Datasheet LTC1871-7 (Analog Devices) - 10

LTC1871-7
operaTion
external frequency (above 1.3fO) can result in inadequate logic circuitry within the LTC1871-7, as shown in Figure 7. slope compensation and possible subharmonic oscillation The INTVCC regulator can supply up to 50mA and must be (or jitter). bypassed to ground immediately adjacent to the IC pins The external clock signal must exceed 2V for at least 25ns, with a minimum of 4.7µF tantalum or ceramic capacitor. and should have a maximum duty cycle of 80%, as shown Good bypassing is necessary to supply the high transient in Figure 5. The MOSFET turn on will synchronize to the currents required by the MOSFET gate driver. rising edge of the external clock signal. The LTC1871-7 contains an undervoltage lockout circuit which protects the external MOSFET from switching at low
Programming the Operating Frequency
gate-to-source voltages. This undervoltage circuit senses The choice of operating frequency and inductor value is the INTVCC voltage and has a 5.6V rising threshold and a a tradeoff between efficiency and component size. Low 4.6V falling threshold. frequency operation improves efficiency by reducing For input voltages that don’t exceed 8V (the absolute MOSFET and diode switching losses. However, lower maximum rating for INTVCC is 9V), the internal low dropout frequency operation requires more inductance for a given regulator in the LTC1871-7 is redundant and the INTVCC amount of load current. pin can be shorted directly to the VIN pin. With the INTVCC The LTC1871-7 uses a constant frequency architecture that pin shorted to VIN, however, the divider that programs the can be programmed over a 50kHz to 1000kHz range with regulated INTVCC voltage will draw 14µA of current from a single external resistor from the FREQ pin to ground, as the input supply, even in shutdown mode. For applications shown in Figure 1. The nominal voltage on the FREQ pin is that require the lowest shutdown mode input supply cur- 0.6V, and the current that flows into the FREQ pin is used rent, do not connect the INTVCC pin to VIN. Regardless to charge and discharge an internal oscillator capacitor. A of whether the INTVCC pin is shorted to VIN or not,
it is
graph for selecting the value of R
always necessary to have the driver circuitry bypassed
T for a given operating frequency is shown in Figure 6.
with a 4.7µF ceramic capacitor to ground immediately adjacent to the INTVCC and GND pins. INTVCC Regulator Bypassing and Operation
In an actual application, most of the IC supply current is An internal, P-channel low dropout voltage regulator used to drive the gate capacitance of the power MOSFET. produces the 7V supply which powers the gate driver and As a result, high input voltage applications in which a large power MOSFET is being driven at high frequencies 2V TO 7V 1000 MODE/ SYNC tMIN = 25ns 0.8T T T = 1/fO 100 GATE (kΩ) D = 40% R T IL 10 0 100 200 300 400 500 600 700 800 9001000 FREQUENCY (kHz) 18717 F05 18717 F06
Figure 5. MODE/SYNC Clock Input and Switching Figure 6. Timing Resistor (RT) Value Waveforms for Synchronized Operation
18717fd 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts