FS8860

Rev. 2.2

11/15

12. Detail Description

The FS8860 is a low-dropout linear regulator. The device provides preset 1.8V and 3.3V output voltages for

output current up to 1.0A. Adjustable output voltage and other mask options for special output voltages are also

available. As illustrated in function block diagram, it consists of a 1.25V bandgap reference, an error amplifier, a

P-channel pass transistor and an internal feedback voltage divider (fixed voltage types).

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 feed back through an internal resistive divider (or external resistive divider for adjustable

output voltage type) connected to OUT pin. Additional blocks include an output current limiter, thermal sensor,

and shutdown logic.

Internal P-channel Pass Transistor

The FS8860 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 FS8860 does not suffer from these problems and consumes only 65μA (Typ.) of ground

pin current under heavy loads as well as in dropout conditions.

Output Voltage Selection

For fixed voltage type of FS8860, the output voltage is preset at an internally trimmed voltage. The first two

digits of part number suffix identify the output voltage (see Ordering Information). For example, the

FS8860-33CJ has a preset 3.3V output voltage.

For adjustable voltage type of FS8860, the output voltage is set by comparing the feedback voltage at adjust

terminal to the internal bandgap reference voltage. The reference voltage VREF is 1.25V. The output voltage

is given by the equation:

VOUT=VREF*(1+R2/R1)+IADJ*R2

(see Typical Application Schematic)

Current Limit

The FS8860 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 1.3A.

Thermal Overload Protection

Thermal overload protection limits total power dissipation in the FS8860. When the junction temperature

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.