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CS3844GD14 Datasheet(PDF) 6 Page - ON Semiconductor |
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CS3844GD14 Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 12 page CS2844, CS3844, CS2845, CS3845 http://onsemi.com 6 CIRCUIT DESCRIPTION CSX845 8.4 V 7.6 V CSX844 16 V 10 V VON VOFF ON/OFF Command to reset of IC VCC < 1.0 mA < 15 mA VON VOFF VCC ICC Figure 3. Startup Voltage for CSX844 and CSX845 Undervoltage Lockout During Undervoltage Lockout (Figure 3), the output driver is biased to sink minor amounts of current. The output should be shunted to ground with a resistor to prevent activating the power switch with extraneous leakage currents. PWM Waveform To generate the PWM waveform, the control voltage from the error amplifier is compared to a current sense signal which represents the peak output inductor current (Figure 4). An increase in VCC causes the inductor current slope to increase, thus reducing the duty cycle. This is an inherent feed–forward characteristic of current mode control, since the control voltage does not have to change during changes of input supply voltage. Figure 4. Timing Diagram VOSC OSC RESET Toggle F/F Output EA Output Switch Current VCC IO VO Figure 5. Duty Cycle Parameters Vupper Vlower ton tc toff td ton = tc toff = tc + 2td When the power supply sees a sudden large output current increase, the control voltage will increase allowing the duty cycle to momentarily increase. Since the duty cycle tends to exceed the maximum allowed to prevent transformer saturation in some power supplies, the internal oscillator waveform provides the maximum duty cycle clamp as programmed by the selection of OSC components. Setting the Oscillator The times tc and td can be determined as follows: tc + RTCT ln VREF * Vlower VREF * Vupper td + RTCT ln VREF * IdRT * Vlower VREF * IdRT * Vupper Substituting in typical values for the parameters in the above formulas: VREF = 5.0 V, Vupper = 2.7 V, Vlower = 1.0 V, Id = 8.3 mA then: tc [ 0.5534RTCT td + RTCT ln 2.3 * 0.0083RT 4.0 * 0.0083RT For better accuracy RT should be ≥ 10 kΩ. Grounding High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypass capacitors should be connected close to Gnd in a single point ground. The transistor and 5.0 k Ω potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to Sense. |
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