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

部品番号

コンポーネント説明

メーカー

AN723

Design A High Performance Buck or Boost Converter With Si9165

Vishay Semiconductors 

AN723

Vishay Siliconix

Start-Up/UVLO

The internal under voltage lock out (UVLO) circuit keeps most

of the IC function blocks off until the supply voltage (VDD)

increases above 2.4 V. A 100-mV hysteresis is built into the

UVLO point, so the controller will be functional until VDD drops

below 2.3 V. This helps to eliminate the IC from bouncing

between ON and OFF stages. After the IC is turned ON, it

takes about 4 ms for the POR to be ready, the error amp

output to charge up, and the output voltage to start ramping

up. The output voltage will need an additional 3 to 4 ms to

reach regulation, depending on load condition.

By-Pass Mode

When using the Si9165, the output voltage regulation point

can be set within the input voltage range, regardless of

whether a buck or boost configuration is being used. For

instance, for an input range of 2.7 V to 4.2 V, the output

voltage could be set to 3.3 V.

For a boost converter, when the input is higher than 3.3 V, the

duty cycle of the switch stays at 0%, and the output voltage

follows the input voltage by a voltage drop consisting of

inductor resistance and MOSFET (in PWM mode) or diode (in

PSM mode) drop. When the input decreases and approaches

3.3 V, the output drops to the regulation point, and the main

switch starts to switch at a minimum duty cycle to keep the

output regulated at 3.3 V. This duty cycle increases as the

input voltage decreases. In some instances, noise can be

generated during the transition because there is a minimum

controllable duty cycle for any PWM controller. The frequency

and amplitude of this transition noise vary depending on the

compensation network. The wider the loop bandwidth (BW),

the higher the switching frequency and the lower the output

ripple.

For a buck converter, when input voltage is higher than 3.3 V,

it is stepped down to 3.3 V at the output. As the input

decreases and approaches 3.3 V, the switching duty cycle

increases to the maximum duty cycle, jumping to 100% and

making the high-side switch work like a saturated linear

regulator. The output voltage will simply follow the input

voltage by the saturation voltage until the input drops below

the UVLO voltage or until another user-defined control signal

disables the converter. The same noise considerations as for

a boost converter apply in this case.

Buck/Boost Configuration

The Si9165 can be easily configured to function as a step-

down (buck) or a step-up (boost) converter. Figures 2 and 3

show the typical application circuit for buck and boost

converters, respectively. The list in Table 1 shows the key IC

connection differences in the two topologies.

TABLE 1. Buck-Boost Pin Connection Comparison

Name of Pin

Buck

Boost

VIN/VOUT

MODE

VS

Input

Low

Input

Output

High

Output

FIGURE 2. Typical Application Circuit-Buck

FaxBack 408-970-5600, request 70823

2

www.siliconix.com

Share Link: