optimum design, the reflected secondary voltage on the primary side will force the drain voltage to zero. Thus, zero voltage switching is very possible, preventing large capacitive switching losses , and allowing high frequency operation, which results in small and cost effective inductors. Demagnetization The system will be in discontinuous conduction mode all the time. The oscillator will not start a new primary stroke until the secondary stroke has ended. Demagnetization features a cycle-by-cycle output short-circuit protection by immediately lowering the frequency (longer off-time), thereby reducing the power level. Demagnetization recognition is suppressed during the first tsuppr time. This suppression may be necessary in applications where the transformer has a large leakage inductance and at low output voltages/start-up. Minimum and maximum ‘on-time’ The minimum ‘on-time’ of the SMPS is determined by the Leading Edge Blanking (LEB) time. The IC limits the ‘on-time’ to 50 µs. When the system desires an ‘on-time’ longer than 50 µs, a fault condition is assumed (e.g. removed Ci), the IC will stop switching and enter the safe restart mode. 1 2 π × L p C d × () × () ---------------------------------------------------- P 1 2 --- CV 2 × f × × = handbook, halfpage VCTRL 1 V (typ) 0.5 V 1.5 V (typ) MGU233 Vsense(max) Fig.5 The Vsense(max) voltage as function of VCTRL. handbook, halfpage Vsense(max) MGU234 f 6 kHz 175 kHz 50 mV (typ) 75 mV (typ) Fig.6 The VCO frequency as function of Vsense(max) |
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