CS5151

http://onsemi.com

12

Figure 17. CS5150H Gate Drive Waveforms Depicting

Rail to Rail Swing

M 1.00 μs

Channel 2− Inductor Switching Node

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:

Power + ILOAD2

RDSON

duty cycle

Schottky diode:

Power + VFORWARD

ILOAD

(1 * duty cycle)

Duty Cycle =

VOUT ) VFORWARD

VIN ) VFORWARD * (ILOAD

RDSON OF SYNCH FET)

TOFF + COFF

4848.5

calculated as follows:

TOFF + COFF

24, 242.5

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

COFF +

Perioid

(1 * duty cycle)

4848.5

where:

Period +

1

switching frequency

“Droop” Resistor for Adaptive Voltage Positioning

Adaptive voltage positioning is used to reduce output

voltage excursions during abrupt changes in load current.

Regulator output voltage is offset +40 mV when the

regulator is unloaded, and −40 mV at full load. This results

in increased margin before encountering minimum and

maximum transient voltage limits, allowing use of less

capacitance on the regulator output (see Figure 9).

To implement adaptive voltage positioning, a “droop”

resistor must be connected between the output inductor and

output capacitors and load. This is normally implemented by

a PC board trace of the following value:

RDROOP + 80 mV

IMAX

Adaptive voltage positioning can be disabled for

to the load 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

provide acceptable ripple on the input supply lines and

regulator 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

inductor value will decrease output voltage ripple, but

degrade transient response.

THERMAL MANAGEMENT

Thermal Considerations for Power

MOSFETs and Diodes

In order to maintain good reliability, the junction

temperature of the semiconductor components should be

kept to a maximum of 150°C or lower. The thermal

impedance (junction to ambient) required to meet this

requirement can be calculated as follows:

Thermal Impedance +

TJUNCTION(MAX) * TAMBIENT

Power

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 area can be used to improve the power handling

capability of surface mount components.

EMI Management

As a consequence of large currents being turned on and off

at high frequency, switching regulators generate noise as a

consequence of their normal operation. When designing for

compliance with EMI/EMC regulations, additional

components may be added to reduce noise emissions. These

components are not required for regulator operation and

experimental results may allow them to be eliminated. The

input filter inductor may not be required because bulk filter