MIC5191

Micrel

April 2004

10

M9999-042804

ource threshold specified in most MOSFET data sheets

refers to the minimumvoltage needed to fully enhance the

MOSFET. Although for the most part, the MOSFET will be

operating in the linear region and the V

voltage) will be less than the fully enhanced V

recommended the V

V

voltage of the MIC5191 output driver.

V

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

fully enhanced at 4.5V

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 it is 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 regu-

lator 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.

Figure 9. Post Regulator

4

5

3

1

2

VIN

EN/UVLO

CSH

VOUT

FB

LSD

560pF

8.06k

BST

COMP

6

HSD

12

VSW

11

GND

D1

SD103BWS

2.2

µF

10V

0.1

µF

10

8

VDD

7

U1 MIC2198-BML

9

100pF

11.5k

100k

CSH

J2

EN

J1

CSH

10k

10k

10

Ω

10

Ω

1

µF

25V

330

µF

16V

10

µF 10µF

10

µF

10

µF

10

µF

22

µF

D2

1N5819HW

IRF7821

IRF7821

L1

1.8

µH

CDEP134-1R8MC-H

12.4k

330

µF

Tantalum

OUT

VCC1

VCC2

VIN

FB

ISENSE

COMP

GND

10nF

100

Ω

10

Ω

MIC5191

1

µF

12V