LTC3890 datasheet(20/38 Pages) LINER

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LTC3784

3784fb

For more information www.linear.com/LTC3784

applicaTions inForMaTion

Multiple capacitors placed in parallel may be needed to

meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic and

ceramic capacitors are all available in surface mount

packages. Ceramic capacitors have excellent low ESR

characteristics but can have a high voltage coefficient.

Capacitors are now available with low ESR and high ripple

current ratings (e.g., OS-CON and POSCAP).

PolyPhase Operation

For output loads that demand high current, multiple

LTC3784s can be cascaded to run out-of-phase to provide

more output current and at the same time to reduce input

and output voltage ripple. The PLLIN/MODE pin allows the

LTC3784 to synchronize to the CLKOUT signal of another

LTC3784. The CLKOUT signal can be connected to the

PLLIN/MODE pin of the following LTC3784 stage to line

up both the frequency and the phase of the entire system.

Tying the PHASMD pin to INTVCC, SGND or floating

generates a phase difference (between PLLIN/MODE

and CLKOUT) of 240°, 60° or 90°, respectively, and a

phase difference (between CH1 and CH2) of 120°, 180°

or 180°. Figure 4 shows the connections necessary for

3-, 4-, 6- or 12-phase operation. A total of 12 phases can

be cascaded to run simultaneously out-of-phase with

respect to each other.

Setting Output Voltage

The LTC3784 output voltage is set by an external feedback

resistordividercarefullyplacedacrosstheoutput,asshown

in Figure 5. The regulated output voltage is determined by:

VOUT =1.2V 1+

⎛

⎝⎜

⎞

⎠⎟

Great care should be taken to route the VFB line away

from noise sources, such as the inductor or the SW line.

Also place the feedback resistor divider close to the VFB

pin and keep the VFB node as small as possible to avoid

noise pickup.

Soft-Start (SS Pin)

The start-up of VOUT is controlled by the voltage on the

SS pin. When the voltage on the SS pin is less than the

internal 1.2V reference, the LTC3784 regulates the VFB

pin voltage to the voltage on the SS pin instead of 1.2V.

Soft-startisenabledbysimplyconnectingacapacitorfrom

the SS pin to ground, as shown in Figure 6. An internal

10μA current source charges the capacitor, providing a

linear ramping voltage at the SS pin. The LTC3784 will

regulate the VFB pin (and hence, VOUT) according to the

voltage on the SS pin, allowing VOUT to rise smoothly

from VIN to its final regulated value. The total soft-start

time will be approximately:

tSS =CSS •

1.2V

10µA

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