MIC61300 datasheet(13/22 Pages) MICREL

13 / 22 page

Micrel, Inc.

MIC61300

September 2010

M9999-092910-A

Adjustable Regulator Design

The MIC61300 adjustable version allows programming

the output voltage from 0.5V to 3.0V by placing a resistor

divider network (R1, R2) from VOUT to GND (see

Application Circuit). The high side of R1 should be

connected at the point-of-load for high-accuracy Kelvin

sensing. VOUT is determined by the following equation:

⎟

⎠

⎞

⎜

⎝

⎛

0.5

VOUT

Eq. 1

where VOUT is the desired output voltage.

The resistor (R2) value between the FB pin and GND is

selected to maintain a minimum 10mA load on the

output.

The resistor values are calculated from the previous

equation, resulting in the following:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

0.5

OUT

Eq. 2

Table 1 is a list of resistor combinations to set the output

voltage. A 1% tolerance is recommended for both R1

and R2. For a unity gain, 0.5V output voltage, connect

the FB pin directly to the output.

VOUT

0.5V

−

49.9Ω

0.6V

10.0Ω

49.9Ω

0.7V

20.0Ω

49.9Ω

0.8V

30.1Ω

49.9Ω

0.9V

40.2Ω

49.9Ω

1.0

49.9Ω

1.1V

60.4Ω

49.9Ω

1.2V

69.8Ω

49.9Ω

1.5V

100Ω

49.9Ω

1.8V

130Ω

49.9Ω

2.2V

169Ω

49.9Ω

Table 1. Resistor Selection for Specific VOUT

Thermal Design

Linear regulators are simple to use. The most

complicated design parameters to consider are thermal

characteristics. To help reduce the thermal resistance,

the ePad (underneath the IC) should be soldered to the

PCB ground and the placement of thermal vias either

underneath or near the ePad is highly recommended.

Thermal design requires the following application-

speciﬁc parameters:

• Maximum ambient temperature (TA)

• Output current (IOUT)

• Output voltage (VOUT)

• Input voltage (VIN)

• Ground current (IGND)

First, calculate the power dissipation of the regulator

from these numbers and the device parameters from this

datasheet:

PD = (VIN - VOUT) × IOUT + (VIN × IGND)

Eq. 3

where the ground current is approximated by using

numbers from the Electrical Characteristics or Typical

Characteristics

sections

For example, given an expected maximum ambient

temperature (TA) of 75°C with VIN = 1.2V, VOUT = 0.9V,

and IOUT = 1.5A, first calculate the expected PD using

Equation 1:

PD = (1.2V – 0.9V) × 1.5A + 1.2V × 0.015A

= 0.468W

Eq. 4

Next, determnine the junction temperature for the

expected power dissipation above using the thermal

resistance (θJA) of the 10-pin 3mm × 3mm MLF

® (YML)

adhering to the following criteria for the PCB design:

1oz. copper and 100mm

2 copper area for the

MIC61300.

TJ = (θJA × PD) + TA

= (60.7

°C/W × 0.468W) + 75°C

= 103.4

°C

Eq. 5

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