Tripath Technology, Inc. - Technical Information
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:
VPP
V5
VPP
5V
AGND
VN10
PGND
VNN
10V
VNN
F. BEAD
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 I2RDS(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 I2RDSON
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
25
TK2150 – Rev. 1.0/12.02
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