LT8418 datasheet(21/50 Pages) AD

21 / 50 page

Data Sheet

LTC7892

analog.com

Rev. 0

21 of 50

Dead Time Control (DTCA and DTCB Pins)

The LTC7892 dead time delays can be programmed from 7ns to 60ns through the configuration of the DTCA and DTCB

pins. The DTCA pin programs the dead time associated with the bottom FET turning off and the top FET turning on

(SWx going from low to high). The DTCB pin programs the dead time associated with the top FET turning off and the

bottom FET turning on (SWx going from high to low).

Tying the DTCA pin to ground programs adaptive dead time control, which means the driver logic waits for the

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

approximately 15ns between BGx falling to TGx rising. Placing a resistor between the DTCA pin and ground programs,

the open-loop delay between the bottom FET turning off and the top FET turning on. This delay can be programmed

between 7ns and 60ns. See the Dead Time Control (DTCA and DTCB Pins) section for more information.

Tying the DTCB pin to ground programs adaptive dead time control, which means the driver logic waits for the top

FET to turn off before turning on the bottom FET. Adaptive dead time control results in dead times of approximately

15ns between TGx falling to BGx rising. Placing a resistor between the DTCB pin and ground programs the open-loop

delay between the top FET turning off and the bottom FET turning on. This delay can be programmed between 7ns

and 60ns. See the Dead Time Control (DTCA and DTCB Pins) section for more information.

Start-Up and Shutdown (RUN and SS Pins)

The two channels of the LTC7892 can be independently shut down using the RUN1 and RUN2 pins. Pulling a RUNx

pin below 1.1V shuts down the main control loop for that channel. Pulling both RUNx pins below 0.7V disables both

controllers and most internal circuits, including the INTVCC LDO regulators. In this shutdown state, the LTC7892 draws

only 1μA of current from VBIAS.

The RUNx pin needs to be externally pulled up or driven directly by logic. Each RUNx pin can tolerate up to 100V. See

Absolute Maximum Ratings for more details. Therefore, the RUNx pin can be conveniently tied to VIN or VBIAS in always-

on applications where one or both controllers are enabled continuously and never shut down. Additionally, a

resistive divider from VIN to a RUNx 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 each channel’s output voltage VOUTx is controlled by the voltage on the corresponding SSx pin. When

the voltage on the SSx pin is less than the 1.2V internal reference voltage, the LTC7892 regulates the VFBx voltage to

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

smoothly ramps the output voltage on start-up. An external capacitor from the SSx pin to GND is charged by an

internal 12μA pull-up current, creating a voltage ramp on the SSx pin. As the SSx voltage rises linearly from 0V to 1.2V

(and beyond), the output voltage VOUTx rises smoothly to its final value.

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

Forced Continuous Mode (MODE Pin)

The LTC7892 can be set to enter high-efficiency Burst Mode operation, constant frequency PS mode, or forced

continuous conduction mode at low load currents.

To select the Burst Mode operation, tie the MODE pin to GND. To select forced continuous operation, tie the MODE

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

internal 100kΩ resistor to ground invokes Burst Mode operation when the MODE pin is floating, and PS mode when

the MODE pin is tied to INTVCC through an external 100kΩ resistor.

When the controllers are 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 ITHx pin might indicate a lower value. If

LT8418 Datasheet(PDF) 21 Page - Analog Devices

LT8418 Datasheet(HTML) 21 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Datasheet Download

Link URL