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TS3842B bảng dữ liệu(PDF) 11 Page - List of Unclassifed Manufacturers |
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TS3842B bảng dữ liệu(HTML) 11 Page - List of Unclassifed Manufacturers |
11 / 20 page Undervoltage Lockout(contd.) This provides a low impedance path for filtering the high frequency noised. All high current loops should be kept as short as possible using heavy copper runs to minimize radiated EMI. The Error Amp compensation circuitry and the converter output voltage divider should be located close to the IC and as far as possible from the power switch and other noise generating components. Current mode converters can exhibit subharmonic oscillations when operating at a duty cycle greater than 50% with continuous inductor current. This instability is independent of the regula- tors closed loop characteristics and is caused by the simultaneous operating conditions of fixed frequency and peak current detecting. Figure 19(A) shows the phenomenon graphically. At t 0, switch conduction begins causing the inductor current to rise at a slope of m 1. This slope is a function of the input voltage divided by the inductance. At t1, the Current Sense Input reaches the threshold established by the control voltage. This causes the switch to turn off and the current to decay at a slope of m 2, until the next oscillator cycle. This unstable condition can be shown if a perturbation is added to the control voltage, resulting in a small 1(dashed line). With a fixed oscillator period, the current decay time is reduced, and the minimum current at switch turn-on(t 2) is increased by 1+ 1m 2/m1. The minimum current at the next cycle (t3) decreases to ( 1+ 1m 2/m1)(m2/m1). This perturbation is multiplied by m2/m1 on each succeeding cycle, alternately increasing and decreasing the inductor current at switch turn-on. Several oscillator cycles may be required before the inductor current reaches zero causing the process to commence again. If m 2/ m 1 is greater than 1, the converter will be unstable. Figure 19(B) shows that by adding an artificial ramp that is synchronized with the PWM clock to the control voltage, the 1 perturbation will decrease to zero on succeeding cycles. This compensating ramp (m 3) must have a slope equal to or slightly greater than m 2/2 for stability. With m2/2 slope compensation, the average inductor current follows the control voltage yielding true current mode operation. The compensating ramp can be derived from the oscillator and added to either the Voltage Feedback or Current Sense inputs (Figure 32). Figure 19 - Continuous Current Waveforms + + Control Voltage Inductor Current Oscillator Period Control Voltage Inductor Current Oscillator Period (A) (B) m1 m2 t0 t1 t2 t3 m3 m2 t4 t5 t6 ∆ ∆∆ ∆ ∆ 1 m1 11 m2 m1 11m2 m1 m 2 m 1 |
Số phần tương tự - TS3842B |
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Mô tả tương tự - TS3842B |
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