MIC68401 Datasheet, PDF (11/13 Pages) Micrel Semiconductor – 4A Sequencing LDO with Tracking and Ramp Control™

Micrel, Inc.
T_POR is the time period from the rising of EN to the
assertion (low-to-high transition) of the POR output, and
is calculated by Equation 3:
T_POR = T_DELAY + T_RC
Eq. 3
Where:
T_RC
=
(0.45V
)_{}
C_RC
1μA

At turn off, the DELAY capacitor is discharged via a 1µA
current sink when the EN pin is driven low.
Ramp Control
When the RC pin voltage is less than 0.5V, the output
voltage is controlled by the voltage on the RC pin. When
the RC pin voltage is greater than 0.5V, the output
voltage is held in regulation by the FB pin. The RC
capacitor is charged by an internal 1µA current source
when the EN pin goes high and discharged by an N-
channel when the EN pin is low and the output voltage is
discharged below 0.1V. The size of the capacitor on the
RC pin will control the slew rate of the output voltage
during startup. The startup slew rate at the FB pin may
be calculated from Equation 4:
T_RC
=
0.5
V

C_RC
1μA

Eq. 4
Ramp Down − Turn Off Slew Rate
When the EN pin goes low, the output will begin to turn
off within 5µs. The time that it takes for the output voltage
to fully discharge to 0V is dependent upon the value of
the output capacitor and the load current. In the absence
of a load, the internal discharge current (I_DISCHARGE) will
assure that the output voltage is discharged to 0V. The
RC pin and the DELAY pin are quickly discharged
through an internal N-channel pull down device when the
EN pin goes low and the output voltage is discharged
below 0.1V.
MIC68401
Sequencing Configurations
Sequencing allows a master device to control the start
and stop timing of a single or multiple Slave MIC68401
devices. In a typical sequencing application, the master
device POR output drives the slave EN inputs. The
sequence begins with the master EN driven high. The
master output ramps up and triggers the master DELAY.
When the master output reaches 90% of its final value,
the master POR is released which enables the Slave
device(s).
Ratiometric Tracking
Ratiometric tracking allows independent ramping rates for
multiple MIC68401 regulators, such that their final output
voltages ramp up, and regulation occurs 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 0.5V when the master reaches its
target output voltage.
Ratiometric tracking can also be accomplished by simply
connecting the RC pins of the master and slave(s)
together. When the RC pins are connected, the current
from each of the MIC68401’s RC pin current sources
adds together. In order to maintain the desired slew rate,
the RC capacitor size needs to be increased by the total
number of RC pins that are connected together. For
example: One master and two slaves will require three
times the RC cap size as compared to a single device.
Final Note on Tracking
The MIC68401 does not fully shutdown until the output
voltage is discharged below 100mV. If the 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 MIC68401. This
will not cause damage to the device, but a higher
quiescent will be drawn from the source. A potential
cause for concern in battery operated portable
equipment. When the RC pin is driven in tracking mode,
pulling EN low will cause the output to turn-off. The
MIC68401 cannot enter tracking mode unless EN is
pulled high.
November 7, 2013
11
Revision 1.0