MIC22700 datasheet(13/21 Pages) MICREL

13 / 21 page

Micrel, Inc.

MIC22700

July 2009

M9999-072209-B

Feedback

The MIC22700 provides a feedback pin to adjust the

output voltage to the desired level. This pin connects

internally to an error amplifier. The error amplifier then

compares the voltage at the feedback to the internal

0.7V reference voltage and adjusts the output voltage to

maintain regulation. The resistor divider network for a

desired VOUT is given by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

REF

OUT

where VREF is 0.7V and VOUT is the desired output

voltage. A 10kΩ or lower resistor value from the output

to the feedback is recommended since large feedback

resistor values increase the impedance at the feedback

pin, making the feedback node more susceptible to

noise pick-up. A small capacitor (50pF – 100pF) across

the lower resistor can reduce noise pick-up by providing

a low impedance path to ground.

PWM Operation

The MIC22700 is a voltage mode, pulse width

modulation (PWM) controller. By controlling the ratio of

on-to-off time, or duty cycle, a regulated DC output

voltage is achieved. As load or supply voltage changes,

so does the duty cycle to maintain a constant output

voltage. In cases where the input supply runs into a

dropout condition, the MIC22700 will run at 100% duty

cycle.

The MIC22700 provides constant switching at 1MHz with

synchronous internal MOSFETs. The internal MOSFETs

include a high-side P-Channel MOSFET from the input

supply to the switch pin and an N-Channel MOSFET

from the switch pin-to-ground. Since the low-side N-

Channel MOSFET provides the current during the off

cycle, very low power is dissipated during the off period.

PWM control provides fixed frequency operation. By

maintaining a constant switching frequency, predictable

fundamental and harmonic frequencies are achieved.

Other methods of regulation, such as burst and skip

modes, have frequency spectrums that change with load

that

can

interfere

with

sensitive

communication

equipment.

Sequencing and tracking

The MIC22700 provides additional pins to provide

up/down

sequencing

and

tracking

capability

for

connecting multiple voltage regulators together.

Enable/DLY pin

The Enable pin contains a trimmed, 1µA current source

which can be used with a capacitor to implement a fixed

desired delay in some sequenced power systems. The

threshold level for power on is 1.24V with a hysteresis of

20mV.

Delay Pin

The Delay pin also has a 1µA trimmed current source

and a 1µA current sink which acts with an external

capacitor to delay the operation of the Power On Reset

(POR) output. This can be used also in sequencing

outputs in a sequenced system, but with the addition of a

conditional delay between supplies; allowing a first up,

last down power sequence.

After Enable is driven high, VOUT will start to rise (rate

determined by RC capacitor). As the FB voltage goes

above 90% of its nominal set voltage, Delay begins to

rise as the 1µA source charges the external capacitor.

When the threshold of 1.24V is crossed, POR is

asserted high and Delay continues to charge to a

voltage VDD. When FB falls below 90% of nominal, POR

is asserted low immediately. However, if enable is driven

low, POR will fall immediately to the low state and Delay

will begin to fall as the external capacitor is discharged

by the 1µA current sink. When the threshold of VDD-

1.24V is crossed, VOUT will begin to fall at a rate

determined by the RC capacitor. As the voltage change

in both cases is 1.24V, both rising and falling delays are

matched at

−

⋅

DLY

POR

RC pin

The RC pin provides a trimmed 1µA current source/sink

similar to the Delay Pin for accurate ramp up (soft start)

and ramp down control. This allows the MIC22700 to be

used in systems requiring voltage tracking or ratio-metric

voltage tracking at startup.

There are two ways of using the RC pin:

1. Externally driven from a voltage source

2. Externally attached capacitor sets output ramp

up/down rate

In the first case, driving RC with a voltage from 0V to

VREF will program the output voltage between 0 and

100% of the nominal set voltage.

In the second case, the external capacitor sets the ramp

up and ramp down time of the output voltage. The time

is given by

−

⋅

RAMP

where TRAMP is the time

from 0 to 100% nominal output voltage.

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