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LTC3852

3852f

is recommended that there is always a load present during

the drop-out transition to ensure CB is recharged.

Shutdown and Start-Up (RUN, SHDN and TRACK/SS)

The switching regulator section of the LTC3852 can be

shut down using the RUN pin. Pulling this pin below 1.1V

disables the controller and most of the internal circuitry.

Releasing the RUN pin allows an internal 2μA current to

pull up the pin and enable the 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 6V on this pin.

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

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

voltage on the TRACK/SS pin is less than the 0.8V internal

reference, the LTC3852 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 GND. 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 start-up of VOUT to “track” another

supply. Typically, this requires connecting to the TRACK/SS

pin an external resistor divider from the other supply to

ground (see the Applications Information section). When

the RUN pin is pulled low to disable the controller, or when

INTVCC drops below its undervoltage lockout threshold

of 3.2V, the TRACK/SS pin is pulled low by an internal

MOSFET. When in undervoltage lockout, the controller is

disabled and the external MOSFETs are held off.

The charge pump is separately controlled by SHDN. In

shutdown mode, all charge pump circuitry is turned off and it

drawsonlyleakagecurrentfromtheVIN1supply.Furthermore,

VPUMP is disconnected from VIN1. The SHDN pin is a CMOS

input with a threshold voltage of approximately 0.7V. The

charge pump is in shutdown when a logic low is applied to

the SHDN pin. Since the SHDN pin is a very high impedance

CMOS input, it should never be allowed to ﬂoat. To ensure

that its state is deﬁned, it must always be driven with a valid

logic level not exceeding VIN1, even if it is tied to RUN.

OPERATION

Since the output voltage of the charge pump can go above

the input voltage, special circuitry is required to control the

internal logic. Detection logic will draw an input current

of 5μA when in shutdown. However, this current will

be eliminated if the output voltage (VPUMP) is less than

approximately 0.8V.

The charge pump has built-in soft-start circuitry to prevent

excessive current ﬂow during start-up. The soft-start is

achieved by charging an internal capacitor with a very weak

current source. The voltage on this capacitor, in turn, slowly

ramps the amount of current available to the output storage

capacitor from zero to a value of 50mA over a period of

approximately 125μs. The soft-start circuit is reset in the

event of a commanded shutdown or thermal shutdown.

Light Load Current Operation (Burst Mode Operation,

Pulse skipping or Continuous Conduction)

The LTC3852 can be enabled for high efﬁciency Burst

Mode operation, constant frequency pulse skipping mode

or forced continuous conduction mode. To select forced

continuous operation, tie the MODE/PLLIN pin to INTVCC.

To select pulse skipping mode of operation, ﬂoat the

MODE/PLLIN pin or tie it to GND2. To select Burst Mode

operation, tie MODE/PLLIN to INTVCC through a resistor

no less than 50k, but no greater than 250k.

When the controller is enabled for Burst Mode operation,

the peak current in the inductor is set to approximately

one-fourth of the maximum sense voltage even though

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

average inductor current is higher 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 MOSFETs are turned off.

In sleep mode, the load current is supplied by the output

capacitor. As the output voltage decreases, the EA’s output

begins to rise. When the output voltage drops enough, the

sleep signal goes low, and the controller resumes normal

operation by turning on the top external MOSFET on the

next cycle of the internal oscillator. When the controller is

enabled for Burst Mode operation, the inductor current is

not allowed to reverse. The reverse current comparator,

IREV, turns off the bottom external MOSFET just before the

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