Applications Information: continued

11

Figure 16: CS5155 demonstration board during power up. Power Good

signal is activated when output voltage reaches 1.70V.

Selecting External Components

The CS5155 can be used with a wide range of external

power components to optimize the cost and performance of

a particular design. The following information can be used

as general guidelines to assist in their selection.

NFET Power Transistors

Both logic level and standard MOSFETs can be used. The

reference designs derive gate drive from the 12V supply

which is generally available in most computer systems and

utilize logic level MOSFETs. A charge pump may be easily

implemented to support 5V only systems. Multiple

MOSFETs may be paralleled to reduce losses and improve

efficiency and thermal management.

Voltage applied to the MOSFET gates depends on the

application circuit used. Both upper and lower gate driver

outputs are specified to drive to within 1.5V of ground

when in the low state and to within 2V of their respective

bias supplies when in the high state. In practice, the MOS-

FET gates will be driven rail to rail due to overshoot caused

by the capacitive load they present to the controller IC. For

used as the source for the regulator output current, the fol-

lowing gate drive is provided;

Figure 17: CS5155 gate drive waveforms depicting rail to rail swing.

The most important aspect of MOSFET performance is

thermal management requirements.

The power dissipated by the MOSFETs may be estimated

as follows;

Switching MOSFET:

Synchronous MOSFET:

Duty Cycle =

culated as follows:

time out timer, whichever is longer.

The preceding equations for duty cycle can also be used to

calculate the regulator switching frequency and select the

Trace 2 = Inductor Switching Node (5V/div.)

Trace 4 = 5V Input (2V/div.)

Trace 1 = Regulator Output Voltage (1V/div.)

Trace 2 = Power Good Signal (2V/div.)