LT8705 datasheet(11/38 Pages) LINER

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LTC3784

3784fb

For more information www.linear.com/LTC3784

operaTion

Main Control Loop

The LTC3784 uses a constant-frequency, current mode

step-up architecture with the two controller channels

operating out of phase. During normal operation, each

external bottom MOSFET is turned on when the clock for

that channel sets the RS latch, and is turned off when the

main current comparator, ICMP, resets the RS latch. The

peak inductor current at which ICMP trips and resets the

latch is controlled by the voltage on the ITH pin, which is

the output of the error amplifier EA. The error amplifier

compares the output voltage feedback signal at the VFB

pin (which is generated with an external resistor divider

connected across the output voltage, VOUT, to ground), to

the internal 1.200V reference voltage. In a boost converter,

the required inductor current is determined by the load

current, VIN and VOUT. When the load current increases,

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

which causes the EA to increase the ITH voltage until the

average inductor current in each channel matches the new

requirement based on the new load current.

After the bottom MOSFET is turned off each cycle, the

top MOSFET is turned on until either the inductor current

starts to reverse, as indicated by the current comparator,

IREV, or the beginning of the next clock cycle.

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 tied to a voltage less than 4.8V,

the VBIAS LDO (low dropout linear regulator) supplies

5.4V from VBIAS to INTVCC. If EXTVCC is taken above

4.8V, the VBIAS LDO is turned off and an EXTVCC LDO is

turned on. Once enabled, the EXTVCC LDO supplies 5.4V

from EXTVCC to INTVCC. Using the EXTVCC pin allows the

INTVCC power to be derived from an external source, thus

removing the power dissipation of the VBIAS LDO.

Shutdown and Start-Up (RUN and SS Pins)

The two internal controllers of the LTC3784 can be shut

down using the RUN pin. Pulling this pin below 1.28V

shuts down the main control loops for both phases. Pull-

ing this pin below 0.7V disables both channels and most

internal circuits, including the INTVCC LDOs. In this state,

the LTC3784 draws only 4μA of quiescent current.

NOTE:Donotapplyaheavyloadforanextendedtimewhile

the chip is in shutdown. The top MOSFETs are turned off

during shutdown and the output load may cause excessive

dissipation in the body diodes.

The RUN pin may be externally pulled up or driven directly

by logic. When driving the RUN pin with a low impedance

source, do not exceed the absolute maximum rating of

8V. The RUN pin has an internal 11V voltage clamp that

allows the RUN pin to be connected through a resistor to

a higher voltage (for example, VIN), as long as the maxi-

mum current into the RUN pin does not exceed 100μA.

An external resistor divider connected to VIN can set the

threshold for converter operation. Once running, a 4.5μA

current is sourced from the RUN pin allowing the user to

program hysteresis using the resistor values.

The start-up of the controller’s output voltage VOUT is

controlled by the voltage on the SS pin. When the voltage

on the SS pin is less than the 1.2V internal reference, the

LTC3784 regulates the VFB voltage to the SS pin voltage

instead of the 1.2V reference. This allows the SS pin to

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

capacitor from the SS pin to SGND. An internal 10μA

pull-up current charges this capacitor creating a voltage

ramp on the SS pin. As the SS voltage rises linearly from

0V to 1.2V (and beyond up to INTVCC), the output voltage

rises smoothly to its final value.

Light Load Current Operation—Burst Mode Operation,

Pulse-Skipping or Continuous Conduction

(PLLIN/MODE Pin)

TheLTC3784canbeenabledtoenterhighefficiencyBurst

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 ground (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 1.2V and less

than INTVCC – 1.3V.

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