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LTC3853

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APPLICATIONS INFORMATION

TheoptionalSchottkydiodesconductduringthedeadtime

betweentheconductionofthetwopowerMOSFETs.These

preventthebodydiodesofthebottomMOSFETsfromturn-

ing on, storing charge during the dead time and requiring

a reverse recovery period that could cost as much as 3%

in efficiency at high VIN. A 1A to 3A Schottky is generally

a good compromise for both regions of operation due to

the relatively small average current. Larger diodes result

in additional transition losses due to their larger junction

capacitance.

Soft-Start and Tracking

The LTC3853 has the ability to either soft-start by itself

with a capacitor or track the output of another channel or

externalsupply.Whenoneparticularchannelisconfigured

to soft-start by itself, a capacitor should be connected to

its TK/SS pin. This channel is in the shutdown state if its

RUN pin voltage is below 1.2V. Its TK/SS pin is actively

pulled to ground in this shutdown state.

Once the RUN pin voltage is above 1.2V, the channel pow-

ers up. A soft-start current of 1.3µA then starts to charge

its soft-start capacitor. Note that soft-start or tracking is

achieved not by limiting the maximum output current of

the controller but by controlling the output ramp voltage

according to the ramp rate on the TK/SS pin. Current

foldback is disabled during this phase to ensure smooth

soft-start or tracking. The soft-start or tracking range is

defined to be the voltage range from 0V to 0.8V on the

TK/SS pin. The total soft-start time can be calculated as:

tSOFTSTART = 0.8 •

CSS

1.3µA

Regardless of the mode selected by the MODE/PLLIN pin,

the regulator will always start in pulse skipping mode up

to TK/SS = 0.64V. Between TK/SS = 0.64V and 0.74V, it

will operate in forced continuous mode and revert to the

selected mode once TK/SS > 0.74V. The output ripple is

minimized during the 100mV forced continuous mode

window ensuring a clean PGOOD signal.

When the channel is configured to track another supply,

the feedback voltage of the other supply is duplicated by

a resistor divider and applied to the TK/SS pin. Therefore,

the voltage ramp rate on this pin is determined by the

ramp rate of the other supply’s voltage. Note that the

small soft-start capacitor charging current is always

flowing, producing a small offset error. To minimize this

error, select the tracking resistive divider value to be small

enough to make this error negligible.

In order to track down another channel or supply after

the soft-start phase expires, the LTC3853 is forced into

continuous mode of operation as soon as VFB is below the

undervoltage threshold of 0.74V regardless of the setting

of the MODE/PLLIN pin. However, the LTC3853 should

always be set in force continuous mode tracking down

when there is no load. After TK/SS drops below 0.1V, its

channel will operate in discontinuous mode.

Output Voltage Tracking

The LTC3853 allows the user to program how its out-

put ramps up and down by means of the TK/SS pins.

Through these pins, the output can be set up to ei-

ther coincidentally or ratiometrically track another

supply’s output, as shown in Figure 5. In the following

discussions, VOUT1 refers to the LTC3853’s output 1 as a

masterchannelandVOUT2referstotheLTC3853’soutput 2

as a slave channel. In practice though, any phase can be

used as the master. To implement the coincident track-

ing in Figure 5a, connect an additional resistive divider to

VOUT1 and connect its midpoint to the TK/SS pin of the

slave channel. The ratio of this divider should be the same

as that of the slave channel’s feedback divider shown in

Figure 6a. In this tracking mode, VOUT1 must be set higher

thanVOUT2.Toimplementtheratiometrictracking,theratio

of the slave’s divider should be exactly the same as the

master channel’s feedback divider. By selecting different

resistors, the LTC3853 can achieve different modes of

tracking including the two in Figure 5.

So which mode should be programmed? While either

modeinFigure6satisfiesmostpracticalapplications,there

are some trade-offs. The ratiometric mode saves a pair

of resistors, but the coincident mode offers better output

regulation. This can be better understood with the help

of Figure 7. At the input stage of the slave channel’s error

amplifier, two common anode diodes are used to clamp

the equivalent reference voltage and an additional diode

is used to match the shifted common mode voltage. The

top two current sources are of the same amplitude. In the

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