Datasheet LTC3704 (Analog Devices) - 10

LTC3704
U U W U APPLICATIO S I FOR ATIO INTVCC Regulator Bypassing and Operation
2V TO 7V MODE/ SYNC An internal, P-channel low dropout voltage regulator pro- tMIN = 25ns duces the 5.2V supply which powers the gate driver and 0.8T T T = 1/fO logic circuitry within the LTC3704, as shown in Figure 7. The INTVCC regulator can supply up to 50mA and must be GATE D = 40% 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. ISW INPUT VIN SUPPLY 2.5V TO 30V 3404 F05 – 1.230V
Figure 5. MODE/SYNC Clock Input and Switching
P-CH
Waveforms for Synchronized Operation
+ CIN
Programming the Operating Frequency
R1 5.2V INTVCC The choice of operating frequency and inductor value is a R2 + CVCC tradeoff between efficiency and component size. Low 4.7μF GATE frequency operation improves efficiency by reducing LOGIC DRIVER M1 MOSFET and diode switching losses. However, lower frequency operation requires more inductance for a given GND GND amount of load current. PLACE AS CLOSE AS The LTC3704 uses a constant frequency architecture that POSSIBLE TO DEVICE PINS 3704 F07 can be programmed over a 50kHz to 1000kHz range with
Figure 7. Bypassing the LDO Regulator and Gate Driver Supply
a single external resistor from the FREQ pin to ground, as shown in Figure 1. The nominal voltage on the FREQ pin is For input voltages that don’t exceed 7V (the absolute 0.6V, and the current that flows into the FREQ pin is used maximum rating for this pin), the internal low dropout to charge and discharge an internal oscillator capacitor. A regulator in the LTC3704 is redundant and the INTVCC pin graph for selecting the value of RT for a given operating can be shorted directly to the VIN pin. With the INTVCC pin frequency is shown in Figure 6. shorted to VIN, however, the divider that programs the 1000 regulated INTVCC voltage will draw 10μA of current from the input supply, even in shutdown mode. For applications that require the lowest shutdown mode input supply current, do not connect the INTVCC pin to VIN. Regardless ) of whether the INTV Ω CC pin is shorted to VIN or not,
it is
(k 100
always necessary to have the driver circuitry bypassed
TR
with a 4.7
μ
F tantalum or low ESR ceramic capacitor to ground immediately adjacent to the INTVCC and GND pins.
10 In an actual application, most of the IC supply current is 0 100 200 300 400 500 600 700 800 900 1000 used to drive the gate capacitance of the power MOSFET. FREQUENCY (kHz) 3704 F06 As a result, high input voltage applications in which a large
Figure 6. Timing Resistor (RT) Value
power MOSFET is being driven at high frequencies can 3704fb 10