LTC3826-1 datasheet(10/32 Pages) LINER

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LTC3826-1

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OPERATION (Refer to Functional Diagram)

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

other internal circuitry is derived from the INTVCC pin.

When the EXTVCC pin is left open or tied to a voltage less

than 4.7V, an internal 5.25V low dropout linear regulator

supplies INTVCC power from VIN. If EXTVCC is taken above

4.7V, the 5.25V regulator is turned off and a 7.5V low

dropout linear regulator is enabled that supplies INTVCC

power from EXTVCC. If EXTVCC is less than 7.5V (but

greater than 4.7V), the 7.5V regulator is in dropout and

INTVCC is approximately equal to EXTVCC. When EXTVCC

is greater than 7.5V (up to an absolute maximum rating

of 10V), INTVCC is regulated to 7.5V. Using the EXTVCC

pin allows the INTVCC power to be derived from a high

efﬁciency external source such as one of the LTC3826-1

switching regulator outputs.

Each top MOSFET driver is biased from the ﬂoating boot-

strap capacitor CB, which normally recharges during each

off cycle through an external diode when the top MOSFET

turns off. If the input voltage VIN decreases to a voltage

close to VOUT, the loop may enter dropout and attempt

to turn on the top MOSFET continuously. The dropout

detector detects this and forces the top MOSFET off for

about one twelfth of the clock period every tenth cycle to

allow CB to recharge.

Shutdown and Start-Up (RUN1, RUN2 and TRACK/

SS1, TRACK/SS2 Pins)

The two channels of the LTC3826-1 can be independently

shut down using the RUN1 and RUN2 pins. Pulling either

of these pins below 0.7V shuts down the main control

loop for that controller. Pulling both pins low disables

both controllers and most internal circuits, including the

INTVCC regulator, and the LTC3826-1 draws only 4μA of

quiescent current.

Releasing either RUN pin allows an internal 0.5μA current

to pull up the pin and enable that controller. Alternatively,

the RUN pin may be externally pulled up or driven directly

by logic. Be careful not to exceed the Absolute Maximum

rating of 7V on this pin.

The start-up of each controller’s output voltage VOUTiscon-

trolled by the voltage on the TRACK/SS1 and TRACK/SS2

pin. When the voltage on the TRACK/SS pin is less than

the 0.8V internal reference, the LTC3826-1 regulates the

VFB voltage to the TRACK/SS pin voltage instead of the

0.8V reference. This allows the TRACK/SS pin to be used

to program a soft start by connecting an external capacitor

from the TRACK/SS pin to SGND. An internal 1μA pull-up

current charges this capacitor creating a voltage ramp on

the TRACK/SS pin. As the TRACK/SS voltage rises linearly

from 0V to 0.8V (and beyond), the output voltage VOUT

rises smoothly from zero to its ﬁnal value.

Alternatively the TRACK/SS pin can be used to cause the

startup of VOUT to “track” that of another supply. Typically,

this requires connecting to the TRACK/SS pin an external

resistor divider from the other supply to ground (see

Applications Information section).

When the corresponding RUN pin is pulled low to disable

a controller, or when VIN drops below its undervoltage

lockout threshold of 3.5V, the TRACK/SS pin is pulled low

by an internal MOSFET. When in undervoltage lockout,

both controllers are disabled and the external MOSFETs

are held off.

Light Load Current Operation (Burst Mode Operation,

Pulse Skipping, or Continuous Conduction)

(PLLIN/MODE Pin)

The LTC3826-1 can be enabled to enter high efﬁciency

Burst Mode operation, constant frequency pulse skipping

mode, or forced continuous conduction mode at low load

currents. To select Burst Mode operation, tie the PLLIN/

MODE pin to a DC voltage below 0.7V (e.g., SGND). To

select forced continuous operation, tie the PLLIN/MODE

pin to INTVCC. To select pulse-skipping mode, tie the

PLLIN/MODE pin to a DC voltage greater than 0.9V and

less than INTVCC – 1.2V.

When a controller is enabled for Burst Mode operation,

the peak current in the inductor is set to approximately

one-tenth of the maximum sense voltage even though the

voltage on the ITH pin indicates a lower value. If the aver-

age inductor current is lower than the load current, the

error ampliﬁer EA will decrease the voltage on the ITH pin.

When the ITH voltage drops below 0.4V, the internal sleep

signal goes high (enabling “sleep” mode) and both external

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