CS5161, CS5161H

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14

Schottky Diode for Synchronous MOSFET

A Schottky diode may be placed in parallel with the

synchronous MOSFET to conduct the inductor current upon

turn off of the switching MOSFET to improve efficiency.

For a design operating at 200 kHz or so, the low non−overlap

time combined with Schottky forward recovery time may

make the benefits of this device not worth the additional

expense (see Figure 8, channel 2). The power dissipation in

the synchronous MOSFET due to body diode conduction

can be estimated by the following equation:

Power + VBD

ILOAD

conduction time

switching frequency

diode. For the CS5161/5161H demonstration board as

shown in Figure 8;

Power + 1.6 V

13 A

100 ns

233 kHz + 0.48 W

This is only 1.3% of the 36.4 W being delivered to the

load.

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

and bypass capacitors, as well as other loads located on the

board will tend to reduce regulator di/dt effects on the circuit

board and input power supply. Placement of the power

component to minimize routing distance will also help to

reduce emissions.

Figure 21. Filter Components

1000 pF

33 Ω

2.0 μH

Figure 22. Input Filter

1200 pF × 3.0/16 V

2.0 μH

+