APW7158 datasheet(18/24 Pages) ANPEC

18 / 24 page

Rev. A.2 - Aug., 2009

APW7158

www.anpec.com.tw

Application Information (Cont.)

Where Fs is the switching frequency of the regulator. Al-

though increase the inductor value and frequency reduce

the ripple current and voltage, but there is a tradeoff ex-

ists between the inductor’s ripple current and the regula-

tor load transient response time.

A smaller inductor will give the regulator a faster load

transient response at the expense of higher ripple current.

Increasing the switching frequency (F

S) also reduces the

ripple current and voltage, but it will increase the switch-

ing loss of the MOSFET and the power dissipation of the

converter. The maximum ripple current occurs at the

maximum input voltage. A good starting point is to choose

the ripple current to be approximately 30% of the maxi-

mum output current.

Once the inductance value has been chosen, select an

inductor that is capable of carrying the required peak cur-

rent without going into saturation. In some types of

inductors, especially core that is made of ferrite, the ripple

current will increase abruptly when it saturates. This will

result in a larger output ripple voltage.

Output Inductor Selection (Cont.)

Output Capacitor Selection

Higher Capacitor value and lower ESR reduce the output

ripple and the load transient drop. Therefore select high

performance low ESR capacitors that are intended for

switching regulator applications. In some applications,

multiple capacitors have to be parallel to achieve the de-

sired ESR value. A small decoupling capacitor in parallel

for bypassing the noise is also recommended, and the

voltage rating of the output capacitors are also must be

considered. If tantalum capacitors are used, make sure

they are surge tested by the manufactures. If in doubt,

consult the capacitors manufacturer.

Input Capacitor Selection

The input capacitor is chosen based on the voltage rating

and the RMS current rating. For reliable operation, select

the capacitor voltage rating to be at least 1.3 times higher

than the maximum input voltage.

The maximum RMS current rating requirement is approxi-

mately I

OUT/2, where IOUT is the load current. During power

up, the input capacitors have to handle large amount of

surge current. If tantalum capacitors are used, make sure

they are surge tested by the manufactures. If in doubt,

consult the capacitors manufacturer. For high frequency

decoupling, a ceramic capacitor 1uF can be connected

between the drain of upper MOSFET and the source of

lower MOSFET.

MOSFET Selection

The selection of the N-channel power MOSFETs are de-

termined by the R

DS(ON), reverse transfer capacitance (CRSS)

and maximum output current requirement. The losses in

the MOSFETs have two components: conduction loss and

transition loss. For the upper and lower MOSFET, the

losses are approximately given by the following :

UPPER=IOUT(1+TC)(RDS(ON))D+(0.5)(IOUT)(VIN)(tSW)FS

LOWER=IOUT(1+TC)(RDS(ON))(1-D)

Where

OUT

is the load current

TC is the temperature dependency of RDS(ON)

is the switching frequency

is the switching interval

D is the duty cycle

Note that both MOSFETs have conduction losses while

the upper MOSFET include an additional transition loss.

The switching internal, t

sw, is a function of the reverse

transfer capacitance C

RSS.

The (1+TC) term is to factor in the temperature depen-

dency of the R

DS(ON) and can be extracted from the “RDS(ON)

vs Temperature” curve of the power MOSFET.

Short Circuit Protection

The APW7158 provides a simple short circuit protection

function, and it is not easy to predict its performance, since

many factors can affect how well it works. Therefore, the

limitations and suggestions of this method must be pro-

vided for users to understand how to work it well.

• The short circuit protection was not designed to work

for the output in initial short condition. In this case, the

short circuit protection may not work, and damage the

MOSFETs. If the circuit still works, remove the short can

cause an inductive kick on the phase pin, and it may

damage the IC and MOSFETs.

• If the resistance of the short is not low enough to

cause protection, the regulator will work as the load has

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