LT3757 datasheet(16/48 Pages) AD

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LTC7841

Rev. 0

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OPERATION

Main Control Loop

The LTC7841 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 reference voltage. In a boost converter, the

required inductor current is determined by the load cur-

rent, 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 (RUN Pin)

The two internal controllers of the LTC7841 can be shut

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

shuts down the main control loops for both phases and

the serial interface. Pulling this pin below 0.7V disables

both channels and most internal circuits, including the

INTVCC LDOs. In this state, the LTC7841 draws only 8μA

of quiescent current.

NOTE: Do not apply a heavy load for an extended time

while 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.

Standby Mode

Standby mode is invoked by clearing the ON bit in the

OPERATION command. Standby mode shuts down the

main control loops for both phases like the RUN pin shut-

down, however leaves the ADC and PMBus still active.

In standby mode, the LTC7841 will still respond to the

PMBus host but will only refresh the telemetry at a 1Hz

rate instead of 25Hz. Exiting standby mode (setting ON bit

back to 1) resets all faults and the ALERT pin. Data writ-

ten to internal registers is not affected by standby mode.

Pulling RUN pin below 1.28V overrides the Standby mode

and turns off the ADC and PMBus. When RUN returns

above 1.28V, the ON bit is initialized to 0 and thus must

be set to 1 to turn the control loop back on.

Table 1. Summary of Shutdown Modes

Input Conditions

ON/OFF States

RUN Pin

ON bit VOUT PMBus

ADC

RUN < 0.7V

OFF

8µA

0.7V < RUN < 1.28V

OFF

350µA

RUN > 1.28V

OFF

1Hz Refresh

1mA

RUN > 1.28V

25Hz

Refresh

1mA to

2mA

(Note 5)

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