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CD54-101 Scheda tecnica(PDF) 9 Page - Linear Technology |
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CD54-101 Scheda tecnica(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC1504A APPLICATIONS INFORMATION phase margin. The typical applications in this data sheet show compensation values that work with several combi- nations of external components—use them as a starting point. For complex cases or stubborn oscillations, contact the LTC Applications Department. External Schottky Diode An external Schottky diode can be included across the internal N-channel switch (Q2) to improve efficiency at heavy loads. The diode carries the inductor current during the nonoverlap time while the LTC1504A turns Q1 off and Q2 on and prevents current from flowing in the intrinsic body diode in parallel with Q2. This diode will improve efficiency by a percentage point or two as output current approaches 500mA and can help minimize erratic behav- ior at very high peak current levels caused by excessive parasitic current flow through Q2. A Motorola MBRS0530L is usually adequate, with the cathode connected to SW and the anode connected to GND. Note that this diode is not required for normal operation and has a negligible effect on efficiency at low (< 250mA) output currents. COMP FB CC *ADJUSTABLE PARTS ONLY 1504A • F04a LTC1504A RFB2* RFB1* VOUT COMP FB CC *ADJUSTABLE PARTS ONLY 1504A • F04b LTC1504A CF CFF* RC RFB2* RFB1* VOUT Figure 4a. Minimum Compensation Network Figure 4b. Optimum Compensation Network Soft Start and Current Limit Soft start and current limit are linked in the LTC1504A. Soft start works in a straightforward manner. An internal 12 µA current source connected to the SS pin will pull up an external capacitor connected from SS to GND at a rate determined by the capacitor value. COMP is clamped to a voltage one diode drop above SS; as SS rises, COMP will rise at the same rate. When COMP reaches roughly 2V below VCC, the duty cycle will slowly begin to increase until the output comes into regulation. As SS continues to rise, the feedback amplifier takes over at COMP, the clamp releases and SS rises to VCC. Current limit operates by pulling down on the soft start pin when it senses an overload condition at the output. The current limit amplifier (ILIM) compares the voltage drop across the internal P-channel switch (Q1) during its on time to the voltage at the IMAX pin. IMAX includes an internal 12 µA pull-down, allowing the voltage to be set by a single resistor between VCC and IMAX . When the IR drop across Q1 exceeds the drop across the IMAX resistor, ILIM pulls current out of the external soft start capacitor, reducing the voltage at SS. A soft start capacitor should always be used if current limit is enabled. SS, in turn, pulls down on COMP, limiting the output duty cycle and controlling the output current. When the current overload is removed, the ILIM amplifier lets go of SS and allows it to rise again as if it were completing a soft start cycle. The size of the external soft start capacitor controls both how fast the current limit responds once an overload is detected and how fast the output recovers once the overload is re- moved. The soft start capacitor also compensates the feedback loop created by the ILIM amplifier. Because the ILIM loop is a current feedback loop, the additional phase shift due to the output inductor and capacitor do not come into play and the loop can be adequately compensated with a single capacitor. Usually a 0.1 µF ceramic capacitor from SS to GND provides adequate soft start behavior and acceptable current limit response. This type of current limit circuit works well with mild current overloads and eliminates the need for an external current sensing resistor, making it attractive for LTC1504A applications. These same features also handicap the cur- rent limit circuit under severe short circuits when the |
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