Applications Information: continued

11

Figure 15: CS5156 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 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

,

where:

Period =

“Droop” Resistor for Adaptive Voltage Positioning

Adaptive voltage positioning is used to reduce output volt-

age excursions during abrupt changes in load current.

Regulator output voltage is offset +40mV when the regula-

tor is unloaded, and -40mV at full load. This results in

increased margin before encountering minimum and maxi-

mum transient voltage limits, allowing use of less capaci-

tance on the regulator output (see Figure 7).

To implement adaptive voltage positioning, a “droop”

resistor must be connected between the output inductor

and output capacitors and load. This is normally imple-

mented by a PC board trace of the following value:

Adaptive voltage positioning can be disabled for improved

using a separate, non-load current carrying circuit trace.

Input and Output Capacitors

These components must be selected and placed carefully to

yield optimal results. Capacitors should be chosen to pro-

vide acceptable ripple on the input supply lines and regula-

tor output voltage. Key specifications for input capacitors

are their ripple rating, while ESR is important for output

capacitors. For best transient response, a combination of

low value/high frequency and bulk capacitors placed close

to the load will be required.

Output Inductor

The inductor should be selected based on its inductance,

current capability, and DC resistance. Increasing the induc-

tor value will decrease output voltage ripple, but degrade

transient response.

Thermal Considerations for Power MOSFETs and Diodes

In order to maintain good reliability, the junction tempera-

ture of the semiconductor components should be kept to a

maximum of 150°C or lower. The thermal impedance (junc-

tion to ambient) required to meet this requirement can be

calculated as follows:

Thermal Impedance =

A heatsink may be added to TO-220 components to reduce

their thermal impedance. A number of PC board layout

techniques such as thermal vias and additional copper foil

Power

Thermal Management

80mV

1

switching frequency

Period × (1 - duty cycle)

4848.5

Channel 2 = Inductor Switching Node