NCP5173 Ver la hoja de datos (PDF) - ON Semiconductor

Número de pieza

componentes Descripción

Fabricante

NCP5173

1.5 A 560 kHz−1.0 MHz Boost Regulator

ON Semiconductor 

NCP5173

The internal control circuitry, including the oscillator and

linear regulator, requires a small amount of power even

when the switch is turned off. The specifications section of

this datasheet reveals that the typical operating current, IQ,

due to this circuitry is 5.5 mA. Additional guidance can be

found in the graph of operating current vs. temperature. This

graph shows that IQ is strongly dependent on input voltage,

VIN, and temperature. Then:

PBIAS + VINIQ

Since the onboard switch is an NPN transistor, the base

drive current must be factored in as well. This current is

drawn from the VIN pin, in addition to the control circuitry

current. The base drive current is listed in the specifications

as DICC/DISW, or switch transconductance. As before, the

designer will find additional guidance in the graphs. With

that information, the designer can calculate:

PDRIVER + VINISW

ICC

DISW

D

where:

ISW = the current through the switch;

D = the duty cycle or percentage of switch on−time.

ISW and D are dependent on the type of converter. In a

boost converter,

ISW(AVG) ^ ILOAD

D

1

Efficiency

D

^

VOUT * VIN

VOUT

In a flyback converter,

ISW(AVG)

^

VOUTILOAD

VIN

1

Efficiency

D

^

VOUT

VOUT

)

NS

NP

VIN

The switch saturation voltage, V(CE)SAT, is the last major

source of on−chip power loss. V(CE)SAT is the

collector−emitter voltage of the internal NPN transistor

when it is driven into saturation by its base drive current. The

value for V(CE)SAT can be obtained from the specifications

or from the graphs, as “Switch Saturation Voltage.” Thus,

PSAT ^ V(CE)SATISW D

Finally, the total on−chip power losses are:

PD + PBIAS)PDRIVER)PSAT

Power dissipation in a semiconductor device results in the

generation of heat in the junctions at the surface of the chip.

This heat is transferred to the surface of the IC package, but

a thermal gradient exists due to the resistive properties of the

package molding compound. The magnitude of the thermal

gradient is expressed in manufacturers’ data sheets as qJA,

or junction−to−ambient thermal resistance. The on−chip

junction temperature can be calculated if qJA, the air

temperature near the surface of the IC, and the on−chip

power dissipation are known.

TJ + TA)(PDqJA)

where:

TJ = IC or FET junction temperature (°C);

TA = ambient temperature (°C);

PD = power dissipated by part in question (W);

qJA = junction−to−ambient thermal resistance (°C/W).

For the NCP5173, qJA = 35°C/W.

Once the designer has calculated TJ, the question of

whether the IC can be used in an application is settled. If TJ

exceeds 150°C, the absolute maximum allowable junction

temperature, the NCP5173 is not suitable for that

application.

If TJ approaches 150°C, the designer should consider

possible means of reducing the junction temperature.

Perhaps another converter topology could be selected to

reduce the switch current. Increasing the airflow across the

surface of the chip might be considered to reduce TA. A

copper “landing pad” can be connected to the ground pin −

Designers are referred to ON Semiconductor Application

Note AND8036/D for more information on properly sizing

a copper area.

Circuit Layout Guidelines

In any switching power supply, circuit layout is very

important for proper operation. Rapidly switching currents

combined with trace inductance generates voltage

transitions that can cause problems. Therefore the following

guidelines should be followed in the layout.

1. In boost circuits, high AC current circulates within the

loop composed of the diode, output capacitor, and

on−chip power transistor. The length of associated

traces and leads should be kept as short as possible. In

the flyback circuit, high AC current loops exist on both

sides of the transformer. On the primary side, the loop

consists of the input capacitor, transformer, and

on−chip power transistor, while the transformer,

rectifier diodes, and output capacitors form another

loop on the secondary side. Just as in the boost circuit,

all traces and leads containing large AC currents

should be kept short.

2. Separate the low current signal grounds from the

power grounds. Use single point grounding or ground

plane construction for the best results.

3. Locate the voltage feedback resistors as near the IC as

possible to keep the sensitive feedback wiring short.

Connect feedback resistors to the low current analog

ground.

http://onsemi.com

15

Share Link: