APW1172 datasheet(14/21 Pages) ANPEC

14 / 21 page

Rev. A.4 - Aug., 2005

APW1172

www.anpec.com.tw

Application Description (Cont.)

)

)(

(

)

(

(7)

)

)(

(

)

(

(7)

generate a zero to provide a positive phase for control

loop.This zero improved the stability without extra PID

compensator, if the zero is lower enough.

Switch diode

APW1172 is an non-synchronous type buck regulator

and needs a Shottky diode as the wheeling diode.

This diode will conduct when the pass element turned

off.Current flows through the diode in the conducted

period, the order of the maximum peak current

reaches few Amperes. The diode requires the ability

to flow the great forward current. The peak forward

current of the diode denote in the specification must

great than 15A, and the conducting time in this

situation must great than 8ms. 1N5818 is a suitable

component.

Thermal Consideration

APW1172 is a switching regulator whose pass element

inside, it have the ability to provide 3 Amperes.As the

show in the block diagram, the structure of the pass

element consist of a NPN and a PNP transistors. The

voltage across the pass element, VCE, is about 0.8V

to 1.3V in the light load to heavy load. The product of

VCE and IL, where IL is current flowing through the

inductor, generate thermal cause the junction

temperature increased. The thermal stream conduct

via the thermal pad of SOP-8-P to the printed circuit

board.The power dissipation of APW1172 can be

approximated by (7)

Where VCE is the voltage across the pass element, IL

is the current flowing through the inductor, D is the

duty. TR and TF are the transition time.

The wheeling diode is another thermal source. It’s

power dissipation approximated by (8)

Output Capacitor (Cont.)

Where VD is the forward voltage of the wheeling diode,

ID is current flowing through the wheeling diode when

it conducting. In the PCB layout,usually place the

wheeling diode near the APW1172, the power

dissipation of wheeling diode will increase the ambient

temperature and limit the maximum power dissipation

of APW1172.These power dissipations are the major

energy loss in the voltage conversion.

To improve the thermal resistance by increasing

copper area is a suitable method. Design a copper

area according to the following curve to improve the

thermal resistance.

Frequency Compensation

In the Buck converter,there is a LPF (Low Pass Filter)

in the output stage to filtering the switching noise.

The LPF consist of an inductor and a capacitor. These

two components generate the double poles in the

frequency domain.

Where L is the inductance of the LPF and C is the

capacitance of the output capacitor. These double poles

natural

(9)

natural

(9)

Top Copper Area (cm^2)

)

(

−

(8)

APW1172 Datasheet(PDF) 14 Page - Anpec Electronics Coropration

APW1172 Datasheet(HTML) 14 Page - Anpec Electronics Coropration

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL