KB3440 datasheet(13/20 Pages) KINGBOR

13 / 20 page

Output Voltage > 4.3V

A Schottky diode from SW to VOUT is required for output

voltages over 4.3V. The diode must be located as close to

the pins as possible in order to reduce the peak voltage on

SW2 due to the parasitic lead and trace inductance.

Input Voltage > 4.5V

For applications with input voltages above 4.5V which

could exhibit an overload or short-circuit condition, a 2 /

1nF series snubber is required between the SW1 pin and

GND. A Schottky diode such as the Phillips PMEG2010EA

or equivalent from SW1 to VIN should also be added as

close to the pins as possible. For the higher input voltages

VIN bypassing becomes more critical, therefore, a ceramic

bypass capacitor as close to the VIN and GND pins as

possible is also required.

Operating Frequency Selection

There are several considerations in selecting the operating

frequency of the converter. The first is, what are the

sensitive frequency bands that cannot tolerate any spec-

tral noise? For example, in products incorporating RF

communications, the 455kHz IF frequency is sensitive to

any noise, therefore switching above 600kHz is desired.

Some communications have sensitivity to 1.1MHz and in

that case a 2MHz converter frequency may be employed.

Other considerations are the physical size of the converter

and efficiency. As the operating frequency goes up, the

inductor and filter capacitors go down in value and size.

The trade off is in efficiency since the switching losses due

to gate charge are going up proportional with frequency.

Additional quiescent current due to the output switches

GATE charge is given by:

Buck: 500e–12 • VIN • F

Boost: 250e–12 • (VIN + VOUT) • F

Buck/Boost: F • (750e–12 • VIN + 250e–12 • VOUT)

where F = switching frequency

Closing the Feedback Loop

The kB3440 incorporates voltage mode PWM control.

The control to output gain varies with operation region

(Buck, Boost, Buck-Boost), but is usually no greater than

15. The output filter exhibits a double pole response is

given by:

Hz in Buck

FILTER POLE

OUT

••

•

mod

()

Hz in Boost

FILTER POLE

OUT

••

mod

()

where COUT is the output filter capacitor.

The output filter zero is given by:

FILTER ZERO

ESR

OUT

••

•

where RESR is the capacitor equivalent series resistance.

A troublesome feature in Boost mode is the right-half

plane zero (RHP), and is given by:

IL V

RHPZ

OUT

2• •

• •

The loop gain is typically rolled off before the RHP zero

frequency.

A simple Type I compensation network can be incorpo-

rated to stabilize the loop but at a cost of reduced band-

width and slower transient response. To ensure proper

phase margin, the loop requires to be crossed over a

decade before the LC double pole.

The unity-gain frequency of the error amplifier with the

Type I compensation is given by:

RCP

••

•

Most applications demand an improved transient response

to allow a smaller output filter capacitor. To achieve a

higher bandwidth, Type III compensation is required. Two

zeros are required to compensate for the double-pole

response.

kB3440

Kingbor Technology Co.,Ltd

TEL:(86)0755-26508846 FAX:(86)0755-26509052

APPLICATIONS INFORMATION

KB3440 датащи(PDF) 13 Page - Kingbor Technology Co

KB3440 датащи(HTML) 13 Page - Kingbor Technology Co

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

датащи скачать

ссылки URL