Datasheet LTC3610 (Analog Devices) - 10

LTC3610 applications inForMation The basic LTC3610 application circuit is shown on the
Operating Frequency
front page of this data sheet. External component selection The choice of operating frequency is a trade-off between is primarily determined by the maximum load current. efficiency and component size. Low frequency operation The LTC3610 uses the on-resistance of the synchronous improves efficiency by reducing MOSFET switching losses power MOSFET for determining the inductor current. The but requires larger inductance and/or capacitance in order desired amount of ripple current and operating frequency to maintain low output ripple voltage. also determines the inductor value. Finally, CIN is selected for its ability to handle the large RMS current into the The operating frequency of LTC3610 applications is de- converter and COUT is chosen with low enough ESR to meet termined implicitly by the one-shot timer that controls the the output voltage ripple and transient specification. on-time tON of the top MOSFET switch. The on-time is set by the current into the ION pin and the voltage at the VON
VON and PGOOD
pin according to: The LTC3610 has an open-drain PGOOD output that VVON indicates when the output voltage is within ±10% of the tON = ( pF 10 ) IION regulation point. The LTC3610 also has a VON pin that allows the on-time to be adjusted. Tying the VON pin high Tying a resistor, RON, from VIN to the ION pin yields an results in lower values for RON which is useful in high VOUT on-time inversely proportional to VIN. The current out of applications. The VON pin also provides a means to adjust the ION pin is: the on-time to maintain constant frequency operation in V applications where V IN OUT changes and to correct minor IION = R frequency shifts with changes in load current. ON For a step-down converter, this results in approximately
VRNG Pin and ILIMIT Adjust
constant frequency operation as the input supply varies: The VRNG pin is used to adjust the maximum inductor V valley current, which in turn determines the maximum f OUT = [HZ] average output current that the LTC3610 can deliver. The VVON RON( pF 10 ) maximum output current is given by: To hold frequency constant during output voltage changes, IOUT(MAX) = IVALLEY(MAX) + 1/2 ΔIL tie the VON pin to VOUT or to a resistive divider from VOUT The I when VOUT > 2.4V. The VON pin has internal clamps that VALLEY(MAX) is shown in the figure “Maximum Valley Current Limit vs V limit its input to the one-shot timer. If the pin is tied below RNG Voltage” in the Typical Performance Characteristics. 0.7V, the input to the one-shot is clamped at 0.7V. Similarly, if the pin is tied above 2.4V, the input is clamped at 2.4V. An external resistor divider from INTVCC can be used to In high VOUT applications, tying VON to INTVCC so that the set the voltage on the VRNG pin from 0.5V to 1V, or it can comparator input is 2.4V results in a lower value for RON. be simply tied to ground force a default value equivalent Figures 1a and 1b show how RON relates to switching to 0.7V. When setting current limit, ensure that the junc- frequency for several common output voltages. tion temperature does not exceed the maximum rating of 125°C. Do not float the VRNG pin. 3610ff 0 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts