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LTC3853

3853f

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 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 INTVCC voltage

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 moni-

tor 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 then 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

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 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.

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:

quired I

RMS

MAX

OUT

–

≈

()(

)

⎡⎣

⎤⎦⎦

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

offer much relief. Note that capacitor manufacturers’ 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

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