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ã 2000 Semtech Corp.

www.semtech.com

SC1454

POWER MANAGEMENT

PIN Descriptions

Applications Information

Theory Of Operation

The SC1454 is intended for applications where very low

dropout voltage, low supply current and low output noise

are critical. Furthermore, the SC1454, by combining two

ultra low dropout (ULDO) regulators, along with enable

controls and output voltage adjustability for one output,

provides a very space efficient solution for multiple

supply requirements.

The SC1454 contains two ULDOs, both of which are

supplied by one input supply, between IN and GND. Each

ULDO has its own active high enable pin (ENA/ENB).

Pulling this pin low causes that specific ULDO to enter a

very low power shutdown state.

The SC1454 contains an internal bandgap reference

which is fed into the inverting input of two error

amplifiers, one for each output. The output voltage of

each regulator is divided down internally using a resistor

divider and compared to the bandgap voltage. The error

amplifier drives the gate of a low R

MOSFET pass device.

Output A has both a fixed and adjustable output voltage

mode. Grounding the SETA pin (pulling it below the Sense/

Select threshold of 40mV) will connect the internal

resistor divider to the error amplifier resulting with the

internally preset output voltage. If SETA is pulled above

this threshold, then the Sense/Select switch will

connect the SETA pin to the error amplifier. Output A will

then be regulated such that the voltage at SETA will equal

V

A bypass pin (BYP) is provided to decouple the bandgap

reference to reduce output noise (on both outputs) and

also to improve power supply rejection.

Each regulator has its own current limit circuitry to

ensure that the output current will not damage the

device during output short, overload or start-up. The

current limit is guaranteed to be greater than 400mA to

allow fast charging of the output capacitor and high

initial currents for DSP initialization.

The SC1454 has a fast start-up circuit to speed up the

initial charging time of the bypass capacitor to enable

the output voltage to come up quicker.

The SC1454 includes thermal shutdown circuitry to turn

off the device if T

device remaining off until T

Reverse battery protection circuitry ensures that the

device cannot be damaged if the input supply is

accidentally reversed, limiting the reverse current to less

than 1.5mA.

Component Selection - General

Output capacitor - Semtech recommends a minimum

capacitance of 1µF at the output with an equivalent

series resistance (ESR) of < 1W over temperature. While

the SC1454 has been designed to be used with ceramic

capacitors, it does not have to be used with ceramic

capacitors, allowing the designer a choice. Increasing the

bulk capacitance will further reduce output noise and

improve the overall transient response.

Input capacitor - Semtech recommends the use of a 1µF

ceramic capacitor at the input. This allows for the device

being some distance from any bulk capacitance on the

rail. Additionally, input droop due to load transients is

reduced, improving overall load transient response.

Bypass capacitor - Semtech recommends the use of a

10nF ceramic capacitor to bypass the bandgap

reference. Increasing this capacitor to 100nF will

further improve power supply rejection. Reducing this

capacitor below 1nF may result in output overshoot at

turn-on.

Component Selection - Externally Setting Output

Referring to the circuit above, the output voltage of

output A can be externally adjusted anywhere within the

range from 1.25V to (V

will be in accordance with the following equation:

1% tolerance resistors are recommended. The values of

OUTPUT A: 2.5V

OUTPUT B: 2.8V

VIN

ENABLE OUTPUT A

ENABLE OUTPUT B

C3

10nF

C4

1uF

C1

1uF

C2

1uF

R1

100k

R2

100k

U1

SC1454CIMS

1

2

3

4

5

7

6

8

OUTA

OUTB

GND

SETA

ENB

ENA

BYP

IN











 +

•

=

2

R

1

R

1

250

.

1

V

OUTA