Applications Information: continued

10

Figure 14: CS5151 demonstration board during power up. Power Good

signal is activated when output voltage reaches 1.70V.

Selecting External Components

The CS5151 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

use 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 gate depends on the appli-

cation circuit used. The gate driver output is specified to

drive to within 1.5V of ground when in the low state and to

within 2V of its bias supply when in the high state. In prac-

tice, the MOSFET gate will be driven rail to rail due to

overshoot caused by the capacitive load it presents to the

= 12V and 5V is used as the source for the regulator output

current, the following gate drive is provided;

Figure 15: CS5151 gate drive waveforms depicting rail to rail swing.

The most important aspect of MOSFET performance is

thermal management requirements.

The power dissipated by the MOSFET and the Schottky

diode may be estimated as follows;

Switching MOSFET:

Schottky diode:

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

Channel 2 = Inductor Switching Node

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

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

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