LTC3855_15 Datasheet, PDF(25/44 Page) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

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LTC3855_15 Datasheet, PDF (25/44 Pages) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

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LTC3855

Applications Information

due to the dropout voltage. Make sure the INTVCC voltage

is at or exceeds the RDS(ON) test voltage for the MOSFET

which is typically 4.5V for logic level devices.

LTC3855 VIN

INTVCC

RVIN

CINTVCC

1â¦

4.7ÂµF

CIN

3855 F07

Figure 10. Setup for a 5V Input

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors CB connected to the BOOST

pins supply the gate drive voltages for the topside MOSFETs.

Capacitor CB in the Functional Diagram is charged though

external diode DB from INTVCC when the SW pin is low.

When one of the topside MOSFETs is to be turned on,

the driver places the CB voltage across the gate source

of the desired MOSFET. This enhances the MOSFET and

turns on the topside switch. The switch node voltage, SW,

rises to VIN and the BOOST pin follows. With the topside

MOSFET on, the boost voltage is above the input supply:

VBOOST = VIN + VINTVCC. The value of the boost capacitor

CB needs to be 100 times that of the total input capa-

citance of the topside MOSFET(s). The reverse break-

down of the external Schottky diode must be greater

than VIN(MAX). When adjusting the gate drive level, the

final arbiter is the total input current for the regulator. If

a change is made and the input current decreases, then

the efficiency has improved. If there is no change in input

current, then there is no change in efficiency.

Undervoltage Lockout

The LTC3855 has two functions that help protect the

controller in case of undervoltage conditions. A precision

UVLO comparator constantly monitors the INTVCC voltage

to ensure that an adequate gate-drive voltage is present. It

locks out the switching action when INTVCC is below 3.2V.

To prevent oscillation when there is a disturbance on the

INTVCC, the UVLO comparator has 600mV of precision

hysteresis.

Another way to detect an undervoltage condition is to

monitor the VIN supply. Because the RUN pins have a

precision turn-on reference of 1.2V, one can use a resistor

divider to VIN to turn on the IC when VIN is high enough.

An extra 4.5ÂµA of current flows out of the RUN pin once

the RUN pin voltage passes 1.2V. One can program the

hysteresis of the run comparator by adjusting the values

of the resistive divider. For accurate VIN undervoltage

detection, VIN needs to be higher than 4.5V.

CIN and COUT Selection

The selection of CIN is simplified by the 2-phase architec-

ture and its impact on the worst-case RMS current drawn

through the input network (battery/fuse/capacitor). It can be

shown that the worst-case capacitor RMS current occurs

when only one controller is operating. The controller with

the highest (VOUT)(IOUT) product needs to be used in the

formula below to determine the maximum RMS capacitor

current requirement. Increasing the output current drawn

from the other controller will actually decrease the input

RMS ripple current from its maximum value. The out-of-

phase technique typically reduces the input capacitorâs RMS

ripple current by a factor of 30% to 70% when compared

to a single phase power supply solution.

In continuous mode, the source current of the top MOSFET

is a square wave of duty cycle (VOUT)/(VIN). To prevent

large voltage transients, a low ESR capacitor sized for the

maximum RMS current of one channel must be used. The

maximum RMS capacitor current is given by:

( )( ) CIN

quired

IRMS

IMAX

VIN

ï£®ï£°

VOUT

VIN â VOUT ï£¹ï£»1/2

This formula has a maximum at VIN = 2VOUT, where IRMS =

IOUT/2. This simple worst-case condition is commonly used

for design because even significant deviations do not of-

fer much relief. Note that capacitor manufacturersâ ripple

current ratings 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 be paralleled to meet

size or height requirements in the design. Due to the high

operating frequency of the LTC3855, ceramic capacitors

3855f

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