MIC2172 datasheet(13/20 Pages) MICREL

13 / 20 page

Micrel

MIC2172/3172

April 2006

M9999-041806

(408) 955-1690

clamp to the COMP output (figure 9). This feature can be

useful in applications requiring either a complete

shutdown of Q1’s switching action or a form of current

fold-back limiting. This use of the COMP output does not

disable the oscillator, amplifiers or other circuitry,

therefore

the

supply

current

never

less

than

approximately 5mA.

Thermal Management

Although the MIC2172/3172 family contains thermal

protection circuitry, for best reliability, avoid prolonged

operation with junction temperatures near the rated

maximum.

The

junction

temperature

determined

first

calculating the power dissipation of the device. For the

MIC2172/3172, the total power dissipation is the sum of

the device operating losses and power switch losses.

The

device

operating

losses

are

the

losses

associated with biasing all of the internal functions plus

the losses of the power switch driver circuitry. The dc

losses are calculated from the supply voltage (VIN) and

device supply current (IQ). The MIC2172/3172 supply

current is almost constant regardless of the supply

voltage (see “Electrical Characteristics”). The driver

section losses (not including the switch) are a function of

supply voltage, power switch current, and duty cycle.

() ()

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

0.004

driver

bias

where:

P(bias+driver) = device operating losses

VIN = supply voltage

IQ = quiescent supply current

ISW = power switch current

(see “Design Hints: Switch Current Calculations”)

δ = duty cycle

OUT

VOUT = output voltage

VF = D1 forward voltage drop

As a practical example refer to figure 1.

VIN = 5.0V

IQ = 0.006A

ISW = 0.625A

δ = 60% (0.6)

Then:

() ()

() 0.068W

0.6

0.004

0.625

0.006

driver

bias

driver

bias

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

Power

switch

dissipation

calculations

are

greatly

simplified by making two assumptions which are usually

fairly accurate. First, the majority of losses in the power

switch are due to on-losses. To find these losses, assign

a resistance value to the collector/emitter terminals of

the device using the saturation voltage versus collector

current

curves

(see

Typical

Performance

Characteristics). Power switch losses are calculated by

modeling the switch as a resistor with the switch duty

cycle modifying the average power dissipation.

PSW = (ISW)

2 R

SW δ

From the Typical performance Characteristics:

RSW = 1Ω

Then:

PSW = (0.625)

2 × 1 × 0.6

PSW = 0.234W

P(total) = 0.068 + 0.234

P(total) = 0.302W

The junction temperature for any semiconductor is

calculated using the following:

TJ = TA + P(total) θJA

Where:

TJ = junction temperature

TA = ambient temperature (maximum)

P(total) = total power dissipation

JA = junction to ambient thermal resistance

For the practical example:

TA = 70

°C

JA = 130

°C/W (for plastic DIP)

Then:

TJ = 70 + 0.30

⋅ 130

TJ = 109

°C

This junction temperature is below the rated maximum of

150

°C.

Grounding

Refer to figure 10. Heavy lines indicate high current

paths.

MIC2172 Datasheet(PDF) 13 Page - Micrel Semiconductor

MIC2172 Datasheet(HTML) 13 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL