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TD1509PR

2A 150KHz PWM Buck DC/DC Converter

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Techcode®

2A 150KHz PWM Buck DC/DC Converter

Function Description(Cont.)

DATASHEET

TD1509

this is the output (switch) pin, which should not have

large areas of copper.) Large areas of copper provide the

best transfer of heat (lower thermal resistance) to the

surrounding air, and moving air lowers the thermal

resistance even further.

L = (VOUT) * (VIN-VOUT) / VIN * f * ∆I

Where VOUT is the output voltage, VIN is the input

voltage, f is the switching frequency, and ∆I is the

peak-to-peak inductor ripple current.

Setting the Output Voltage

Input Capacitor

The output voltage is set using a resistive voltage divider

from the output voltage to FB(TD1509-ADJ) The voltage

divider divides the output voltage down by the ratio:

VFB = VOUT * R1 / (R1 + R2)

Thus the output voltage is:

VOUT = 1.235 * (R1 + R2) / R1

R1 can be as high as 100KΩ, but a typical value is 10KΩ.

Using that value, R2 is determined by:

R2 ~= 8.18 * (VOUT – 1.235) (KΩ)

For example, for a 3.3V output voltage, R1 is 10KΩ, and

R2is 17KΩ.

Inductor

The inductor is required to supply constant current to the

output load while being driven by the switched input

voltage. A larger value inductor results in less ripple

current that in turn results in lower output ripple voltage.

However, the larger value inductor has a larger physical

size, higher series resistance, and/or lower saturation

current. Choose an inductor that does not saturate under

the worst-case load conditions. A good rule for

determining the inductance is to allow the peak-to-peak

ripple current in the inductor to be approximately 30% of

the maximum load current. Also, make sure that the peak

inductor current (the load current plus half the peak to

peak inductor ripple current) is below the TBDA minimum

current limit. The inductance value can be calculated by

the equation:

The input current to the step-down converter is

discontinuous, and so a capacitor is required to supply

the AC current to the step-down converter while

maintaining the DC input voltage. A low ESR capacitor is

required to keep the noise at the IC to a minimum.

Ceramic capacitors are preferred, but tantalum or

low-ESR electrolytic capacitors may also suffice.

The input capacitor value should be greater than 10μF.

The capacitor can be electrolytic, tantalum or ceramic.

However since it absorbs the input switching current it

requires an adequate ripple current rating. Its RMS

current rating should be greater than approximately

1/2 of the DC load current.

For insuring stable operation should be placed as close

to the IC as possible. Alternately a smaller high quality

ceramic 0.1μF capacitor may be placed closer to the IC

and a larger capacitor placed further away. If using this

technique, it is recommended that the larger capacitor be

a tantalum or electrolytic type. All ceramic capacitors

should be places close to the TD1509.

Output Capacitor

The output capacitor is required to maintain the DC

output voltage. Low ESR capacitors are preferred to

keep the output voltage ripple low. The characteristics of

the output capacitor also affect the stability of the

regulation control system. Ceramic, tantalum, or low

ESR electrolytic capacitors are recommended. In the

case of ceramic capacitors, the impedance at the

switching frequency is dominated by the capacitance,

December, 20, 2005.

Techcode Semiconductor Limited

12

www.techcodesemi.com

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