10

loop is operating open circuit and the output voltage varies

with the output load resistance, Ro, as given by:

Where Vin is the 5V Main voltage, Dmax = 0.33, L is the

value of the boost inductor, L3, and F = 100kHz. This

provides automatic output current limiting. When the

maximum duty cycle has been reached and for a given

inductor, a further reduction in Ro by one-half will pull the

output voltage down to 0.707 of nominal and cause an

under-voltage condition.

The 12V converter starts to operate at the same time as the

5V Main converter. The rising voltage on the 5V Main output

and the 33% duty cycle limit provides a similar soft-start, as

the 5V Main, for the 12V output.

3V ALWAYS, 5V ALWAYS Linear

Regulators

The 3.3V ALWAYS and 5V ALWAYS outputs are derived

from the battery voltage and are the first voltages available

in the notebook when power on is initiated. The 5V ALWAYS

output is generated directly from the battery voltage by a

linear regulator. It is used to power the system micro-

controller and to internally power the chip and the gate

drivers. The 3.3V ALWAYS output is generated from the 5V

ALWAYS output and may be used to power the keyboard

controller or other peripherals. The combined current

capability of these outputs is 50mA. When the 5V Main

output is greater than it’s undervoltage level, it is switched to

the 5V ALWAYS output via an internal 1.3

switch. Simultaneously, the 5V ALWAYS linear regulator is

disabled to prevent excessive power dissipation.

The rise time of the 5V ALWAYS is determined by the value

of the output capacitance on the 5V and 3.3V ALWAYS

outputs. The internal regulator is current limited to about

180mA, so the start up time is approximately:

3.3V ALWAYS outputs.

Power Good Status

The IPM6220 monitors all the output voltages except for the

3.3V ALWAYS. A single power-good signal, PGOOD, is

issued when soft-start is completed and all monitored

outputs are within 10% of their respective set points. After

the soft-start sequence is completed, undervoltage

protection latches the chip off when any of the monitored

outputs drop below 75% of its set point.

A ‘soft-crowbar’ function is implemented for an overvoltage

on the 3.3V Main or 5V Main outputs. If the output voltage

goes above 115% of their nominal output level, the upper

MOSFET is turned off and the lower MOSFET is turned on.

This ‘soft-crowbar’ condition will be maintained until the

output voltage returns to the regulation window and then

normal operation will continue.

This ‘soft-crowbar’ and monitoring of the output, prevents the

output voltage from ringing negative as the inductor current

flows in the ‘reverse’ direction through the lower MOSFET

and output capacitors.

Over-Temperature Protection

The IC incorporates an over-temperature protection circuit

that shuts all the outputs down when the die temperature

Component Selection Guidelines

Output Capacitor Selection

The output capacitors for each output have unique

requirements. In general, the output capacitors should be

selected to meet the dynamic regulation requirements

including ripple voltage and load transients.

3.3V Main and 5V Main PWM Output Capacitors

Selection of the output capacitors is also dependent on the

output inductor so some inductor analysis is required to

select the output capacitors.

One of the parameters limiting the converter’s response to a

load transient is the time required for the inductor current to

slew to it’s new level. Given a sufficiently fast control loop

design, the IPM6220 will provide either 0% or 94% duty

cycle in response to a load transient. The response time is

the time interval required to slew the inductor current from an

initial current value to the load current level. During this

interval the difference between the inductor current and the

transient current level must be supplied by the output

capacitor(s). Minimizing the response time can minimize the

output capacitance required. Also, if the load transient rise

time is slower than the inductor response time, as in a hard

drive or CD drive, this reduces the requirement on the output

capacitor.

The maximum capacitor value required to provide the full,

rising step, transient load current during the response time of

the inductor is:

the drop in output voltage allowed during the load transient.

High frequency capacitors initially supply the transient

current and slow the load rate-of-change seen by the bulk

capacitors. The bulk filter capacitor values are generally

Vo

Vin

Dmax

×

Ro

2LxF

()

-------------------





=

tC

OUT

5V

180mA

-------------------

×

=

C

OUT

L

O

I

TRAN

×

V

IN

V

OUT

–

() 2

×

----------------------------------------------

I

TRAN

DV

OUT

--------------------

×

=

IPM6220