CS5155

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13

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

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.

The CS5155 reference circuit does not use this device due

to it’s excellent design. Instead, the body diode of the

synchronous MOSFET is utilized to reduce cost and

conducts the inductor current. 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 CS5155 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.

“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

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 20. Filter Components

1000 pF

33 Ω

2.0 μH