AP1516

150KHz, 3A PWM Buck DC/DC Converter + 1A Ultra Low

Drop Regulator

Anachip Corp.

www.anachip.com.tw

Rev.1.1 May 23, 2005

9/10

Function Description (Continued)

Since the minimum load current requirement of the

channel 2 is 10mA, R1 is typically selected to be a

121Ω resistor so that it automatically satisfies this

the range of 50uA it only adds a small error to the

output voltage and should be considered when very

precise output voltage setting is required.

Load Regulation

Since the channel 2 has separate pins for the

providing true remote sensing of the output voltage

at the load. This means that the voltage drops due

to parasitic resistance such as PCB traces between

the regulator and the load are compensated for

using remote sensing. Figure following shows a

typical application of the channel 2 with remote

sensing.

Channel 2-ADJ

Vin3

Vin2

Vin3

Vin2

Vout2

Vsense2

FB2

R1

R2

R

L

Stability

The channel 2 requires the use of an output

capacitor as part of the frequency compensation in

order to make the regulator stable. Typical designs

for the microprocessor applications use standard

electrolytic capacitors with typical ESR in the range

of 50 to 100mΩ and an output capacitance of

100uF to 1000uF. Fortunately as the capacitance

increases, the ESR decreases resulting in a fixed

RC time constant. The channel 2 takes advantage

of the phenomena in making the overall regulator

loop stable.

For most applications a minimum of 100uF

aluminum

electrolytic

capacitor

insures

both

stability and good transient response.

Thermal Considerations

The SOP-16 package needs a heat sink under

most conditions. The size of the heatsink depends

on the input voltage, the output voltage, the load

current and the ambient temperature. The AP1516

junction

temperature

rises

above

ambient

temperature for a 3A load and different input and

output voltages.

For the best thermal performance, wide copper

traces and generous amounts of printed circuit

board copper should be used in the board layout.

(Once exception to this is the output (switch) pin,

which should not have large areas of copper.)

Large areas of copper provide the best transfer of

heat (lower thermal resistance) to the surrounding

air, and moving air lowers the thermal resistance

even further.

Package

thermal

resistance

and

junction

temperature rise numbers are all approximate, and

there are many factors that will affect these

numbers.

Some of these factors include board

size, shape, thickness, position, location, and even

board temperature. Other factors are, trace width,

total printed circuit copper area, copper thickness,

single or double-sided, multi-layer board and the

amount of solder on the board.

The effectiveness of the PC board to dissipate heat

also depends on the size, quantity and spacing of

other components on the board, as well as whether

the surrounding air is still or moving. Furthermore,

some of these components such as the catch diode

will add heat to the PC board and the heat can vary

as the input voltage changes. For the inductor,

depending on the physical size, type of core

material and the DC resistance, it could either act

as a heat sink taking heat away from the board, or

it could add heat to the board.