Texas Instruments HPA070 Component Selection, TPS40055 Device Selection, Frequency of Operation

SLUU186 − March 2004

4 Component Selection

4.1TPS40055 Device Selection

The TPS4005x family of parts offers a range of output current configurations including source only (TPS40054), source/sink (TPS40055), or source/sink with VOUT prebias (TPS40057). In this converter the TPS40055 with source/sink capability is selected. This serves to maintain continuous inductor ripple current all the way to zero load to improve the small signal loop response by preventing the inductor current from transitioning to the discontinuous current mode.

The TPS4005x family is packaged in TI’s PWP PowerPAD thermally enhanced package which should be soldered to the PCB using standard solder flow techniques. The PowerPAD technology uses a thermally conductive epoxy to attach the integrated circuit die to the leadframe die pad, which is exposed on the bottom of the completed package. The PWP PowerPAD package has a θJC = 2°C/W which helps keep the junction temperature rise relatively low even with the power dissipation inherent in the onboard MOSFET drivers. This power loss is proportional to switching frequency, drive voltage, and the gate charge needed to enhance the N-channel MOSFETs. Effective heat removal allows the use of ultra-small packaging while maintaining high component reliability.

The technical brief, PowerPAD Thermally Enhanced Package[2] contains more information on the PowerPAD package.

4.2Frequency of Operation

The clock oscillator frequency for the TPS40055 is programmed with a single resistor from RT (pin 2) to signal ground. The following equation (1) from the datasheet allows selection of RT in kΩ for a given switching frequency in kHz.

		1		W		(1)
RT + R2 + f	SW	17.82	10*6 * 23 (k		)

For 300-kHz operation, R2 is selected to be 165 kΩ.

For a particular operating frequency, the PWM ramp time must be programmed via the resistor RKFF connected to VIN. Also, the selection of RKFF programs the VIN voltage at which the circuit starts operation. This prevents the circuit from starting at low voltages, which can lead to current flow larger than desired. RKFF is programmed using equation (2).

RKFF + R6 + ￿VIN(min) * 3.5￿ ￿58.14 RT ) 1340￿ (kW) (2)

Where VIN(min) is the minimum startup input voltage, and RT is in kΩ. Note that internal tolerances have been incorporated into this equation, so the actual VIN(min) of the input voltage should be used. For an oscillator frequency of 300-kHz, the RKFF value of 71.5 kΩ is selected.

6TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)