CS5159

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12

used as the source for the regulator output current, the

following gate drive is provided;

VGATE(H) + 12 V * 5.0 V + 7.0 V, VGATE(L) + 12 V

(see Figure 19.)

Figure 19. CS5159 Gate Drive Waveforms Depicting

Rail to Rail Swing

M 1.00 μs

Trace 2− Inductor Switching Nodes (5.0 V/div.)

The most important aspect of MOSFET performance is

thermal management requirements.

The power dissipated by the MOSFETs may be estimated

as follows;

Switching MOSFET:

Power + ILOAD2

RDSON

duty cycle

Synchronous MOSFET:

Power + ILOAD2

RDSON

(1 * duty cycle)

Duty Cycle =

VOUT ) (ILOAD

RDSON OF SYNCH FET)

VIN)(ILOAD

RDSON OF SYNCH FET)

* (ILOAD

RDSON OF SWITCH 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

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 CS5159 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 CS5159 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