MIC5191BML datasheet(11/15 Pages) MICREL

11 / 15 page

Micrel, Inc.

MIC5191

December 2006

M9999-122206

Referring to Application Hint 17, Designing PCB Heat

Sinks

, the minimum amount of copper area for a D

2pack

at 28.8°C/W is 2750mm

2 (or 0.426in2). The solid line

denotes convection heating only (2 oz. copper) and the

dotted line shows thermal resistance with 250LFM air-

flow. The copper area can be significantly reduced by

increasing airflow or by adding external heat sinks.

Figure 8. PC Board Heat Sink

Another important characteristic is the amount of gate

capacitance. Large gate capacitance can reduce

transient performance by reducing the ability of the

MIC5190 to slew the gate. It is recommended that the

MOSFET used has an input capacitance <10nF (CISS).

Source threshold specified in most MOSFET data sheets

refers to the minimum voltage needed to fully enhance

the MOSFET. Although for the most part, the MOSFET

will be operating in the linear region and the VGS (gate-

source voltage) will be less than the fully enhanced VGS,

it is recommended the VCC voltage has 2V over the

minimum VGS and output voltage. This is due to the

saturation voltage of the MIC5191 output driver.

VCC1, 2 ≥ 2V + VGS + VOUT

For our example, with a 1.5V output voltage, our

MOSFET is fully enhanced at 4.5VGS, our VCC voltage

should be greater or equal to 8V.

Input Capacitor

Good input bypassing is important for improved perfor-

mance. Low ESR and low ESL input capacitors reduce

both the drain of the N-Channel MOSFET, as well as the

source impedance to the MIC5191. When a load

transient on the output occurs, the load step will also

appear on the input. Deviations on the input voltage will

be reduced by the MIC5191’s PSRR, but nonetheless

appear on the output. There is no minimum input

capacitance,

but

for

optimal

performance

recommended that the input capacitance be equal to or

greater than the output capacitance.

Output Capacitor

The MIC5191 is stable with any type or value of output

capacitor (even without any output capacitor!). This

allows the output capacitor to select which parameters of

the regulator are important. In cases where transient

response is the most important, low ESR and low ESL

ceramic capacitors are recommended. Also, the more

capacitance on the output, the better the transient

response.

VIN

EN/UVLO

CSH

VOUT

LSD

560pF

8.06k

BST

COMP

HSD

VSW

GND

SD103BWS

2.2µF

10V

0.1µF

VDD 7

U1 MIC2198-BML

100pF

11.5k

100k

VOUT

CSH

+VIN

VIN

CSH

VOUT

10k

1µF

25V

330µF

16V

10µF

22µF

1VOUT @10A

1N5819HW

IRF7821

1.8µH

CDEP134-1R8MC-H

12.4k

330µF

Tantalum

OUT

VCC1

VCC2

VIN

ISENSE

COMP

GND

10nF

MIC5191

1µF

VOUT

12V

Figure 9. Post Regulator

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