MIC68400 datasheet(13/15 Pages) MICREL

13 / 15 page

Micrel, Inc.

MIC68400

September 2007

M9999-090607-C

Tracking Configurations

Normal Tracking

In normal tracking the Slave RC pin is driven from the

Master output. The internal control buffering assures

that the output of the Slave is always slightly above

the Master to guarantee that the Slave properly

regulates (based on its own internal reference) if

Master and Slave are both fixed voltage devices of the

same output voltage. The schematic and plot below

show a 1.2 volt device tracking a 1.8 volt device

through the entire turn-on / turn-off sequence. Note

that since the RC pin will overdrive the target voltage

(to assure proper regulation) the ramp down delay is

longer than the POR delay during turn-on.

Fixed Voltage Devices

Fixed voltage versions of MIC68400 have two internal

voltage dividers: one for setting the output voltage and

the other for driving the tracking circuitry. Adjustable

parts have up to two external dividers: one from

output to SNS (to set the output voltage) and one from

the output to the Slave RC pin (in tracking

configurations). Also, the RC pin in fixed parts

operates at the same voltage as the output, whereas

the RC pin in adjustable parts operates at the 0.5V

reference. To setup a normal tracking configuration,

the divider driving the Slave RC pin is the same ratio

(or nearly the same – if both Master and Slave are set

to the same output voltage, the Slave RC divider

should be adjusted 2% to 4% higher) as the divider

driving the Slave SNS pin. This is shown below.

Adjustable Voltage devices

MIC68400 Datasheet(PDF) 13 Page - Micrel Semiconductor