MIC68400_09 Datasheet, PDF (14/15 Pages) Micrel Semiconductor – 4A Sequencing LDO with Tracking and Ramp Control

Micrel, Inc.
Ratiometric Tracking
Ratiometric tracking allows independent ramping
speeds for both regulators so that the regulation
voltage is reached at the same time. This is
accomplished by adding a resistor divider between the
Master output pin and the Slave RC pin. The divider
should be scaled such that the Slave RC pin reaches
or exceeds the target output voltage of the Slave as
the Master reaches its target voltage.
MIC68400
Ratiometric tracking may be used with adjustable
parts by simply connecting the RC pins of the Master
and Slave. Use a single RC capacitor of twice the
normal value (since twice the current is injected into
the single RC cap). Alternatively, fixed parts may use
ratiometric tracking in a manner similar to adjustable
normal tracking, with the tracking divider changed to
the same resistor ratio driving the Master Adj/Sns pin.
Fixed Voltage Devices
Adjustable Voltage Devices
Final Note on Tracking
The MIC68400 does not fully shutdown until the output load is discharged to near zero. If RC is driven from an
external source in a tracking configuration, and the external source does not go to zero on shutdown it may prevent
complete shutdown of the MIC68400. This will cause no damage, but some Q current will remain and may cause
concern in battery operated portable equipment. Also, when RC is driven in tracking mode, pulling EN low will not
cause the output to drop. Maintaining low EN in tracking mode simply means that the MIC68400 will shutdown when
the tracking voltage gets near zero. In no case can the MIC68400 enter the tracking mode unless EN is pulled high.
August 2009
14
M9999-081809-C