CS5421

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8

3. output voltage change due to the ESR and ESL of the

bulk and high frequency decoupling capacitors,

circuit traces, and vias;

4. output voltage ripple and noise.

Budgeting the tolerance is left up to the designer who must

take into account all of the above effects and provide an

output voltage that will meet the specified tolerance at the

load.

The designer must also ensure that the regulator

component temperatures are kept within the manufacturer’s

specified ratings at full load and maximum ambient

temperature.

Selecting Feedback Divider Resistors

Figure 6. Selecting Feedback Divider Resistors

R1

R2

externalresistor dividers to set the output voltages. The error

amplifier is referenced to 1.0 V and the output voltage is

determined by selecting resistor divider values. Resistor R1

is selected based on a design trade−off between efficiency

and output voltage accuracy. The output voltage error can be

estimated due to the bias current of the error amplifier

neglecting resistor tolerance:

Error% + 1

10*6

R1

1.0

100%

R2 can be sized after R1 has been determined:

R2 + R1

VOUT

1.0 *

1.0

Calculating Duty Cycle

The duty cycle of a buck converter (including parasitic

losses) is given by the formula:

Duty Cycle + D +

VOUT ) (VHFET ) VL)

VIN ) VLFET * VHFET * VL

where:

DC resistance;

Figure 7. Switching Frequency

10

20

30

40

50

60

100

200

300

400

500

600

700

800

Selecting the Switching Frequency

Selecting the switching frequency is a trade−off between

component size and power losses. Operation at higher

switching frequencies allows the use of smaller inductor and

capacitor values. Nevertheless, it is common to select lower

frequency operation because a higher frequency results in

lower efficiency due to MOSFET gate charge losses.

Additionally, the use of smaller inductors at higher

frequencies results in higher ripple current, higher output

voltage ripple, and lower efficiency at light load currents.

The value of the oscillator resistor is designed to be

linearly related to the switching period. If the designer

prefers not to use Figure 7 to select the necessary resistor, the

following equation quite accurately predicts the proper

resistance for room temperature conditions.

ROSC +

21700 * fSW

2.31fSW

where:

Selection of the 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. There are many factors to

consider in selecting the inductor including cost, efficiency,

EMI and ease of manufacture. The inductor must be able to

handle the peak current at the switching frequency without

saturating, and the copper resistance in the winding should

be kept as low as possible to minimize resistive power loss.

There are a variety of materials and types of magnetic

cores that could be used for this application. Among them

are ferrites, molypermalloy cores (MPP), amorphous and

powdered iron cores. Powdered iron cores are very

commonly used. Powdered iron cores are very suitable due