RT9178
To assure proper operation, the signal source used to
drive the EN pin must be able to swing above and below
the specified turn-on/off voltage thresholds listed in the
“Electrical
Characteristics” under V
IH
and V
IL
.The ON/
OFF signal may comes from either CMOS output, or an
open-collector output with pull-up resistor to the device
input voltage or another logic supply. The high-level voltage
may exceed the device input voltage, but must remain
within the absolute maximum ratings for the EN pin.
Quick Start-Up Time
The start-up time is determined by the time constant of
the bypass capacitor. The smaller the capacitor value, the
shorter the power up time, but less noise gets reduced.
As a result, start-up time and noise reduction need to be
taken into design consideration when choosing the value
of the bypass capacitor.
Input-Output (Dropout) Voltage
A regulator's minimum input-to-output voltage differential
(dropout voltage) determines the lowest usable supply
voltage. In battery-powered systems, this determines the
useful end-of-life battery voltage. Because the device uses
a PMOS, its dropout voltage is a function of drain-to-
source on-resistance, R
DS(ON)
, multiplied by the load
current:
V
DROPOUT
= V
IN
- V
OUT
= R
DS(ON)
×
I
OUT
Current Limit
The RT9178 monitors and controls the PMOS’ gate
voltage, limiting the output current to 400mA (typ). The
output can be shorted to ground for an indefinite period of
time without damaging the part.
Short-Circuit Protection
The device is short circuit protected and in the event of a
peak over-current condition, the short-circuit control loop
will rapidly drive the output PMOS pass element off. Once
the power pass element shuts down, the control loop will
rapidly cycle the output on and off until the average power
dissipation causes the thermal shutdown circuit to
respond to servo the on/off cycling to a lower frequency.
Please refer to the section on thermal information for power
dissipation calculations.
Capacitor Characteristics
It is important to note that capacitance tolerance and
variation with temperature must be taken into
consideration when selecting a capacitor so that the
minimum required amount of capacitance is provided over
the full operating temperature range. In general, a good
tantalum capacitor will show very little capacitance
variation with temperature, but a ceramic may not be as
good (depending on dielectric type).
Aluminum electrolytics also typically have large temperature
variation of capacitance value.
Equally important to consider is a capacitor's ESR change
with temperature: this is not an issue with ceramics, as
their ESR is extremely low. However, it is very important
in Tantalum and aluminum electrolytic capacitors. Both
show increasing ESR at colder temperatures, but the
increase in aluminum electrolytic capacitors is so severe
they may not be feasible for some applications.
Ceramic:
For values of capacitance in the 10μF to 100μF range,
ceramics are usually larger and more costly than tantalums
but give superior AC performance for by-passing high
frequency noise because of very low ESR (typically
less than 10mΩ). However, some dielectric types do not
have good capacitance characteristics as a function of
voltage and temperature.
Z5U and Y5V dielectric ceramics have capacitance that
drops severely with applied voltage. A typical Z5U or Y5V
capacitor can lose 60% of its rated capacitance with
half of the rated voltage applied to it. The Z5U and Y5V
also exhibit a severe temperature effect, losing more than
50% of nominal capacitance at high and low limits of the
temperature range.
X7R and X5R dielectric ceramic capacitors are strongly
recommended if ceramics are used, as they typically
maintain a capacitance range within
±
20% of nominal
over full operating ratings of temperature and voltage. Of
course, they are typically larger and more costly than Z5U/
Y5U types for a given voltage and capacitance.
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DS9178-16 April 2008
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
