LT8705 datasheet(13/38 Pages) LINER

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LTC3784

3784fb

For more information www.linear.com/LTC3784

operaTion

the PLL loop only needs to make slight changes to the

VCO input in order to synchronize the rising edge of the

external clock’s to the rising edge of BG1. The ability to

prebias the loop filter allows the PLL to lock-in rapidly

without deviating far from the desired frequency.

The typical capture range of the LTC3784’s PLL is from

approximately 55kHz to 1MHz, and is guaranteed to lock

to an external clock source whose frequency is between

75kHz and 850kHz.

The typical input clock thresholds on the PLLIN/MODE

pin are 1.6V (rising) and 1.2V (falling). The recommended

maximumamplitudeforlowlevelandminimumamplitude

forhighlevelofexternalclockare0Vand2.5V,respectively.

PolyPhase Applications (CLKOUT and PHASMD Pins)

The LTC3784 features two pins, CLKOUT and PHASMD,

that allow other controller ICs to be daisy-chained with

the LTC3784 in PolyPhase applications. The clock output

signal on the CLKOUT pin can be used to synchronize

additional power stages in a multiphase power supply

solution feeding a single, high current output or multiple

separate outputs. The PHASMD pin is used to adjust the

phase of the CLKOUT signal as well as the relative phases

between the two internal controllers, as summarized in

Table 1. The phases are calculated relative to the zero

degrees phase being defined as the rising edge of the

bottom gate driver output of controller 1 (BG1). Depend-

ing on the phase selection, a PolyPhase application with

multiple LTC3784s can be configured for 2-, 3-, 4- , 6- and

12-phase operation.

Table 1.

VPHASMD

CONTROLLER 2 PHASE (°)

CLKOUT PHASE (°)

GND

180

Floating

180

INTVCC

240

120

CLKOUT is disabled when the controller is in shutdown

or in sleep mode.

Operation When VIN > Regulated VOUT

WhenVINrisesabovetheregulatedVOUTvoltage,theboost

controller can behave differently depending on the mode,

inductor current and VIN voltage. In forced continuous

mode, the control loop works to keep the top MOSFET on

continuouslyonceVINrisesaboveVOUT.Theinternalcharge

pump delivers current to the boost capacitor to maintain

a sufficiently high TG voltage. The amount of current the

charge pump can deliver is characterized by two curves

in the Typical Performance Characteristics section.

In pulse-skipping mode, if VIN is between 100% and

110% of the regulated VOUT voltage, TG turns on if the

inductor current rises above a certain threshold and turns

off if the inductor current falls below this threshold. This

threshold current is set to approximately 6%, 4% or

3% of the maximum ILIM current when the ILIM pin is

grounded, floating or tied to INTVCC, respectively. If the

controller is programmed to Burst Mode operation under

this same VIN window, then TG remains off regardless of

the inductor current.

If the OVMODE pin is grounded and VIN rises above 110%

of the regulated VOUT voltage in any mode, the controller

turns on TG regardless of the inductor current. In Burst

Mode operation, however, the internal charge pump turns

off if the chip is asleep. With the charge pump off, there

would be nothing to prevent the boost capacitor from

discharging, resulting in an insufficient TG voltage needed

to keep the top MOSFET completely on. To prevent exces-

sive power dissipation across the body diode of the top

MOSFET in this situation, the chip can be switched over

to forced continuous mode to enable the charge pump;

a Schottky diode can also be placed in parallel with the

top MOSFET.

Power Good

The PGOOD pin is connected to an open drain of an

internal N-channel MOSFET. The MOSFET turns on and

pulls the PGOOD pin low when the VFB pin voltage is not

within ±10% of the 1.2V reference voltage. The PGOOD

pin is also pulled low when the corresponding RUN pin

is low (shut down). When the VFB pin voltage is within

the ±10% requirement, the MOSFET is turned off and the

pin is allowed to be pulled up by an external resistor to a

source of up to 6V (abs max).

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