TD1509PR datasheet(13/17 Pages) TECHCODE

13 / 17 page

December, 20, 2005. Techcode Semiconductor Limited www.techcodesemi.com

Techcode®

2A 150KHz PWM Buck DC/DC Converter

TD1509

DATASHEET

Function Description(Cont.)

and so the output voltage ripple is mostly

independent of the ESR. The output voltage

ripple is estimated to be:

VRIPPLE ~= 1.4 * VIN * (fLC/fSW)^2

Where VRIPPLE is the output ripple voltage, VIN

is the input voltage, fLC is the resonant

frequency of the LC filter, fSW is the switching

frequency. In the case of tanatalum or low-

ESR electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency, and so the output ripple is

calculated as:

VRIPPLE ~= ∆I * RESR

Where VRIPPLE is the output voltage ripple, ∆I is

the inductor ripple current, and RESR is the

equivalent series resistance of the output

capacitors.

Output Rectifier Diode

The output rectifier diode supplies the current to the

inductor when the high-side switch is off. To reduce

losses due to the diode forward voltage and recovery

times, use a Schottky rectifier.

Table 1 provides the Schottky rectifier part numbers

based on the maximum input voltage and current rating.

Choose a rectifier who’s maximum reverse voltage rating

is greater than the maximum input voltage, and who’s

current rating is greater than the maximum load current.

Feedforward Capacitor (CFF)

For output voltages greater than approximately 8V, an

additional capacitor is required. The compensation

capacitor is typically between 100 pF and 33 nF, and is

wired in parallel with the output voltage setting resistor,

R2. It provides additional stability for high output

voltages, low input-output voltages, and/or very low ESR

output capacitors, such as solid tantalum capacitors.

This capacitor type can be ceramic, plastic, silver mica,

etc.(Because of the unstable characteristics of ceramic

capacitors made with Z5U material, they are not

recommended.)

Note:In PCB layout. Reserved an area for CFF.

Over Current Protection (OCP)

The cycle by cycle current limit threshold is set between

3.8A and 4A. When the load current reaches the current

limit threshold, the cycle by cycle current limit circuit

turns off the high side switch immediately to terminate

the current duty cycle. The inductor current stops rising.

The cycle by cycle current limit protection directly limits

inductor peak current. The average inductor current is

also limited due to the limitation on peak inductor current.

When the cycle by cycle current limit circuit is triggered,

the output voltage drops as the duty cycle is decreasing.

Thermal Management and Layout

Consideration

In the TD1509 buck regulator circuit, high pulsing current

flows through two circuit loops. The first loop starts from

the input capacitors, to the VIN pin, to the VOUT pins, to

the filter inductor, to the output capacitor and load, and

then returns to the input capacitor through ground.

Current flows in the first loop when the high side switch is

on. The second loop starts from the inductor, to the

output capacitors and load, to the GND pin of the

TD1509, and to the VOUT pins of the TD1509. Current

flows in the second loop when the low side diode is on.

In PCB layout, minimizing the two loops area reduces the

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