LTC7899 datasheet(21/50 Pages) AD

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Data Sheet

LTC7899

analog.com

Rev. 0

21 of 50

Tying the DTCx pin to VIN– or INTVCC programs adaptive dead time control, which means the driver logic waits for one

MOSFET to turn off before deciding to turn on the other MOSFET. Adaptive dead time control results in dead times

between BG/(TG) falling to TG/(BG) rising of approximately 80ns with DTCx = VIN– and 30ns with DTCx = INTVCC. Placing

a resistor between the DTC pin and VIN– programs the open-loop delay between one MOSFET turning off and the other

turning on. This delay can be programmed between 13ns and 100ns. See the Applications Information section on

dead time control for more information.

Start-Up and Shutdown (RUN and SS Pins)

The LTC7899 can be shut down using the RUN pin. Pulling the RUN pin below 1.1V shuts down the main control loop.

Pulling the RUN pin below 0.7V disables the controller and most internal circuits, including the DRVCC and INTVCC

LDOs. In this shutdown state, the LTC7899 draws only 1μA of quiescent current.

The RUN pin needs to be externally pulled up or driven directly by logic. The RUN pin can tolerate up to 140V

(absolute maximum), so it can be conveniently tied to VBIAS in always-on applications where the controller is enabled

continuously and never shut down. Additionally, a resistive divider between VBIAS and VIN– with a tap to the RUN pin

can be used to set a precise input undervoltage lockout so that the power supply does not operate below a user

adjustable level.

The start-up of the output voltage VOUT with respect to ground is controlled by the voltage on the SS pin. When the

voltage on the SS pin is less than the 1.2V internal reference voltage, the LTC7899 regulates the VFBB voltage to the SS

pin voltage instead of the 1.2V reference voltage. This allows the SS pin to be used as a soft-start, which smoothly

ramps the output voltage on start-up. An external capacitor from the SS pin to VIN– is charged by an internal 9μA pull-

up current, creating a voltage ramp on the SS pin. As the SS voltage rises linearly from 0V to 1.2V (and beyond), the

output voltage VOUT rises smoothly from zero to its final value.

Light Load Operation: Burst Mode Operation, Pulse-Skipping Mode, or Forced

Continuous Mode (MODE Pin)

The LTC7899 can be enabled to enter high efficiency burst mode operation, constant frequency pulse-skipping mode

or forced continuous conduction mode at low load currents.

To select burst mode operation, tie the MODE pin to VIN–. To select forced continuous operation, tie the MODE pin to

INTVCC. To select pulse-skipping mode, tie the MODE pin to a DC voltage greater than 1.2V and less than INTVCC – 1.3V.

An internal 100k resistor to VIN– invokes burst mode operation when the MODE pin is floating and pulse-skipping

mode when the MODE pin is tied to INTVCC through an external 100k resistor.

When the controller is enabled for burst mode operation, the minimum peak current in the inductor is set to

approximately 25% of its maximum value, even though the voltage on the ITH pin may indicate a lower value. If the

average inductor current is higher than the load current, the error amplifier, EA, decreases the voltage on the ITH

pin. When the ITH voltage drops below 0.425V, the internal sleep signal goes high (enabling sleep mode) and both

external MOSFETs are turned off. The ITH pin is then disconnected from the output of the EA and parked at 0.45V.

In sleep mode, much of the internal circuitry is turned off, reducing the quiescent current that the LTC7899 draws to

15µA. In sleep mode, the load current is supplied by the output capacitor. As the output voltage decreases with

respect to ground, the EA’s output begins to rise. When the output voltage drops enough, the ITH pin is reconnected

to the output of the EA, the sleep signal goes low, and the controller resumes normal operation by turning on the

bottom 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 (IR) turns off the top MOSFET just before the inductor current reaches zero, preventing it from

reversing and going negative. Thus, the controller operates in discontinuous operation.

LTC7899 Datasheet(PDF) 21 Page - Analog Devices

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