LTC3860 datasheet(13/36 Pages) LINER

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LTC3860

3860f

Alternatively, the TRACK/SS pin can be used to force the

start-up of VOUT to track the voltage of another supply.

Typically this requires connecting the TRACK/SS pin to an

external divider from the other supply to ground (see Ap-

plications Information). It is only possible to track another

supply that is slower than the internal soft-start ramp.

The TRACK/SS pin also has an internal open-drain NMOS

pull-down transistor that turns on to reset the TRACK/SS

voltage when the channel is shut down (RUN = 0V or VCC

< UVLO threshold) or during an OC fault condition.

In multiphase operation, one master error amplifier is

used to control all of the PWM comparators. The FB pins

for the unused error amplifiers are connected to VCC in

ordertothree-statetheseamplifieroutputs,andtheCOMP

pins are connected together. The TRACK/SS pins should

also be connected together so that the slave phases can

detect when soft-start is complete and enable the bottom

MOSFET.

Frequency Selection and the Phase-Locked Loop (PLL)

The selection of the switching frequency is a tradeoff

between efficiency, transient response and component

size. High frequency operation reduces the size of the

inductor and output capacitor as well as increasing the

maximum practical control loop bandwidth. However,

efficiency is generally lower due to increased transition

and switching losses.

The LTC3860’s switching frequency can be set in three

ways: using an external resistor to linearly program the

frequency, synchronizing to an external clock, or simply

selecting one of two fixed frequencies (400kHz and

600kHz). Table 1 highlights these modes.

Table 1. Frequency Selection

CLKIN PIN

FREQ PIN

FREQUENCY

Clocked

RFREQ to GND

250kHz to 1.25MHz

High

RFREQ to GND

250kHz to 1.25MHz

Low

400kHz

Low

High

600kHz

No external PLL filter is required to synchronize the

LTC3860 to an external clock. Applying an external clock

signal to the CLKIN pin will automatically enable the PLL

with internal filter.

Constant frequency operation brings with it a number of

benefits: inductor and capacitor values can be chosen for

a precise operating frequency and the feedback loop can

besimilarlytightlyspecified.Noisegeneratedbythecircuit

will always be at known frequencies. Subharmonic oscil-

lation and slope compensation, common headaches with

constant frequency current mode switchers, are absent in

voltage mode designs like the LTC3860.

Using the CLKOUT and PHSMD Pins in

Multiphase Applications

The LTC3860 features CLKOUT and PHSMD pins that al-

low multiple LTC3860 ICs to be daisy-chained together in

multiphase applications. The clock output signal on the

CLKOUT pin can be used to synchronize additional ICs in

a 3-, 4-, 6- or 12-phase power supply solution feeding a

single high current output, or even several outputs from

the same input supply.

The PHSMD pin is used to adjust the phase relationship

between channel 1 and channel 2, as well as the phase

relationship between channel 1 and CLKOUT, as sum-

marized in Table 2. The phases are calculated relative to

zero degrees, defined as the rising edge of PWM1. Refer

to Applications Information for more details on how to

create multiphase applications.

Table 2. Phase Selection

PHSMD PIN

CH-1 to CH-2 PHASE

CH-1 to CLKOUT PHASE

Float

180°

90°

Low

180°

60°

High

120°

240°

operaTion (Refer to Functional Diagram)

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