MIC2800-ADJSLASH1.2SLASH3.3YML datasheet(18/20 Pages) MICREL

18 / 20 page

Micrel, Inc.

MIC2800

September 2007

M9999-090507-C

The switch (SW) pin connects directly to the inductor

and provides the switching current necessary to operate

in PWM mode. Due to the high speed switching on this

pin, the switch node should be routed away from

sensitive nodes.

PGND

Power ground (PGND) is the ground path for the high

current PWM mode. The current loop for the power

ground should be as small as possible. Refer to the

layout considerations for more details.

SGND

Signal ground (SGND) is the ground path for the biasing

and control circuitry. The current loop for the signal

ground should be as small as possible. Refer to the

layout considerations for more details.

CSET

The SET pin is a current source output that charges a

capacitor that sets the delay time for the power-on reset

output from low to high. The delay for POR high to low

(detecting an undervoltage on any of the outputs) is

always minimal. The current source of 1.25µA charges a

capacitor up from 0V. When the capacitor reaches

1.25V, the output of the POR is allowed to go high. The

delay time in micro seconds is equal to the Cset in

picofarads.

POR Delay (µs) = CSET (pF)

CBYP

The internal reference voltage can be bypassed with a

capacitor to ground to reduce output noise and increase

power supply rejection (PSRR). A quick-start feature

allows for quick turn-on of the output voltage. The

recommended nominal bypass capacitor is 0.1µF, but it

can be increased, which will also result in an increase to

the start-up time.

Output Capacitor

LDO1 and LDO2 outputs require a 2.2µF ceramic output

capacitor for stability. The DC/DC switch mode regulator

requires a 2.2µF ceramic output capacitor to be stable.

All output capacitor values can be increased to improve

transient response, but performance has been optimized

for a 2.2µF ceramic on the LDOs and the DC/DC.

X7R/X5R

dielectric-type

ceramic

capacitors

are

recommended

because

their

temperature

performance. X7R-type capacitors change capacitance

by 15% over their operating temperature range and are

the most stable type of ceramic capacitors. Z5U and

Y5V dielectric capacitors change value by as much as

50%

60%

respectively

over

their

operating

temperature ranges.

Input Capacitor

A minimum 1µF ceramic is recommended on the VIN pin

for bypassing. X5R or X7R dielectrics are recommended

for the input capacitor. Y5V dielectrics lose most of their

capacitance over temperature and are therefore, not

recommended. A minimum 1µF is recommended close

to the VIN and PGND pins for high frequency filtering.

Smaller case size capacitors are recommended due to

their lower ESR and ESL. Please refer to layout

recommendations for proper layout of the input

capacitor.

Inductor Selection

The MIC2800 is designed for use with a 2.2µH inductor.

Proper selection should ensure the inductor can handle

the maximum average and peak currents required by the

load. Maximum current ratings of the inductor are

generally given in two methods; permissible DC current

and saturation current. Permissible DC current can be

rated either for a 40°C temperature rise or a 10% to 20%

loss in inductance. Ensure that the inductor selected can

handle the maximum operating current. When saturation

current is specified, make sure that there is enough

margin that the peak current will not saturate the

inductor. Peak inductor current can be calculated as

follows:

OUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

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