AN723 データシートの表示（PDF） - Vishay Semiconductors

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AN723

Design A High Performance Buck or Boost Converter With Si9165

Vishay Semiconductors 

AN723

Vishay Siliconix

As one may have noticed, the switching frequency needs to

be determined at the beginning of the design process. A high

switching frequency allows the use of a smaller L/C power

stage filter without any sacrifice to current/voltage ripple and

conduction losses. In addition, a fast switching cycle helps

speed up transient response times. However, one drawback of

the high switching frequency is high gate charge and

crossover switching losses, which in turn impair converter

efficiency. Since the Si9165 features internal MOSFETs with

low gate charge, the efficiency penalty is minimal, even at a

2-MHz switching frequency.

With a switching frequency (Fsw) capability as high as 2 MHz,

the Si9165 allows use of small surface-mount inductors which

are essential for compact cellular phone designs. The

recommended inductance at a 2-MHz Fsw is 1.5 µH, which

offers a good balance between size, ripple current, and

efficiency. When a lower switching frequency is chosen,

higher inductance is required to match the efficiency and

ripple performance at 2 MHz. For instance, a 3-µH inductor is

preferred for a 1-MHz switching frequency. In PSM mode,

however, the operation is affected by inductance value but not

the switching frequency.

Input/Output Capacitor Selection

Low ESR (Effective Series Resistance) capacitors are

required on both the input and output to minimize voltage

ripple. The ESR of the output capacitor also changes the loop

stability, and it will be discussed later. At a 2-MHz Fsw, a 10-

µF surface-mount ceramic capacitor is recommended at the

output of the Si9165. A 10-µF ceramic or 22-µF low-ESR

tantalum capacitor is recommended as the input filtering

capacitor. Of course, the voltage rating on capacitor must not

be neglected.

a fixed ramp signal (see Figure 1), and the comparator output

is a controlled pulse width used to drive the switches. As the

switching duty cycle varies, the output voltage is regulated.

This single control loop needs to be compensated so that the

converter meets following specifications:

• Control loop stability margin

• Overshoot/undershoot at the output voltage induced by load

and line transients

• Settling time for overshoot/undershoot

The peak overshoot/undershoot voltage is determined by

closed-loop output impedance (ZO). The higher the output

impedance, the higher the peak. Although heavily dependent

on output capacitance and inductance, ZO is also closely

related to closed loop gain. With fixed power stage

components, a control loop with high bandwidth (BW) has low

ZO. Improving the compensation network is more cost-

effective than increasing the size of the output capacitor and

inductor. Fast settling times also rely on good loop design with

high BW. Adding capacitance at the output of the power

supply can reduce the peak deviation, but it can also produce

several unintended results, including low BW, long settling

times, reduced phase margin, and even system instability.

Diode Selection

To maximize converter efficiency, the use of an external

Schottky diode is strongly recommended over utilizing the

internal body diode of the MOSFET, which will typically have a

higher forward voltage drop by comparison. The Schottky

diode must be connected across the synchronous rectifying

switch. In PWM mode, it carries the inductor current flow

during BBM time; in PSM mode, this diode conducts all the

time during inductor discharge since the rectifier switch is

turned off during PSM. A low forward drop diode is preferable

for its efficiency advantages and fast recovery times, which

help reduce high-frequency noise.

FIGURE 4. Type I Compensation Network

Compensation Network

Voltage mode control is used in the Si9165 for both buck and

boost converter configurations. Output voltage is sensed and

fed back (pin 10, FB) to be compared with a reference voltage.

The difference is amplified by the internal error amplifier. Then

the output of the error amp (pin 11, COMP) is compared with

FIGURE 5. Type III Compensation Network

FaxBack 408-970-5600, request 70823

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