LTC7897 datasheet(21/53 Pages) AD

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Data Sheet

LTC7897

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Rev. A

21 of 53

Tying the DTCB pin to GND or INTVCC programs adaptive dead time control, which means the driver logic waits for

the top MOSFET to turn off before turning on the bottom MOSFET. Adaptive dead time control results in dead times

between TG falling to BG rising of approximately 50ns with DTCB tied to ground and 30ns with DTCB tied to INTVCC.

Placing a resistor between the DTCA pin and GND adds a smart delay between SW rising and BG falling from 13ns to

100ns. See the Applications Information section on dead time control for more information.

Placing a resistor between the DTCB pin and GND adds a smart delay between SW falling and BG rising from 13ns to

100ns. See the Applications Information section on dead time control for more information.

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

The LTC7897 can be shut down using the RUN pin. Pulling the RUN pin below 1.08V 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

LDO regulators. In this shutdown state, the LTC7897 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). Therefore, the pin can be tied to VIN in always-on applications where the controller is enabled

continuously and never shuts down. Additionally, a resistor divider from VIN 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 OVLO pin inhibits switching when the input voltage rises above a programmable operating range. By using a

resistor divider from the input supply to ground, the OVLO pin serves as a precise input supply voltage monitor.

Switching is disabled when the OVLO pin rises above 1.2V, which can be configured to limit switching to a specific

range of input supply voltage. When switching is disabled, the LTC7897 can safely sustain input voltages up to the

absolute maximum rating of 140V. Input supply overvoltage events trigger a soft-start reset, which results in a

graceful recovery from an input supply transient.

The start-up of 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 voltage, the LTC7897 regulates the VFB voltage to the TRACK/SS pin voltage instead

of the 0.8V reference voltage. This method allows the TRACK/SS pin to be used as a soft-start, which smoothly ramps

the output voltage on start-up, limiting the input supply inrush current. An external capacitor from the TRACK/SS pin

to GND is charged by an internal 9μA pull-up current, creating a voltage ramp on the TRACK/SS pin. As the TRACK/SS

voltage rises linearly from 0V to 0.8V (and beyond), VOUT rises smoothly from zero to its final value.

Alternatively, the TRACK/SS pin can be used to make the start-up of VOUT track that of another supply. Typically, this

requires connecting to the TRACK/SS pin through an external resistor divider from the other supply to GND (see the

RUN Pin and Overvoltage/Undervoltage Lockout section and Soft-Start and Tracking (TRACK/SS Pin) section).

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

Continuous Mode (MODE Pin)

The LTC7897 can be set to enter high efficiency Burst Mode operation, constant frequency pulse-skipping mode, or

forced continuous conduction mode at light load currents.

To select Burst Mode operation, tie the MODE pin to GND. 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 100kΩ resistor to GND 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 100kΩ 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 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

turn off. The ITH pin is then disconnected from the output of the EA and parked at 0.45V.

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