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LT1871IMS-7-TRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LT1871IMS-7-TRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 32 page  LTC1871-7 10 18717fc external frequency (above 1.3fO) can result in inadequate slope compensation and possible subharmonic oscillation (or jitter). The external clock signal must exceed 2V for at least 25ns, and should have a maximum duty cycle of 80%, as shown in Figure 5. The MOSFET turn on will synchronize to the rising edge of the external clock signal. Programming the Operating Frequency The choice of operating frequency and inductor value is a tradeoff between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET and diode switching losses. However, lower frequency operation requires more inductance for a given amount of load current. The LTC1871-7 uses a constant frequency architecture that can be programmed over a 50kHz to 1000kHz range with a single external resistor from the FREQ pin to ground, as shown in Figure 1. The nominal voltage on the FREQ pin is 0.6V, and the current that flows into the FREQ pin is used to charge and discharge an internal oscillator capacitor. A graph for selecting the value of RT for a given operating frequency is shown in Figure 6. INTVCC Regulator Bypassing and Operation An internal, P-channel low dropout voltage regulator produces the 7V supply which powers the gate driver and OPERATION logic circuitry within the LTC1871-7, as shown in Figure 7. The INTVCC regulator can supply up to 50mA and must be bypassed to ground immediately adjacent to the IC pins with a minimum of 4.7μF tantalum or ceramic capacitor. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate driver. The LTC1871-7 contains an undervoltage lockout circuit which protects the external MOSFET from switching at low gate-to-source voltages. This undervoltage circuit senses the INTVCC voltage and has a 5.6V rising threshold and a 4.6V falling threshold. For input voltages that don’t exceed 8V (the absolute maximum rating for INTVCC is 9V), the internal low dropout regulator in the LTC1871-7 is redundant and the INTVCC pin can be shorted directly to the VIN pin. With the INTVCC pin shorted to VIN, however, the divider that programs the regulated INTVCC voltage will draw 14μA of current from the input supply, even in shutdown mode. For applications that require the lowest shutdown mode input supply cur- rent, do not connect the INTVCC pin to VIN. Regardless of whether the INTVCC pin is shorted to VIN or not, it is always necessary to have the driver circuitry bypassed with a 4.7μF ceramic capacitor to ground immediately adjacent to the INTVCC and GND pins. In an actual application, most of the IC supply current is used to drive the gate capacitance of the power MOSFET. As a result, high input voltage applications in which a large power MOSFET is being driven at high frequencies Figure 5. MODE/SYNC Clock Input and Switching Waveforms for Synchronized Operation Figure 6. Timing Resistor (RT) Value 18717 F05 2V TO 7V MODE/ SYNC GATE IL tMIN = 25ns 0.8T D = 40% T T = 1/fO FREQUENCY (kHz) 100 300 1000 18717 F06 10 100 200 1000 900 800 700 600 500 400 0 |
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Similar Description - LT1871IMS-7-TRPBF |
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