TS3405CS datasheet(9/10 Pages) TSC

9 / 10 page

Application Guidelines (continued)

The equations below relate the compensation network’s

poles, zeros and gain to the components (R1, R2, R3, C1,

C2 and C3) in Fig. 7. Use these guidelines for locating the

poles and zeros of the compensation network:

1. Pick Gain (R2/R1) for desired converter bandwidth.

2. Place 1

st zero below filter’s double pole (~75% F

LC)

3. Place 2

nd zero at filter’s double pole.

4. Place 1

st pole at the ESR zero.

5. Place 2

nd pole at half the switching frequency

6. Check gain against error amplifier’s open-loop gain.

7. Estimate phase margin – repeat if necessary.

Output Capacitor Selection

An output capacitor is required to filter the output and

supply

the

load

transient

current.

The

filtering

requirements are a function of the switching frequency

and the ripple current. The load transient requirements

are a function of the slew rate (di/dt) and the magnitude

of the transient load current. These requirements are

generally met with a mix of capacitors and careful layout.

Modern components and loads are capable of producing

transient load rates above 1A/nS. High frequency

capacitors initially supply the transient and slow the

current load rate seen by the bulk capacitors. The bulk

filter capacitor values are generally determined by the

ESR (Effective Series Resistance) and voltage rating

requirements

rather

than

actual

capacitance

requirements.

High frequency decoupling capacitors should be placed

as close to the power pins of the load as physically

possible. Be careful not to add inductance in the circuit

board wiring that could cancel the usefulness of these

low

inductance

components.

Consult

with

the

manufacturer of the load on specific decoupling

requirements.

Use only specialized low-ESR capacitors intended for

switching-regulator applications for the bulk capacitors.

The bulk capacitor’s ESR will determine the output ripple

voltage and the initial voltage drop after a high slew-rate

transient. An aluminum electrolytic capacitor’s ESR value

is related to the case size with lower ESR available in

larger case sizes. However, the equivalent Series

inductance (ESL) of these capacitors increases with case

size and can reduce the usefulness of the capacitor to

high slew-rate transient loading. Unfortunately, ESL is

not a specified parameter. Work with your capacitor

supplier and measure the capacitor’s impedance with

frequency to select a suitable component.

In most cases, multiple electrolytic capacitors of small

case size perform better than a single large case

capacitor.

Feedback Divider

The reference of TS3405 is 0.8V. the output voltage can

be set by R1 and R4 as shown in Fig. 4. The equation is

following:

Vout = 0.8 x (1 + R1 / R4)

The R1 should be between 2kΩ to 5kΩ. put the R1, R4

and others compensation component as close to TS3405

as possible.

Shutdown

Pulling low the COMP pin can shutdown the TS3405

PWM controller. You can use a small single transistor as

switch like as JP1 shown in Fig. 4.

Compensation Break Frequency Equations

As in any high frequency switching converter, layout is

very important. Switching current from one power device

to another can generate voltage transients across the

impedances of the interconnecting bond wires and circuit

traces. Using wide, short printed circuit traces should

minimize these interconnecting impedances. The critical

components should be located as close together as

possible, using ground plane construction or single point

grounding.

To minimize the voltage overshoot, the interconnecting

wires indicated by heavy lines should be part of a ground

or power plane in a printed circuit board. Locate the

TS3405 within 3 inches of the MOSFETs. Q1 and Q2.

The circuit traces for the MOSFETs’ gate and source

connections from the TS3405 must be sized to handle up

to 1A peak current. Provide local Vcc decoupling

between Vcc and Gnd pins. Locate the capacitor, CBOOT

as close as practical to the Boot and Phase pins. All

components used for feedback compensation should be

located as close to the IC a practical.

TS3405

9-10

2003/12 rev. A

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