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NCV8614BMNR2G

Automotive System Power Supply, Ultra-Low Iq, IC Power Saving Triple-Output

ON Semiconductor 

NCV8614B

CIRCUIT DESCRIPTION

Auto Switchover Circuitry

The auto switchover circuit is designed to insure

continuous operation of the device, automatically switching

the input voltage from the ASO_RAIL input, to the VIN−B

input, to the VIN−H input depending on conditions. The

primary input voltage pin is ASO_RAIL, which is driven

from the 8 V supply. When this voltage is present it will drive

the output voltages. Regardless of whether the 8 V supply is

available, the reference and core functions of the device will

be driven by the higher of VIN−B and VIN−H. The

switchover control circuitry will be powered solely by the

8 V supply, via VIN−A.

When the 8 V supply is not present, the gates of the 2

P−FET switches will be pulled to ground, turning the

switches on. In this condition, the VIN−B and VIN−H

voltages will be diode or’ed, with the higher voltage

powering the chip. The VIN−H voltage will be one diode

lower than the VIN−B voltage, thereby forcing the VIN−B

voltage to be dominant supply.

In the event that both the 8 V supply and the VIN−B supply

are not present, the VIN−H supply will be powering the

device. The VIN−H supply is then fed from a recommended

1000 mF cap. The duration of VIN−H supply is dependent on

output current. It is intended as protection against temporary

loss of battery conditions.

In the event of a double battery, or prolonged high voltage

condition on the battery line, a bleed transistor has been

included on the VIN−H line. With the large hold−up cap on

VIN−H, the voltage on that pin has the potential to remain

in an elevated position for an extended period of time. The

main result of this condition would be an

Overvoltage Shutdown of the device. In order to avoid this

condition, a transistor that is connected to the

Overvoltage Shutdown signal is tied to the VIN−H line. This

transistor will become active in a high voltage event,

allowing the hold−up cap to discharge the excess voltage in

a timely manner.

In the Block Diagram, Figure 1, CASO_RAIL is listed as a

1 mF capacitor. It is required for proper operation of the

device that CASO_RAIL is no larger than 1 mF.

During a switchover event, a timer in the output stages

prepares the regulator in anticipation of change in input

voltage. The event results in a hitch in the output waveforms,

as can be seen in Figure 3.

IOUT = 100 mA

COUT = 47 mF

Figure 3. VOUTX Response to ASO Switchover Event

VIN−B/VIN−H Minimum Operating Voltage

The internal reference and core functions are powered by

either the VIN−B or VIN−H supply. The higher of the two

voltages will dominate and power the reference. This

provides quick circuit response on start−up, as well as a

stable reference voltage. Since the VIN−B voltage will come

up much more quickly than the VIN−H voltage, initially, the

VIN−B voltage will be running the reference. In the case of

any transient drops on VIN−B, the VIN−H supply, with its

large hold−up capacitor, will then be the dominant voltage,

and will be powering the reference.

For proper operation of the device, VIN−B or VIN−H

must be at least 4.5 V. Below that voltage the reference will

not operate properly, leading to incorrect functioning by the

device. VIN−B or VIN−H must be greater than 4.5 V

regardless of the voltage on the VIN−A pin.

Internal Soft−Start

The NCV8614B is equipped with an internal soft−start

function. This function is included to limit inrush currents

and overshoot of output voltages. The soft−start function

applies to all 3 regulators.

The soft−start function kicks in for start up, start up via

enable, start up after thermal shutdown, and startup after an

over voltage condition.

LDO3 is not subject to soft−start under all conditions. The

LDO3 output is not affected by overvoltage shutdown, and

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