MMM5062 datasheet(13/22 Pages) FREESCALE

13 / 22 page

Micrel, Inc.

MIC22950

February 2010

M9999-021910-B

Alternatively, the Figure 3 can be used as a visual guide:

Switching Frequency

vs. CF Capacitance

250

500

750

1000

1250

1500

1750

2000

2250

250

500

750

1000

CF CAPACITANCE (pF)

Figure 3. Switching Frequency vs. CF Capacitance

This pin should not be over-ridden using an external

clock because the trigger pulses generated when the CF

pin reaches 1V are utilized internally.

Sequencing and Tracking

The MIC22950 provides additional pins to provide

up/down

sequencing

and

tracking

capability

for

connecting multiple voltage regulators together.

EN/DLY Pin

The EN/DLY 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/PG)

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 EN pin is driven high, VOUT will start to rise (rate

determined by RC pin capacitor). As the FB pin voltage

goes above 90% of its nominal set voltage, DELAY pin

begins to rise as the 1µA source charges the external

capacitor. When the threshold of 1.24V is crossed,

POR/PG is asserted high and DELAY continues to

charge to a voltage SVIN. When FB falls below 90% of

nominal, POR/PG is asserted low immediately.

However, if EN pin is driven low, POR/PG will fall

immediately to the low state and DELAY pin will begin to

fall as the external capacitor is discharged by the 1µA

current sink. When the threshold of (VTP +1.24V)-1.24V

is crossed (VTP is internal voltage clamp, VTP ≅ 0.9V),

VOUT will begin to fall at a rate determined by the RC pin

capacitor. As the voltage change in both cases is 1.24V,

both rising and falling delays are matched at:

DELAY

POR

1.24

−

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 MIC22950

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 pin 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 rate of the output voltage. The rate is

given by

RAMP

0.7

−

where TRAMP is the time

from 0% to 100% nominal output voltage. RC pin cannot

be left floating. Use a minimum capacitor value of 120p

for larger.

Tracking and Sequencing Examples

There are four distinct variations which are easily

implemented using the MIC22950. The two Sequencing

variations are Delayed and windowed. The two tracking

variants are ratio Metric and Normal. The following

diagrams

illustrate

methods

for

connecting

two

MIC22950’s to achieve these requirements.

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