Datasheet LTC1871-1 (Analog Devices) - 10

LTC1871-1
APPLICATIONS INFORMATION
When the oscillator’s internal logic circuitry detects a 0.6V, and the current that fl ows into the FREQ pin is used synchronizing signal on the MODE/SYNC pin, the in- to charge and discharge an internal oscillator capacitor. A ternal oscillator ramp is terminated early and the slope graph for selecting the value of RT for a given operating compensation is increased by approximately 30%. As frequency is shown in Figure 6. a result, in applications requiring synchronization, it is recommended that the nominal operating frequency of 1000 the IC be programmed to be about 75% of the external clock frequency. Attempting to synchronize to too high an external frequency (above 1.3fO) can result in inadequate slope compensation and possible subharmonic oscillation 100 (or jitter). (kΩ) TR 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. 10 0 100 200 300 400 500 600 700 800 900 1000 FREQUENCY (kHz) 18711 F06 2V TO 7V MODE/
Figure 6. Timing Resistor (RT) Value
SYNC tMIN = 25ns
INTV
0.8T T T = 1/fO
CC Regulator Bypassing and Operation
An internal, P-channel low dropout voltage regulator pro- GATE D = 40% duces the 5.2V supply which powers the gate driver and logic circuitry within the LTC1871-1, 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. IL Good bypassing is necessary to supply the high transient currents required by the MOSFET gate driver. 18711 F05 For input voltages that don’t exceed 7V (the absolute
Figure 5. MODE/SYNC Clock Input and Switching Waveforms for Synchronized Operation
maximum rating for this pin), the internal low dropout regulator in the LTC1871-1 is redundant and the INTVCC pin can be shorted directly to the V
Programming the Operating Frequency
IN pin. With the INTVCC pin shorted to VIN, however, the divider that programs the The choice of operating frequency and inductor value is regulated INTVCC voltage will draw 10μA of current from a tradeoff between effi ciency and component size. Low the input supply, even in shutdown mode. For applications frequency operation improves effi ciency by reducing that require the lowest shutdown mode input supply cur- MOSFET and diode switching losses. However, lower rent, do not connect the INTVCC pin to VIN. Regardless of frequency operation requires more inductance for a given whether the INTVCC pin is shorted to VIN or not,
it is always
amount of load current.
necessary to have the driver circuitry bypassed with a
The LTC1871-1 uses a constant frequency architecture that
4.7μF tantalum or low ESR ceramic capacitor to ground
can be programmed over a 50kHz to 1000kHz range with
immediately adjacent to the INTVCC and GND pins.
a single external resistor from the FREQ pin to ground, as In an actual application, most of the IC supply current is shown in Figure 1. The nominal voltage on the FREQ pin is used to drive the gate capacitance of the power MOSFET. 18711fb 10