LTC3815 datasheet(20/42 Pages) LINER

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LTC3815

3815fa

For more information www.linear.com/LTC3815

APPLICATIONS INFORMATION

ThebasicLTC3815applicationcircuitisshowninFigure8.

Operating Frequency

Selection of the operating frequency is a trade-off between

efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values.

Operation at lower frequencies improves efficiency by

reducing internal gate charge losses but requires larger

inductance values and/or capacitance to maintain low out-

putripplevoltage.TheoperatingfrequencyoftheLTC3815

is determined by an external resistor that is connected

between the RT pin and ground. The value of the resistor

sets the ramp current that is used to charge and discharge

an internal timing capacitor within the oscillator and can

be calculated by using the following equation:

RT =

1.15 • 1011

fOSC

( )1.11

Ω

Frequencies as high as 3MHz are possible, as long as the

minimum on-time requirement is met (see next section).

Tying the RT pin to VIN sets the default internal operating

frequency to 1MHz ±15%.

TheLTC3815’sinternaloscillatorcanbesynchronizedtoan

external frequency by applying a square wave clock signal

to the MODE/SYNC pin. During synchronization, the top

switch turn-on is locked to the rising edge of the external

frequency source. The synchronization frequency range is

300kHz to 3MHz. During synchronization, discontinuous

operation is disabled.

The internal PLL has a synchronization range of ±30%

around its programmed frequency. Therefore, during

external clock synchronization be sure that the external

clock frequency is within this ±30% range of the RT pro-

grammed frequency.

WhenusingtheRTpintoprogramtheoscillatorfrequency,

asquarewaveclockthatisrunning180°out-of-phasewith

the internal oscillator is available at the CLKOUT pin for

connection to a second LTC3815 for 2-phase operation

(see the 2-Phase section).

Figure 8. 1.8V, 6A Step-Down Regulator

VIN

2.25V TO 5.5V

VOUT

1.8V

PGOOD

LTC3815

MODE/SYNC

RUN_MSTR

RUN_STBY

CLKOUT

PGLIM

MARGIN

VCC_SENSE

VSS_SENSE

REF

ASEL

PGFD

DAOUT

VFB

ITH

SCL, SDA, ALERT

PVIN

VIN

PMBUS

CSS

15nF

CSLEW

1nF

11.8k, 1%

RASEL

RPGFD

3815 F08

300nH

RC2

10k

RC1

CC2

1nF

COUT

100µF

×2

CC1

47pF

CIN

22µF

×4

SS/TRACK

CSLEW

RREF1

14.3k

0.5%

RREF2

3.7k

0.5%

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