PAM2309 datasheet(10/16 Pages) PAM

10 / 16 page

Power Analog Microelectronics Inc

www.poweranalog.com

Application Information

The basic PAM2309 application circuit is shown

in Page 1. External component selection is

determined by the load requirement, selecting L

first and then Cin and Cout.

For most applications, the value of the inductor

will fall in the range of 1μH to 4.7μH. Its value is

chosen based on the desired ripple current.

Large value inductors lower ripple current and

small value inductors result in higher ripple

currents. Higher V

or Vout also increases the

ripple current as shown in equation 1. A

reasonable starting point for setting ripple

current is

I = 400mA (40% of 1A).

(1)

The DC current rating of the inductor should be

at least equal to the maximum load current plus

half the ripple current to prevent core saturation.

Thus, a 1.4A rated inductor should be enough for

most applications (1A + 400mA). For better

efficiency, choose a low DC-resis tance inductor.

In continuous mode, the source current of the top

MOSFET is a square wave of duty cycle

Vout/Vin. To prevent large voltage transients, a

low ESR input capacitor sized for the maximum

RMS current must be used. The maximum RMS

capacitor current is given by:

This formula has a maximum at V

=2Vout,

where I

/2. T his simple worst-case

condition is com monly used for design because

even significant deviations do not offer much

relief. Note that the capacitor manufacturer's

ripple current ratings are often based on 2000

hours of life. This makes it advisable to further

derate the capacitor, or choose a capacitor rated

at a higher temperature than required. Consult

the manufac turer if there is any question.

The selection of Cout is driven by the required

effective series resistance (ESR).

Typically, once the ESR requirement for Cout

has been met, the RMS current rating generally

far exceeds the I

(P-P) requirement. The

output ripple

Vout is determined by:

Where f = operating frequency, C

=output

capacitance and

= ripple current in the

inductor. For a fixed output voltage, the output

ripple is highest at maximum input voltage since

I increases with input voltage.

Higher values, lower cost ceramic capacitors are

now becoming available in smaller case sizes.

Their high ripple current, high voltage rating and

low ESR make them ideal for switching regulator

applications. Using ceramic capacitors can

achieve very low output ripple and small circuit

size.

When choosing the input and output ceramic

capacitors, choose the X5R or X7R dielectric

formulations. These dielectrics have the best

temperature and voltage charac teristics of all

the ceramics for a given value and size.

Inductor Selection

C and C

Selection

Using Ceramic Input and Output Capacitors

RMS

O UT

RIPPLE

OUT

△

OUT

Thermal consideration

Thermal protection limits power dissipation in

the PAM2309. When the junction temperature

exceeds 150°C, the OTP (Over Temperature

Protection) starts the thermal shutdown and

turns the pass transistor off. The pass transistor

resumes

operation

after

t he

junction

temperature drops below 120°C.

For c ontinuous operation, the j unction

temperature should be maintained below 125°C.

The power dissipation is defined as:

I is the step-down converter quiescent current.

The term tsw is used to estimate the full load

step-down converter switching losses.

()

OUT

RMS

OMAX

C required I

éù

ëû

OUT

VI ESR+

8fC

æö

ç÷

èø

PAM2309

1A Step-Down DC-DC Converter

()( )

æö

ç÷

èø

OUT

()

DSONH

DSONL

S O

+ V -V

P=I

+ t

F I +I

1.2V

1.5V

1.8V

2.5V

3.3V

2.2μH

4.7μH

08/2009 Rev 1.1

PAM2309 Datasheet(PDF) 10 Page - Power Analog Micoelectronics

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