APW7085 datasheet(18/24 Pages) ANPEC

18 / 24 page

Rev. A.7 - Sep., 2010

APW7085

www.anpec.com.tw

Application Information (Cont.)

Output Capacitor Selection (Cont.)

(V)

OUT

OSC

COUT

⋅

∆

........... (4)

For the applications, using bulk capacitors, the

∆V

COUT is

much smaller than the V

ESR and can be ignored. Therefore,

the AC peak-to-peak output voltage (

∆V

OUT ) is shown as

below:

(V)

ESR

VOUT

⋅

∆

........... (5)

........... (6)

The Schottky diode carries load current during the off-

time. Therefore, the average diode current is dependent

on the P-channel power MOSFET duty cycle. At high input

voltages, the diode conducts most of the time. As V

IN ap-

proaches V

OUT, the diode conducts only a small fraction of

the time. The most stressful condition for the diode is

when the output is short-circuited. Therefore, it is impor-

tant to adequately specify the diode peak current and av-

erage power dissipation so as not to exceed the diode

ratings.

Under normal load conditions, the average current con-

ducted by the diode is:

OUT

⋅

The APW7085 is equipped with whole protections to re-

duce the power dissipation during short-circuit condition.

Therefore, the maximum power dissipation of the diode

is calculated from the maximum output current as:

D(MAX)

DIODE(MAX)

where V

IN= VIN(MAX)

where V

OUT= VOUT(MAX)

Remember to keep lead length short and observe proper

grounding to avoid ringing and increased dissipation.

1.2

380000

)

OUT

≤

(H)

456000

)

OUT

≥

For the applications, using ceramic capacitors, the V

ESR is

much smaller than the

∆V

COUT an d c an b e i g no r e d .

Therefore, the AC peak-to-peak output voltage (

∆V

OUT ) is

close to

∆V

COUT .

The load transient requirements are the functions of the

slew rate (di/dt) and the magnitude of the transient load

current. These requirements are generally met with a

mix of capacitors and careful layout. High frequency ca-

pacitors initially supply the transient and slow the current

load rate seen by the bulk capacitors. The bulk filter ca-

pacitor values are generally determined by the ESR

(Effective Series Resistance) and voltage rating require-

ments rather than actual capacitance requirements.

High frequency decoupling capacitors should be placed

physically as close to the power pins of the load as

possible. Be careful not to add inductance in the circuit

board wiring that could cancel the usefulness of these

low inductance components. An aluminum electrolytic

capacitor’s ESR value is related to the case size with lower

ESR available in larger case sizes. However, the Equiva-

lent Series Inductance (ESL) of these capacitors increases

with case size and can reduce the usefulness of the ca-

pacitor to high slew-rate transient loading.

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies permit the

use of a smaller inductor for the same amount of inductor

ripple current. However, this is at the expense of efficiency

due to an increase in MOSFET gate charge losses. The

equation (2) shows that the inductance value has a direct

effect on ripple current.

Accepting larger values of ripple current allows the use of

low inductances but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is

∆I ≤ 0.4. I

OUT(MAX). Remember, the

maximum ripple current occurs at the maximum input

voltage. The minimum inductance of the inductor is cal-

culated by using the following equation:

Output Diode Selection

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