NCP2860

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DETAILED OPERATING DESCRIPTION

Internal Pass Transistor

The NCP2860 incorporates a 0.5

W typical P−channel

MOSFET pass transistor. The P−channel MOSFET requires

no drive current and then compared to the PNP based

regulators, this solution drastically reduces the quiescent

current and associated losses.

Shutdown Block

The circuit turns into shutdown mode when the shutdown

pin is in low state. In this mode, the internal biasing current

sources are disconnected so that the pass transistor is off and

the consumption reduced to a minimum value. Practically,

the shutdown consumption is in the range of 10 nA. When

this function is unused, “IN’’ is generally applied to the

shutdown pin.

Current Limitation

The NCP2860 incorporates a short circuit protection that

prevents the pass transistor current from exceeding 465 mA

typically. The current limit is set to 220 mA during the

start−up phase.

Thermal Protection

The thermal protection protects the die against excessive

overheating. Practically, when the junction temperature

exceeds 170°C for the 2.77 V option and 150°C for the 3.0 V

option, an internal thermal sensor sends a logical signal to

the shutdown block so that the circuit enters the shutdown

mode. Once the die has cooled enough (typically 30°C), the

circuit enters a new working phase.

Output Voltage Setting

a) The output voltage is set by internal resistor divider to

2.77 V for NCP2860DM277R2 device. The ”SET” pin must

be grounded or pulled below 0.2 V.

b) The output voltage can also be programmed to a different

value for NCP2860DMADJR2G device. In this case a

portion of the output voltage acquired by the external

resistor divider must be applied to the ”SET” pin. Do not

leave the ”SET” pin unconnected. If a (R1, R2) external

resistors divider is used, then output voltage is given by

equation:

Now if R1 and R2 are high impedance resistors, the

leakage current that is absorbed by the “SET’’ pin, may have

to be taken into account as follows:

“SET’’ pin leakage current.

If the output voltage is directly applied to the “SET’’ pin,

OUT

SET

NCP2860

R1

R2

Regulation

The circuit incorporates a transconductance error

amplifier. The error amplifier output varies in response to

load and input voltage variations to control the pass

transistor current so that the “OUT’’ pin delivers the wished

voltage. No compensation capacitor is required.

Fault Detection Circuitry

The circuit detects when the input−output differential

voltage is too low to ensure a correct load and line regulation

at the output. The input−output differential threshold scales

proportionally with the load current to be always just higher

than the dropout.

When the circuit detects a fault condition, an internal

switch connects “FAULT’’ to ground. In normal operation,

the “FAULT’’ terminal is an open−drain−N−channel

MOSFET and if a pull−up resistor is connected between

“OUT’’ and “FAULT’’, “FAULT’’ goes high. The pullup

resistor is generally selected in the range of 100 k

W to

minimize the current consumption.

Application Information

It is recommended to use 2.2

mF capacitors on the input

and on the output of the NCP2860. Capacitor type is not very

critical. Simply the ESR should be lower than 0.5

W to

ensure a stable operation over the temperature and output

current ranges. It could be convenient to increase the

capacitor size and its quality (lower ESR) only if it was

necessary to further improve the noise performances, the

Power Supply Rejection Ratio or the fast transient response.