TC2001 datasheet(25/34 Pages) TRIPATH

25 / 34 page

Tr i p ath Technol ogy, I n c. - Techni cal In fo rma t io n

TK2150 – Rev. 1.0/12.02

only sink current into the regulator output and will not be capable of sourcing the current required

by VN10. Furthermore, problems can arise since VN10 will not track movements in VNN. The

external VN10 supply must be able to source a maximum of 250mA into the VN10 pin. Thus, a

positive supply must be used and must be referenced to the VNN rail. If the external VN10

supply does not track fluctuations in the VNN supply or is not able to source current into the VN10

pin, the TP2150 will not work and can also become permanently damaged.

Figure 7 shows the correct way to power the TP2150:

Figure 7: Proper Power Supply Connection

Output Transistor Selection

The key parameters to consider when selecting what MOSFET to use with the TK2150 are drain-

source breakdown voltage (BVdss), gate charge (Qg), and on-resistance (RDS(ON)).

The BVdss rating of the MOSFET needs to be selected to accommodate the voltage swing

between VSPOS and VSNEG as well as any voltage peaks caused by voltage ringing due to

switching transients. With a ‘good’ circuit board layout, a BVdss that is 50% higher than the VPP

and VNN voltage swing is a reasonable starting point. The BVdss rating should be verified by

measuring the actual voltages experienced by the MOSFET in the final circuit.

Ideally a low Qg (total gate charge) and low RDS(ON) are desired for the best amplifier

performance.

Unfortunately, these are conflicting requirements since RDS(ON) is inversely

proportional to Qg for a typical MOSFET. The design trade-off is one of cost versus performance.

A lower RDS(ON) means lower I

DS(ON) losses but the associated higher Qg translates into higher

switching losses (losses = Qg x 10 x 1.2MHz). A lower RDS(ON) also means a larger silicon die

and higher cost. A higher RDS(ON) means lower cost and lower switching losses but higher I

DSON

losses.

Gate Resistor Selection

The gate resistors, RG, are used to control MOSFET switching rise/fall times and thereby

minimize voltage overshoots. They also dissipate a portion of the power resulting from moving

the gate charge each time the MOSFET is switched. If RG is too small, excessive heat can be

generated in the driver. Large gate resistors lead to slower MOSFET switching, which requires a

larger break-before-make (BBM) delay.

Break-Before-Make (BBM) Timing Control

VPP

VN10

VNN

PGND

VPP

VNN

10V

AGND

F. BEAD

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