LTC1876_15 Datasheet, PDF(18/36 Page) Linear Technology – High Efficiency, 2-Phase, Dual Synchronous Step-Down Switching Controller and Step-Up Regulator

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LTC1876_15 Datasheet, PDF (18/36 Pages) Linear Technology – High Efficiency, 2-Phase, Dual Synchronous Step-Down Switching Controller and Step-Up Regulator

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LTC1876

APPLICATIO S I FOR ATIO

discharge during the operating frequency period due to

ripple current. The choice of using smaller output capaci-

tance increases the ripple voltage due to the discharging

term but can be compensated for by using capacitors of

very low ESR to maintain the ripple voltage at or below

50mV. The ITH pin OPTI-LOOP compensation compo-

nents can be optimized to provide stable, high perfor-

mance transient response regardless of the output

capacitors selected.

For the boost regulator, the output ripple (âVOUT) is

determined by:

âVOUT

IPKESR +

ï£« 1.5IOUT

ï£ï£¬ f COUT

ï£¶

ï£¸ï£·

Since the boost regulator is operating at high frequency,

the second term will be small even with a small value of

COUT. Hence, all efforts can be concentrated on finding a

low ESR capacitor. A ceramic capacitor can be used for the

output capacitor.

Manufacturers such as Nichicon, United Chemicon and

Sanyo can be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest (ESR) (size)

product of any aluminum electrolytic at a somewhat

higher price. An additional ceramic capacitor in parallel

with OS-CON capacitors is recommended to reduce the

inductance effects.

In surface mount applications multiple capacitors may

need to be used in parallel to meet the ESR, RMS current

handling and load step requirements of the application.

Aluminum electrolytic, dry tantalum and special polymer

capacitors are available in surface mount packages. Spe-

cial polymer surface mount capacitors offer very low ESR

but have lower storage capacity per unit volume than other

capacitor types. These capacitors offer a very cost-effec-

tive output capacitor solution and are an ideal choice when

combined with a controller having high loop bandwidth.

Tantalum capacitors offer the highest capacitance density

and are often used as output capacitors for switching

regulators having controlled soft-start. Several excellent

surge-tested choices are the AVX TPS, AVX TPSV or the

KEMET T510 series of surface mount tantalums, available

in case heights ranging from 2mm to 4mm. Aluminum

electrolytic capacitors can be used in cost-driven applica-

tions providing that consideration is given to ripple current

ratings, temperature and long term reliability. A typical

application will require several to many aluminum electro-

lytic capacitors in parallel. A combination of the above

mentioned capacitors will often result in maximizing per-

formance and minimizing overall cost. Other capacitor

types include Nichicon PL series, NEC Neocap, Pansonic

SP and Sprague 595D series. For high value of ceramic

capacitors, Taiyo Yuden has a series of them. Select the

X5R or X7R series as these retain the capacitance over

wide voltage and temperature range. Consult manufactur-

ers for other specific recommendations.

INTVCC Regulator

An internal P-channel low dropout regulator produces 5V

at the INTVCC pin from the VIN supply pin. INTVCC powers

the drivers and internal circuitry within the LTC1876 step-

down controllers. The INTVCC pin regulator can supply a

peak current of 50mA and must be bypassed to ground

with a minimum of 4.7ÂµF tantalum, 10ÂµF special polymer,

or low ESR type electrolytic capacitor. A 1ÂµF ceramic

capacitor placed directly adjacent to the INTVCC and PGND

IC pins is highly recommended. Good bypassing is neces-

sary to supply the high transient currents required by the

MOSFET gate drivers and to prevent interaction between

channels.

Higher input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the maxi-

mum junction temperature rating for the LTC1876 to be

exceeded. The system supply current is normally domi-

nated by the gate charge current. Additional external

loading of the INTVCC and 3.3V linear regulators also

needs to be taken into account for the power dissipation

calculations. The total INTVCC current can be supplied by

either the 5V internal linear regulator or by the EXTVCC

input pin. When the voltage applied to the EXTVCC pin is

less than 4.7V, all of the INTVCC current is supplied by the

internal 5V linear regulator. Power dissipation for the IC in

this case is highest: (VIN)(IINTVCC), and overall efficiency

is lowered. The gate charge current is dependent on

1876fa

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