APW7079 Ver la hoja de datos (PDF) - Anpec Electronics

Número de pieza

componentes Descripción

Fabricante

APW7079

Low-Supply-Current Synchronous Step-up DC-DC Converter

Anpec Electronics 

APW7079

Application Information (Cont.)

ILX

TJ = TA + TR

ILIM

where

IPEAK

T = the ambient temperature.

A

IIN

The power dissipation can be calculated as below:

PD = POUT x (1-η)/η

where

ISWN

POUT = Output power (VOUT x IOUT)

η = Efficiency

As an example, the APW7079-18 converts an input volt-

age 1.2V to provide a load current of 175mA at ambient

temperature of 85°C. Assume the efficiency (η) is 0.75.

Therefore, the power dissipated on the converter is:

ISWP

PD = 1.8 x 0.175 x (1-0.75)/0.75= 0.105 Watt

IOUT

Since the power dissipation includes the loss of external

VOUT

VOUT

IPEAK x ESR

components, the actual value is slightly lower. For the

SOT-89 package, the θJA is 180°C/W. Thus, the junction

temperature of the regulator is as below:

TJ = 85°C + (PD)(180) = 104 °C

Thermal Consideration

In most applications, the APW7079 does not dissipate

much heat due to its high efficiency. However, in applica-

tions where the APW7079 is running at high ambient tem-

perature with low output voltage, the heat dissipated may

exceed the maximum junction temperature of the part. If

the junction temperature reaches approximately 150°C,

both power switches will be turned off and the LX node

will become high impedance. To avoid the APW7079 from

exceeding the maximum junction temperature, the user

will need to do some thermal analysis. The goal of the

thermal analysis is to determine whether the power dis-

sipated exceeds the maximum junction temperature of

the part. The temperature rise is given by:

TR = (PD)(θJA)

The maximum junction temperature should be less than

125°C. Note that, the junction temperature is lower at

higher output voltages due to reduced switch resistance.

Layout Consideration

For all switching power supplies especially with high peak

currents and switching frequency, the layout is an impor-

tant step in the design. If the layout is not carefully done,

the regulator may show noise problems and duty cycle

jitter.

1.The input capacitor should be placed close to the

device, which can reduce copper trace resistance and

effect input ripple of the IC.

2.The inductor should be placed as close as possible to

the switch pin to minimize the switching noise.

3.The output capacitor should be place closed to the

VOUT and the GND.

where PD is the power dissipated by the regulator and θJA

is the thermal resistance from the junction of the die to

the ambient temperature. The junction temperature, T ,

J

is given by:

Copyright © ANPEC Electronics Corp.

10

Rev. A.3 - Oct., 2008

www.anpec.com.tw

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