LTC1871 datasheet(14/36 Pages) LINER

14 / 36 page

LTC3787

3787fc

OPERATION

multiple LTC3787s can be configured for 2-, 3-, 4- , 6- and

12-phase operation.

Table 1.

VPHASMD

CONTROLLER 2 PHASE (°C)

CLKOUT PHASE (°C)

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

When VIN rises above the regulated VOUT voltage, the boost

controller can behave differently depending on the mode,

inductor current and VIN voltage. In forced continuous

mode, the loop works to keep the top MOSFET on con-

tinuously once VIN rises above VOUT. The internal charge

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 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 excessive 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 or a Schottky diode can also

be placed in parallel to 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).

Operation at Low SENSE Pin Common Mode Voltage

The current comparator in the LTC3787 is powered directly

from the SENSE+ pin. This enables the common mode

voltage of the SENSE+ and SENSE– pins to operate at as

low as 2.5V, which is below the UVLO threshold. The figure

on the first page shows a typical application in which the

controller’s VBIAS is powered from VOUT while the VIN

supply can go as low as 2.5V. If the voltage on SENSE+

drops below 2.5V, the SS pin will be held low. When the

SENSE voltage returns to the normal operating range, the

SS pin will be released, initiating a new soft-start cycle.

BOOST Supply Refresh and Internal Charge Pump

Each top MOSFET driver is biased from the floating

bootstrap capacitor, CB, which normally recharges during

each cycle through an external diode when the bottom

MOSFET turns on. There are two considerations for keep-

ing the BOOST supply at the required bias level. During

start-up, if the bottom MOSFET is not turned on within

100μs after UVLO goes low, the bottom MOSFET will be

forced to turn on for ~400ns. This forced refresh gener-

ates enough BOOST-SW voltage to allow the top MOSFET

ready to be fully enhanced instead of waiting for the initial

few cycles to charge up. There is also an internal charge

pump that keeps the required bias on BOOST. The charge

pump always operates in both forced continuous mode

and pulse-skipping mode. In Burst Mode operation, the

charge pump is turned off during sleep and enabled when

the chip wakes up. The internal charge pump can normally

supply a charging current of 55μA.

LTC1871 Datasheet(PDF) 14 Page - Linear Technology

LTC1871 Datasheet(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL