FS8853-28GL datasheet(11/15 Pages) FORTUNE

11 / 15 page

FS8853

Rev. 2.7

11/15

13. Detail Description

The FS8853 is a low-dropout linear regulator. The

device provides preset 1.8V and 3.3V output

voltages for output current up to 300mA. Other mask

options for special output voltages from 1.5V to 5.0V

with 100mV increment are also available. As

illustrated in function block diagram, it consists of a

1.25V reference, error amplifier, P-channel pass

transistor and an internal feedback voltage divider.

The 1.25V 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 feedback through an internal

resistive divider connected to OUT pin. Additional

blocks include an output current limiter, thermal

sensor, and shutdown logic.

Internal P-channel Pass Transistor

The FS8853 features a P-channel MOSFET pass

transistor. Unlike similar designs using PNP pass

transistors, P-channel MOSFETs require no base

drive, which reduces ground pin current. PNP-based

regulators also waste considerable current in

dropout when the pass transistor saturates, and use

high base-drive currents under large loads. The

FS8853 does not suffer from these problems and

consumes only 30μA (Typ.) of ground pin current

under heavy loads as well as in dropout conditions.

Output Voltage Selection

The FS8853 output voltage is preset at an internally

trimmed voltage 1.8V, 3.3V or can be mask-optioned

from 1.5V to 5.0V with 100mV increment. The first

two digits of part number suffix identify the output

voltage (see Ordering Information). For example, the

FS8853-33CL has a preset 3.3V output voltage.

Current Limit

The FS8853 also include 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 0.6A.

Thermal Overload Protection

Thermal overload protection limits total power

dissipation in the FS8853. 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 20°C, resulting in a pulsed output during

continuous thermal overload conditions.

Thermal overload protection is designed to protect

the FS8853 in the event of fault conditions. For

continuous operation, the maximum operating

junction temperature rating of TJ = +125°C should

not be exceeded.

Operating Region and Power Dissipation

Maximum power dissipation of the FS8853 depends

on the thermal resistance of the case and circuit

board, the temperature difference between the die

junction and ambient air, and the rate of airflow. The

power dissipation across the devices is P = IOUT x

(VIN-VOUT).

The

resulting

maximum

power

dissipation is:

(

) ()

MAX

−

Where (TJ-TA) is the temperature difference

between

the

FS8853

die

junction

and

the

surrounding air, θJC is the thermal resistance of the

package chosen, and θCA is the thermal resistance

through the printed circuit board, copper traces and

other materials to the surrounding air. For better

heat-sinking, the copper area should be equally

shared between the IN, OUT, and GND pins.

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