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MIC3839 Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC3839 Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 11 page August 2003 9 MIC3838/3839 MIC3838/3839 Micrel Soft Start The soft start feature helps reduce surge currents at the power supply input source. An internal current source and capacitor ramp up from 0V to near V DD at a typical rate of 1V/ms. The soft start feature limits the output voltage of the error amplifier at the COMP pin. As the soft start voltage rises, it allows the COMP pin voltage to rise, which in turn allows the duty cycle of the output drivers to increase. The internal soft start voltage is discharged and remains discharged during the following conditions: 1. The V DD voltage drops below the turn-off threshold. 2. The voltage on the CS pin exceeds the overcur- rent comparator threshold. Once the internal soft start discharge FET is turned on, it cannot be turned off until the internal soft start voltage drops down below 0.5V. This insures a clean restart. Oscillator The oscillator operates at twice the switching frequency of either OUTA or OUTB. The oscillator generates a sawtooth waveform on the RC pin. The rising edge of the waveform is controlled by the external resistor/capacitor combination. The fall time is set by the on-resistance of the discharge FET (see Figure 3). The fall time sets the delay (dead time) between the turn-off of one output driver and the turn-on of the other driver. A toggle flip-flop insures that drive signals to OUTA and OUTB are alternated and therefore insures a maximum duty cycle of less than 50% for each output driver. Graphs of component values vs. oscillator frequency and dead time are shown in the typical characteristic section of this specification. Q S R 0.2V VDD 2 VDD OSCILLATOR OUTPUT RC 4 Figure 3. Oscillator The voltage source to the resistor/capacitor timing compo- nents is V DD. The internal turn-off comparator threshold in the oscillator circuit is V DD/2. This allows the oscillator to track changes in V DD and minimize frequency variations in the oscillator. The oscillator frequency can be roughly approxi- mated using the following formula: F 1.41 RC OSCILLATOR = × Where: frequency is in Hz Resistance is in ohms Capacitance is in Farads. Graphs of oscillator frequency and dead time vs component values are shown in the Typical Characteristic section of this specification. The recommended range of timing resistors and capacitors is 7k Ω to 200kΩ and 100pF to 1000pF. To minimize oscillator noise and insure a stable waveform the following layout rules should be followed: 1. The higher impedance of capacitor values less than 100pF may causes the oscillator circuit to become more susceptible to noise. Parasitic pin and etch trace capacitances become a larger part of the total RC capacitance and may influence the desired switching frequency. 2. The circuit board etch between the timing resistor, capacitor, RC pin and ground must be kept as short as possible to minimize noise pickup and insure a stable oscillator waveform. 3. The ground lead of the capacitor must be routed close to the ground lead of the MIC3838/9. Current Sensing and Overcurrent Protection The features are: • Peak current limit • Overcurrent limit • Internal current sense discharge • Front edge blanking In current mode control, a PWM comparator uses the inductor current signal and the error amplifier signal to determine the operating duty cycle. In the MIC3838/9 the signal at the CS pin is level shifted up before it reaches the PWM comparator as shown in Figure 1. This allows operation of the error amplifier and PWM comparator in a linear region. There are two current limit thresholds in the MIC3838/9; peak current limit and overcurrent limit. The normal operating voltage at the I LIM pin is designed less than these thresholds. A pulse-by-pulse current limit occurs when the inductor current signal at the I LIM pin exceeds the peak current limit threshold. The on-time is terminated for the remainder of the switching cycle, regardless of whether OUTA or OUTB is active. If the signal at the I LIM pin goes past the peak threshold and exceeds the overcurrent limit threshold, the overcurrent limit comparator forces the soft start node to discharge and initiates a soft start reset. An internal FET discharges the RAMP and I LIM pins at the end of the oscillator charge time. The FET turns on when the voltage on the RC pin reaches the upper threshold (V DD/2) and remains on for the duration of the RC pin discharge time and for typically 100ns after the start of the next on-time period. The 100ns period at the beginning of the on-time implements a front edge blanking feature that prevents false triggering of the PWM comparator due to noise spikes on the leading edge of the current turn-on signal. The front edge blanking also sets the minimum on-time for OUTA and OUTB. The timing diagram for the RAMP pin is shown in Figure 4. |
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