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LTC1504 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1504 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC1504 APPLICATIONS INFORMATION C of the output stage form a 2nd order roll-off with 180 ° of phase shift; the R due to ESR forms a single zero at a somewhat higher frequency that reduces the roll-off to first order and reduces the phase shift to 90 °. If the output capacitor has a relatively high ESR, the zero comes in well before the initial phase shift gets all the way to 180 ° and the loop only requires a single small capacitor from COMP to GND to remain stable (Figure 4a). If, on the other hand, the output capacitor is a low ESR type to maximize transient response, the ESR zero can increase in frequency by a decade or more and the output stage phase shift can get awfully close to 180 ° before it turns around and comes back to 90 °. Large value ceramic, OS-CON electrolytic and low impedance tantalum capacitors fall into this category. These loops require an additional zero to be inserted at the COMP pin; a series RC in parallel with a smaller C to ground will usually ensure stability. Figure 4b shows a typical compensation network which will optimize transient response with most output capacitors. Adjustable output parts can add a feedforward capacitor across the feedback resistor divider to further improve phase margin. The typical applications in this data sheet COMP FB CC *ADJUSTABLE PARTS ONLY 1504 • F04a LTC1504 RFB2* RFB1* VOUT COMP FB CC *ADJUSTABLE PARTS ONLY 1504 • F04b LTC1504 CF CFF* RC RFB2* RFB1* VOUT Figure 4a. Minimum Compensation Network Figure 4b. Optimum Compensation Network 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 LTC1504 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. Soft Start and Current Limit Soft start and current limit are linked in the LTC1504. 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. During a soft start cycle, the MIN feedback comparator is disabled to prevent it from overriding the COMP pin and forcing the output to maxi- mum duty cycle. 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 |
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