TS3405 Datasheet, PDF (9/10 Pages) Taiwan Semiconductor Company, Ltd – Single Synchronous Buck PWM Controller

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, C_BOOT
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