FS8844 datasheet(9/10 Pages) FORTUNE

9 / 10 page

FS8844

Rev. 1.8

9/10

13. Detail Description

The FS8844 is a low quiescent current LDO linear

regulator. The device provides preset 1.8V, 2.8V and

3.3V output voltages for output current up to 150mA.

Other mask options for special output voltages from

1.2V to 5.0V with 100mV increment are also

available. As illustrated in function block diagram, it

consists of a 1.23V reference, error amplifier, a

P-channel pass transistor, an ON/OFF control logic,

and an internal feedback voltage divider.

The 1.23V bandgap reference is connected to the

error amplifier, which compares this reference with

the feedback voltage and amplifies the voltage

difference. If the feedback voltage is lower than the

reference voltage, the pass-transistor gate is pulled

lower, which allows more current to pass to the

output pin and increases the output voltage. If the

feedback voltage is too high, the pass-transistor gate

is pulled up to decrease the output voltage.

The output voltage is feed back through an internal

resistive divider connected to OUT pin. Additional

blocks include an output current limiter and

shutdown logic.

Internal P-channel Pass Transistor

The FS8844 features a P-channel MOSFET pass

transistor. Unlike similar designs using PNP pass

transistors, P-channel MOSFETs require no base

drive, which reduces quiescent current. PNP-based

regulators also waste considerable current in

dropout conditions when the pass transistor

saturates, and use high base-drive currents under

large loads. The FS8844 does not suffer from these

problems and consumes only 2.0μA (Typ.) of current

consumption under light loads.

Output Voltage Selection

The FS8844 output voltage is preset at an

internally trimmed voltage 1.8V, 2.8V or 3.3V.

The

output

voltage

also

can

mask-optioned from 1.2V to 5.0V with

100mV increment by special order. The first

two digits of part number suffix identify the

output voltage (see Ordering Information).

For example, the FS8844-33GC has a

preset 3.3V output voltage.

Current Limit

The FS8844 also includes a fold back

current limiter. It monitors and controls the

pass transistor’s gate voltage, estimates the

output current, and limits the output current

within 350mA.

Thermal Overload Protection

Thermal overload protection limits total

power dissipation in the FS8844. When the

junction temperature exceeds TJ = +155°C,

a thermal sensor turns off the pass transistor,

allowing the IC to cool down. The thermal

sensor turns the pass transistor on again

after the junction temperature cools down by

10°C, resulting in a pulsed output during

continuous thermal overload conditions.

Thermal overload protection is designed to

protect the FS8844 in the event of fault

conditions. For continuous operation, the

maximum operating junction temperature

rating of TJ = +125°C should not be

exceeded.

Input-Output Voltage

A regulator’s minimum input-output voltage

differential, or dropout voltage, determines

the lowest usable supply voltage. In

battery-powered systems, this will determine

the useful end-of-life battery voltage. The

FS8844 uses a P-channel MOSFET pass

transistor, its dropout voltage is a function of

drain-to-source

on-resistance

(RDS(ON))

multiplied by the load current.

VDROPOUT = VIN-VOUT = RDS(ON) x IOUT

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