LTC3836_15 Datasheet, PDF(10/30 Page) Linear Technology – Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller

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LTC3836_15 Datasheet, PDF (10/30 Pages) Linear Technology – Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller

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LTC3836

OPERATION (Refer to Functional Diagram)

Main Control Loop

The LTC3836 uses a constant-frequency, current mode

architecture with the two controllers operating 180 degrees

out-of-phase. During normal operation, the top external

power MOSFET is turned on when the clock for its chan-

nel sets the RS latch, and turned off when the current

comparator (ICMP) resets the latch. The peak inductor

current at which ICMP resets the RS latch is determined

by the voltage on the ITH pin, which is driven by the output

of the error ampliï¬er (EAMP). The VFB pin receives the

output voltage feedback signal from an external resistor

divider. This feedback signal is compared to the internal

0.6V reference voltage by the EAMP. When the load current

increases, it causes a slight decrease in VFB relative to the

0.6V reference, which in turn causes the ITH voltage to

increase until the average inductor current matches the

new load current. While the top N-channel MOSFET is off,

the bottom N-channel MOSFET is turned on until either

the inductor current starts to reverse, as indicated by the

current reversal comparator, IRCMP, or the beginning of

the next cycle.

Shutdown, Soft-Start and Tracking Start-Up

(RUN/SS and TRACK/SS2 Pins)

The LTC3836 is shut down by pulling the RUN/SS pin

low. In shutdown, all controller functions are disabled and

the chip draws only 6.5Î¼A. The TG and BG outputs are

held low (off) in shutdown. Releasing RUN/SS allows an

internal 0.65Î¼A current source to charge up the RUN/SS

pin. When the RUN/SS pin reaches 0.65V, the LTC3836âs

two controllers are enabled.

The start-up of VOUT1 is controlled by the LTC3836âs

internal soft-start. During soft-start, the error ampliï¬er

EAMP compares the feedback signal VFB1 to the internal

soft-start ramp (instead of the 0.6V reference), which rises

linearly from 0V to 0.6V in about 1ms. This allows the

output voltage to rise smoothly from 0V to its ï¬nal value,

while maintaining control of the inductor current.

The 1ms soft-start time can be increased by con-

necting the optional external soft-start capacitor CSS

between the RUN/SS and SGND pins. As the RUN/SS

pin continues to rise linearly from approximately 0.65V

to 1.3V (being charged by the internal 0.65Î¼A current

source), the EAMP regulates the VFB1 proportionally

from 0V to 0.6V.

The start-up of VOUT2 is controlled by the voltage on the

TRACK/SS2 pin. When the voltage on the TRACK/SS2

pin is less than the 0.6V internal reference, the LTC3836

regulates the VFB2 voltage to the TRACK/SS2 pin voltage

instead of the 0.6V reference. This allows the TRACK/SS2

pin to be used to program a soft-start by connecting an

external capacitor from the TRACK/SS2 pin to SGND.

An internal 1Î¼A pull-up current charges this capacitor,

creating a voltage ramp on the TRACK/SS2 pin. As the

TRACK/SS2 voltage rises linearly from 0V to 0.6V (and

beyond), the output voltage VOUT2 rises smoothly from

zero to its ï¬nal value.

Alternatively, the TRACK/SS2 pin can be used to cause the

start-up of VOUT2 to âtrackâ that of another supply. Typi-

cally, this requires connecting to the TRACK/SS2 pin an

external resistor divider from the other supply to ground

(see Applications Information section).

When the RUN/SS pin is pulled low to disable the

LTC3836, or when VIN drops below its undervoltage

lockout threshold, the TRACK/SS2 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 Operation (Pulse-Skipping or Continuous

Conduction) (SYNC/FCB Pin)

The LTC3836 can be enabled to enter high efï¬ciency pulse-

skipping operation or forced continuous conduction mode

at low load currents. To select pulse-skipping operation, tie

the SYNC/FCB pin to a DC voltage above 0.6V (e.g., VIN).

To select forced continuous operation, tie the SYNC/FCB

to a DC voltage below 0.6V (e.g., SGND).

In forced continuous operation, the inductor current is

allowed to reverse at light loads or under large transient

conditions. The peak inductor current is determined by

the voltage on the ITH pin. The main N-channel MOSFET

is turned on every cycle (constant-frequency) regardless

of the ITH pin voltage. In this mode, the efï¬ciency at light

loads is lower than in pulse-skipping operation. However,

continuous mode has the advantages of lower output ripple

and less interference with audio circuitry.

3836fb

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