LTC7813 datasheet(11/28 Pages) AD

11 / 28 page

LTC7878

Rev. 0

For more information www.analog.com

OPERATION

MAIN CONTROL LOOP

The LTC7878 is a constant-frequency, peak current

mode controller for 4-switch buck-boost converters

which can regulate an output voltage above, below or

equal to the input voltage. The ADI proprietary current

sensing and control architecture employs inductor DCR

(DC Resistance) for current sensing and close loop reg-

ulation. The inductor peak current is controlled by the

voltage on the ITH and ITHB pins, which is the output of

the error amplifier EA. The VFB pin receives the voltage

feedback signal and compared with the internal 1.0V ref-

erence voltage by the EA. If the input or output average

current regulation loop is implemented, the inductor cur-

rent is controlled by either the output feedback voltage or

the input/output average current.

DRVCC POWER

Power for the top and bottom N-channel MOSFET driv-

ers and most other internal circuitry is derived from the

DRVCC pin. When EXTVCC supply is not used, an internal

7V linear regulator supplies DRVCC power from VIN. If

EXTVCC is connected to an external voltage source such

as the output of the buck-boost converter, another 7V

linear regulator under EXTVCC may be enabled and sup-

ply DRVCC power. LTC7878 internally choose VIN linear

regulator or EXTVCC linear regulator to supply the DRVCC

based on the operation mode and supply voltages to min-

imize the IC power loss and reduce the IC temperature.

Either VIN or EXTVCC can supply the IC separately. If one

of power supplies on VIN or EXTVCC fails, LTC7878 can

still operate normally with the other input supply on VIN

or EXTVCC. Both VIN and EXTVCC can take the voltage as

high as 70V.

START-UP AND SHUTDOWN CONTROL (RUN)

The LTC7878 is in the shutdown mode when the RUN pin

is pulled down and lower than 0.5V. In shutdown mode,

most internal circuitry is turned off including the DRVCC/

INTVCC regulators and the LTC7878 consumes less than

100μA current. All the driver outputs are actively low to

turn off the external N-channel MOSFETs. Releasing RUN

allows an internal 1µA current to pull up this pin and

enable the converter. When RUN pin voltage is higher than

0.5V and lower than 1.2V, the LTC7878 is in the standby

mode, the DRVCC/INTVCC regulators and all the internal

circuitry are enabled but no switching. After RUN pin is

higher than 1.2V, the LTC7878 is in the active mode and

starts switching and regulating the output voltage. The

RUN pin may be externally pulled up or driven directly

by logic. Alternatively, the RUN pin can be controlled by

a resistor divider from VIN to SGND to achieve program-

mable VIN UVLO (undervoltage lockout). Customers may

design the converter to start-up at the pre-programmed

VIN voltage with different resistor divider values. Do not

exceed the Absolute Maximum Ratings of 12V on the

RUN pin.

The start-up of the converter’s output, VOUT, is controlled

by the voltage on the SS pin. When the voltage on the SS

pin is less than the 1.0V internal reference, the LTC7878

regulates the VFB voltage to the SS voltage instead of the

1.0V reference. This allows the SS pin to be used to pro-

gram soft-start by connecting an external capacitor from

the SS pin to SGND. An internal 2.5μA pull-up current

charges this capacitor, creating a voltage ramp on the SS

pin. As the SS voltage rises linearly from 0V to 1.0V (and

beyond), the output voltage VOUT rises smoothly from

zero to its final value. During the start-up, the LTC7878

operates in the pulse-skipping mode to avoid the negative

inductor current and output voltage discharging. When

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

is below the undervoltage lockout threshold of 4.0V, the

SS pin is pulled low by an internal MOSFET. In undervolt-

age lockout, the controller is disabled and the external

MOSFETs are held off.

POWER SWITCH CONTROL

The simplified Block Diagram shows how the four power

switches are connected to the inductor, VIN, VOUT and

GND. LTC7878 operates in buck region (VIN >>VOUT),

boost region (VIN << VOUT) and buck-boost region

(VIN ≈ VOUT) without sensing the VIN and VOUT voltage.

The power switches are properly controlled so the transfer

between regions is continuous and smooth.

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