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ML4813CS Folha de dados(PDF) 6 Page - Micro Linear Corporation |
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ML4813CS Folha de dados(HTML) 6 Page - Micro Linear Corporation |
6 / 15 page ML4813 6 FUNCTIONAL DESCRIPTION OSCILLATOR The ML4813 oscillator charges the external capacitor (CT) with a current (ISET) equal to 5/RSET. When the capacitor voltage reaches the upper threshold, the comparator changes state and the capacitor discharges to the lower threshold through Q1. While the capacitor is discharging, Q2 provides a high pulse. The oscillator period can be described by: tt t OSC RAMP DEADTIME =+ (1) where: t C I RAMP T SET = (2) and: t C mA I DEADTIME T SET = - 84 . (3) A graph showing the relationship between RT, CT, the oscillator frequency, and maximum duty cycle is given in Figure 1. A pulse of a duration shorter than tDEADTIME from an external frequency source set to a higher frequency than fOSC can be applied to pin SYNC to synchronize the oscillator. RSYNC and CSYNC shorten longer pulses, as shown in Figure 2. OUTPUT DRIVER STAGE The ML4813 output driver is a 1A peak output high speed totem-pole circuit designed to quickly drive capacitive loads such as MOSFET gates. See Figure 3 for the output saturation characteristics for sourcing and sinking current. ERROR AMPLIFIER The ML4813 error amplifier is a high open loop gain, wide bandwidth amplifier. See Figure 4 for the gain and phase plot. UN-COMMITTED OP-AMP The ML4813 contains an uncommitted op amp which is normally configured as a differencing amplifier to sense the output voltage. The output voltage in the flyback configuration is not ground referenced. The op amp in the ML4813 is a PNP input amplifier similar to the LM324 but with an open emitter class A output stage. REFERENCE The reference output voltage versus output current characteristic is shown in Figure 5. UNDERVOLTAGE LOCKOUT On power-up, the ML4813 is in the UVLO condition; Figure 1. Oscillator Timing Resistance vs. Frequency Figure 2. Oscillator Block Diagram 10 SYNC 9 RT 16 CT EXTERNAL CLOCK + - 5.6V ISET 8.4mA CT RT ISET CSYNC RSYNC CLOCK TD RAMP VALLEY RAMP PEAK V(CT) 10 OSCILLATOR FREQUENCY (kHz) 10 100 1000 8 5 3 2 1 10nF 20nF 5nF 2nF 1nF 85% 80% 70% 90% output low and quiescent current low. The ML4813 becomes operational when VCC reaches 16V. When VCC drops below 10V, the UVLO condition is re-imposed. During UVLO, the VREF pin is off, making it usable as a "flag" for starting up a down-stream PWM converter. |
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