LTC3833 datasheet(13/50 Pages) LINER

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LTC3839

3839fa

Characteristics Table), then the internal 5.3V LDO is en-

abled. If the EXTVCC pin is tied to an external voltage source

greater than this EXTVCC switchover voltage, then the LDO

is shut down and the internal EXTVCC switch shorts the

EXTVCC pin to the DRVCC2 pin, thereby powering DRVCC

and INTVCC with the external voltage source and helping

to increase overall efficiency and decrease internal self

heating from power dissipated in the LDO. This external

power source could be the output of the step-down con-

verter itself (if the output is programmed to higher than the

switchover voltage’s higher limit, 4.8V). The VIN pin still

needs to be powered up but now draws minimum current.

Power for most internal control circuitry other than gate

drivers is derived from the INTVCC pin. INTVCC can be

powered from the combined DRVCC pins (either directly,

or through an external RC filter to SGND to filter out noises

due to switching).

Shutdown and Start-Up

The RUN pin has an internal proportional-to-absolute-

temperature (PTAT) pull-up current source (around 2.5μA

at 25°C). Taking the RUN pin below a certain threshold

voltage (around 0.8V at 25°C) shuts down all bias of

INTVCCandDRVCCandplacestheLTC3839intomicropower

shutdown mode with a minimum IQ at the VIN pin. The

LTC3839’s DRVCC (through the internal 5.3V LDO regula-

tor or EXTVCC) and the corresponding channel’s internal

circuitry off INTVCC will be biased up when either or both

RUN pins are pulled up above the 0.8V threshold, either by

the internal pull-up current or driven directly by external

voltage source such as logic gate output.

Neither of the two channels will start switching until

the RUN pin is pulled up to 1.2V. When the RUN pin

rises above 1.2V, both channels’ TG and BG drivers are

enabled, and TRACK/SS released. An additional 10μA

temperature-independent pull-up current is connected

internally to the RUN pin. To turn off TG, BG and the ad-

ditional 10μA pull-up current, RUN needs to be pulled down

below 1.2V by about 100mV. These built-in current and

voltage hystereses prevent false jittery turn-on and turn-off

due to noise. Such features on the RUN pin allow input

undervoltage lockout (UVLO) to be set up using external

voltage divider from VIN.

The start-up of the 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 0.6V internal reference, the

differential feedback voltage is regulated to the TRACK/SS

voltage instead of the 0.6V reference. The TRACK/SS

pin can be used to program the output voltage soft-start

ramp-up time by connecting an external capacitor from the

TRACK/SS pin to signal ground. An internal temperature-

independent 1μA pull-up current charges this capacitor,

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

TRACK/SS voltage rises linearly from ground to 0.6V, the

switching starts, VOUT ramps up smoothly to its final value

and the feedback voltage to 0.6V. TRACK/SS will keep

rising beyond 0.6V, until being clamped to around 3.7V.

Alternatively, the TRACK/SS pin can be used to track an

external supply like in a master slave configuration. Typi-

cally, this requires connecting a resistor divider from the

master supply to the TRACK/SS pin (see the Applications

Information section).

TRACK/SS is pulled low internally when the RUN pin is

pulled below the 1.2V threshold (hysteresis applies), or

when INTVCC or DRVCC drop below their undervoltage

lockout (UVLO) threshold.

Light Load Current Operation

If the MODE/PLLIN pin is tied to INTVCC or an external clock

is applied to MODE/PLLIN, the LTC3839 will be forced to

operate in continuous mode. With load current less than

one-half of the full load peak-to-peak ripple, the inductor

current valley can drop to zero or become negative. This

allows constant-frequency operation but at the cost of low

efficiency at light loads.

If the MODE/PLLIN pin is left open or connected to signal

ground, the channel will transition into discontinuous mode

operation, where a current reversal comparator (IREV)shuts

off the bottom MOSFET (MB) as the inductor current ap-

proaches zero, thus preventing negative inductor current

and improving light-load efficiency. In this mode, both

switches can remain off for extended periods of time. As

the output capacitor discharges by load current and the

output voltage droops lower, EA will eventually move the

ITH voltage above the zero current level (0.8V) to initiate

another switching cycle.

OPERATION (Refer to Functional Diagram)

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