6

 2006 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC1453

Applications Information

Theory Of Operation

The SC1453 is intended for applications where very low

dropout voltage, low supply current and low output noise

are critical. It provides a very simple, low cost solution

that uses very little pcb real estate. Only three external

capacitors are required for operation (two if a low noise

output is not required).

The SC1453 contains a bandgap reference trimmed for

optimal temperature coefficient which is fed into the

inverting input of an error amplifier. The output voltage

of the 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

DS(ON)

P-channel MOSFET pass device.

An active high enable pin (EN) allows the regulator to be

shut down. Pulling this pin low causes the device to

enter a very low power shutdown mode, where it will draw

typically 0.1µA from the input supply.

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

reference to reduce output noise and also to improve

power supply rejection. This pin can be left open if low

noise operation is not required.

The 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 SC1453 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 (typically 1.3ms

with C

The SC1453 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 < 1

Ω over temperature. While

the SC1453 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 and overall

output noise. C

is not required.

Thermal Considerations

The worst-case power dissipation for this part is given

by:

()

)

MAX

(

Q

)

MAX

(

IN

)

MAX

(

OUT

)

MIN

(

OUT

)

MAX

(

IN

)

MAX

(

D

I

V

I

V

V

P

•

+

•

−

=

(1)

For all practical purposes, equation (1) can be reduced

to the following expression:

()

)

MAX

(

OUT

)

MIN

(

OUT

)

MAX

(

IN

)

MAX

(

D

I

V

V

P

•

−

=

(2)

Looking at a typical application, 3.3V to 2.8V at 150mA:

V

V

I

T