LTC3814-5_15 Datasheet, PDF(16/30 Page) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LTC3814-5_15 Datasheet, PDF (16/30 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

◁

LTC3814-5

APPLICATIONS INFORMATION

For many designs it is possible to choose a single capacitor

type that satisï¬es both the ESR and bulk C requirements

for the design. In certain demanding applications, however,

the ripple voltage can be improved signiï¬cantly by con-

necting two or more types of capacitors in parallel. For

example, using a low ESR ceramic capacitor can minimize

the ESR step, while an electrolytic capacitor can be used

to supply the required bulk C.

Once the output capacitor ESR and bulk capacitance

have been determined, the overall ripple voltage wave-

form should be veriï¬ed on a dedicated PC board (see PC

Board Layout Checklist section for more information on

component placement). Lab breadboards generally suffer

from excessive series inductance (due to inter-component

wiring), and these parasitics can make the switching

waveforms look signiï¬cantly worse than they would be

on a properly designed PC board.

The output capacitor in a boost regulator experiences high

RMS ripple currents, as shown in Figure 8d. The RMS

output capacitor ripple current is:

IRMS(COUT) IO(MAX) â¢

VO â VIN(MIN)

VIN(MIN)

Note that the ripple current ratings from capacitor manu-

facturers are often based on only 2000 hours of life. This

makes it advisable to further derate the capacitor or to

choose a capacitor rated at a higher temperature than

required. Several capacitors may also be placed in parallel

to meet size or height requirements in the design.

Manufacturers such as Nichicon, Nippon Chemi-con

and Sanyo should be considered for high performance

throughhole capacitors. The OS-CON (organic semicon-

ductor dielectric) capacitor available from Sanyo has the

lowest product of ESR and size of any aluminum electrolytic

at a somewhat higher price. An additional ceramic capaci-

tor in parallel with OS-CON capacitors is recommended

to reduce the effect of their lead inductance.

In surface mount applications, multiple capacitors placed

in parallel may be required to meet the ESR, RMS current

handling and load step requirements. Dry tantalum, special

polymer and aluminum electrolytic capacitors are available

in surface mount packages. Special polymer capacitors

offer very low ESR but have lower capacitance density

than other types. Tantalum capacitors have the highest

capacitance density but it is important to only use types

that have been surge tested for use in switching power

supplies. Several excellent surge-tested choices are the

AVX TPS and TPSV or the KEMET T510 series. Aluminum

electrolytic capacitors have signiï¬cantly higher ESR, but

can be used in cost-driven applications providing that

consideration is given to ripple current ratings and long

term reliability. Other capacitor types include Panasonic

SP and Sanyo POSCAPs. In applications with VOUT > 30V,

however, choices are limited to aluminum electrolytic and

ceramic capacitors.

VIN

VOUT

COUT RL

8a. Circuit Diagram

IIN

8b. Inductor and Input Currents

ISW

tON

8c. Switch Current

tOFF

8d. Diode and Output Currents

VOUT

(AC)

ÎVCOUT

ÎVESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

8e. Output Voltage Ripple Waveform 38145 F08

Figure 8. Switching Waveforms for a Boost Converter

38145fc

▷