LTC3853_15 datasheet(21/36 Pages) LINER

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LTC3853

3853fc

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Topside MOSFET Driver Supply (CB, DB)

Externalbootstrapcapacitors,CB,connectedtotheBOOST

pins supply the gate drive voltages for the topside MOS-

FETs. 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 VINandtheBOOSTpinfollows.

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 capacitance of the topside MOSFET(s).

The reverse breakdown 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 LTC3853 has two functions that help protect the

controller in case of undervoltage conditions. A precision

UVLO comparator constantly monitors the INTVCCvoltage

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

It locks out the switching action when INTVCC is below

3.35V. To prevent oscillation when there is a disturbance

on the INTVCC, the UVLO comparator has 500mV of preci-

sion 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 4.5µA 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 using the RUN pin, VIN needs

to be higher than 4V.

CIN and COUT Selection

The selection of CIN is simplified by the 3-phase architec-

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

throughtheinputnetwork(battery/fuse/capacitor).Itcanbe

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 controllers will actually decrease the input

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

phase technique typically reduces the input capacitor’s

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

compared to a single phase power supply solution.

Incontinuousmode,thesourcecurrentofthetopMOSFET

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 Required IRMS ≈

IMAX

VIN

VOUT

(

) VIN – VOUT

(

)





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

offermuchrelief.Notethatcapacitormanufacturers’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 LTC3853, ceramic capacitors

can also be used for CIN. Always consult the manufacturer

if there is any question.

The benefit of the LTC3853 3-phase operation can be cal-

culated by using the equation above for the higher power

controller and then calculating the loss that would have

resulted if all controller channels switched on at the same

time. The total RMS power lost is lower when more than

one controller is operating due to the reduced overlap of

current pulses required through the input capacitor’s ESR.

This is why the input capacitor’s requirement calculated

above for the worst-case controller is adequate for the

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