ADP1147AR-3.3 データシートの表示（PDF） - Analog Devices

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ADP1147AR-3.3

High Efficiency Step-Down Switching Regulator Controllers

Analog Devices 

ADP1147-3.3/ADP1147-5

The formula used to calculate the continuous operating fre-

quency is:

f = 1−VVOIUNT++VVDD

tOFF

tOFF

= 1.3 × 104

× CT

×V REG

V OUT

VREG is the value of the desired output voltage. VOUT is the ac-

tual measured value of the output voltage. When in regulation

VREG/VOUT is equal to 1. The switching frequency of the ADP1147

decreases as the input voltage decreases. The ADP1147 will

reduce the tOFF time by increasing the discharge current in ca-

pacitor CT if the input to output voltage differential falls below

1.5 volts. This is to eliminate the possible occurrence of audible

switching prior to dropout.

Now that the operating frequency has been determined and the

value selected for CT, the required inductance for inductor L

can be computed. The inductor L should be chosen so it will

generate no more than 25 mV/RSENSE of peak-to-peak inductor

ripple current.

The following equation is used to determine the required value

for inductor L:

25 mV = (VOUT +V D ) × tOFF or

RSENSE

LMIN

LMIN

= (VOUT

+V D ) × tOFF

25 mV

× RSENSE

Substituting for tOFF above gives the minimum required induc-

tor value of:

LMIN = 5.1 × 105× RSENSE × CT × VREG

The ESR requirements for the output storage capacitor can be

relaxed by increasing the inductor value, but efficiency due to

copper losses will be reduced. Conversely, the use of too low an

inductance may allow the inductor current to become discon-

tinuous, causing the device to enter the power savings mode

prematurely. As a result of this the power savings threshold is

lowered and the efficiency at lower current levels is severely

reduced.

Inductor Core Considerations

Now that the minimum inductance value for L has been deter-

mined, the inductor core selection can be made. High efficiency

converters generally cannot afford the core losses found in low

cost powdered iron cores. This forces the use of a more expen-

sive ferrite, molypermalloy, or Kool Mu® cores. The typical

efficiency in Figure 1 reflects the use of a molypermalloy core.

The cost of the inductor can be cut in half by Using a Kool Mu

core type CTX 50-4 by Coiltronics, but the efficiency will be

approximately 1%–2% less. The actual core losses are not de-

pendent on the size of the core, but on the amount of induc-

tance. An increase in inductance will yield a decrease in the

amount of core loss. Although this appears to be desirable, more

inductance requires more turns of wire with added resistance

and greater copper losses.

Kool Mu is a registered trademark of Magnetics, Inc.

Using a ferrite cores in a design can produce very low core

losses, allowing the designer to focus on minimizing copper loss

and core saturation problems. Ferrite cores exhibit a condition

known as “Hard Saturation,” which results in an abrupt collapse

of the inductance when the peak design current is exceeded.

This causes the inductor ripple current to rise sharply, the out-

put ripple voltage to increase and the power savings mode of

operation to be erroneously activated. To prevent this from

occurring the core should never be allowed to saturate.

Molypermalloy (from Magnetics, Inc.) is a very good, low loss

core material for a toroids, but is more expensive than a ferrite

core. A reasonable compromise between price and performance,

from the same manufacturer is Kool Mu. Toroidal cores are

extremely desirable where efficient use of available space and

several layers of wire are required. They are available in various

surface mount configurations from Coiltronics Inc. and other

companies.

Power MOSFET Selection and Considerations

The ADP1147 requires the use of an external P-channel

MOSFET. The major parameters to be considered when select-

ing the power MOSFET are the threshold voltage VGS(TH) and

the on resistance of the device RDS(ON).

The minimum input voltage determines if the design requires a

logic level or a standard threshold MOSFET. In applications

where the input voltage is > 8 volts, a standard threshold

MOSFET with a VGS(TH) of < 4 volts can be used. In designs

where VIN is < 8 volts, a logic level MOSFET with a VGS(TH) of

< 2.5 volts is recommended. Note: If a logic level MOSFET

is selected, the supply voltage to the ADP1147 must not

exceed the absolute maximum for the VGS of the MOSFET

(e.g., < ± 8 volts for IRF7304).

The RDS(ON) requirement for the selected power MOSFET is

determined by the maximum output current (IMAX). An as-

sumption is made that when the ADP1147 is operating in the

continuous mode, either the Schottky Diode or the MOSFET

are always conducting the average load current. The following

formulas are used to determine the duty cycle of each of the

components.

P − Channel MOSFET Duty Cycle =VOUT +V D

V IN +V D

Schottky Diode Duty Cycle = V IN –V D

V IN +V D

Once the Duty Cycle is known, the RDS(ON) requirement for the

Power MOSFET can be determined by:

RDS(ON )

=

(V OUT

(V IN +V D ) × PP

+V D ) × I MAX 2 × (1+ δP

)

where PP is the max allowable power dissipation and where δP is

the temperature dependency of RDS(ON) for the MOSFET. Effi-

ciency and thermal requirements will determine the value of PP,

(refer to Efficiency section). MOSFETS usually specify the 1+ δ

as a normalized RDS(ON ) vs. temperature trace, and δ can be

approximated to 0.007/°C for most low voltage MOSFETs.

Output Diode Considerations

When selecting the output diode careful consideration should be

given to peak current and average power dissipation so the

maximum specifications for the diode are not exceeded.

–8–

REV. 0

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