MIC2827 datasheet(13/27 Pages) MICREL

13 / 27 page

Micrel, Inc.

MIC2827

July 2009

M9999-072709-A

Functional Description – Power Control

and Sequencing

Two Types of Part: Sequence-Enabled and No-

Sequence

•

Sequence-Enabled

parts

support

automatic

sequencing of the three supplies. Sequence-

Enabled parts all have a default sequence (activated

by asserting the EN pin). These parts also allow

sequencing to be disabled.

While very flexible, sequence-enabled parts require

more care in operation. See the later section

“Ensuring Clean Switching in Sequence-Enabled

Parts”.

•

No-Sequence parts have no built-in sequencing

capability. Their default startup turns on only one

supply, which requires no sequencing. If the host

needs more supplies to come on, this can be

accomplished with I²C writes which allows a

sequence activated by software to be performed.

Power-up State

When battery power is first applied to the MIC2827, all

I²C registers are loaded with their default (POR) values.

If EN is high, a default startup is executed; otherwise,

the part remains in a quiescent state waiting to be

started by EN or an I²C command.

Enable Pin-Initiated Default Startup

When EN is asserted, a default startup is executed. This

is defined below:

•

The voltage registers are loaded with their default

values.

•

In sequence-enabled parts, the Sequence Control

bit is set to low (to allow sequencing to occur). No-

sequence parts always have zero for the Sequence

Control bit

•

The correct set of supply enable bits is loaded into

the Enable Register, and the appropriate sequence

is then executed.

•

The Power-On After Fault (POAF) bit is set to its

default state, high.

Turning on the Power Supplies

After power is applied, the MIC2827 offers two methods

of turning the three supply outputs on and off:

1. Default startup sequencing or shutdown via the

EN pin;

2. Flexible startup sequencing or shutdown via the

I²C interface

Power-Up via the EN Pin

The EN pin is transition sensitive and not level sensitive

(with the exception of hot enable—please see the

description below). If the EN pin is toggled low-to-high,

the MIC2827 will execute the default startup sequence.

During the startup sequence, the appropriate set of

supply enables is loaded into the Enable Register. This

allows the part to present a consistent interface to the

I²C host; if the host reads the Enable Control register, it

will see one or more enables on, which is consistent with

one or more active supplies.

Individual control of the supplies is now possible via the

I²C interface.

“Hot Enable” Startup

Some systems may choose to tie the EN pin to DVIN, so

that the MIC2827 registers an active EN pin as it

completes power-on. This is perfectly legal and

produces a default startup immediately after power is

applied. Depending on the rise time of the input power

being applied, the UVLO flag may be set.

Power-Down via the EN Pin

If the EN pin is toggled high-to-low, then the MIC2827

will shut down all outputs simultaneously. For reasons

similar to those above, at the conclusion of the shutdown

sequence, all three individual supply enables will be

clear in the Enable Control register and the bias will be

switched off.

If the MIC2827 startup is initiated by asserting EN and

later shutdown is initiated by clearing the Enable

currents disabled) but EN will still be high. In this case,

de-asserting EN will have no effect, since the part has

already completed its shutdown.

Power-Up and Power-Down via the Enable Register

The three individual power supply enable bits in the

Enable Register (LDO2-EN, LDO1-EN, and DC-EN) may

be used to enable and disable individual supplies. If the

part is sequenced-enabled, and sequencing is permitted

by the Sequence Control bit, enabled supplies are

turned on in sequence. Any disabled outputs will not

participate in the sequence and will be ignored.

See also the “Ensuring Clean Switching in Sequence-

Enabled Parts” section.

Under no circumstances should the EN and I²C control

be used simultaneously.

The results would not be

deterministic.

If a supply output is enabled and its Voltage Control

changes immediately at the I²C acknowledge.

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