RT9179A Ver la hoja de datos (PDF) - Richtek Technology
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RT9179A Datasheet PDF : 11 Pages
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Application Information
Like any low-dropout regulator, the RT9179A requires input
and output decoupling capacitors. These capacitors must
be correctly selected for good performance (see Capacitor
Characteristics Section). Please note that linear regulators
with a low dropout voltage have high internal loop gains
which require care in guarding against oscillation caused
by insufficient decoupling capacitance.
Input Capacitor
An input capacitance of ≅1μF is required between the device
input pin and ground directly (the amount of the capacitance
may be increased without limit).
There are no requirements for the ESR on the input
capacitor, but tolerance and temperature coefficient must
be considered when selecting the capacitor to ensure the
capacitance will be ≅1μF over the entire operating
temperature range.
Output Capacitor
The RT9179A is designed specifically to work with very
small ceramic output capacitors. The recommended
minimum capacitance is 3.3μF ceramic or tantalum
capacitor between LDO output and GND for stability. But
for output voltage lower than 1.35V, to use a minimum of
3.3μF tantalum or electrolyte capacitor. Higher capacitance
values help to improve transient. The output capacitor's
ESR is critical because it forms a zero to provide phase
lead which is required for loop stability.
No Load Stability
The device will remain stable and in regulation with no
external load. This is specially important in CMOS RAM
keep-alive applications
RT9179A
Region of Stable COUT ESR vs. Load Current
10.0100
1.0010
Region of Instable
0.100.01
Region of Stable
0.010
0.001
0
Region of Instable
100
200
300
400
500
Load Current (mA)
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, RDS(ON), multiplied by the load current :
× VDROPOUT = VIN - VOUT = RDS(ON) IOUT
Current Limit
The RT9179A monitors and controls the PMOS’ gate
voltage, minimum limiting the output current to 700mA. 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.
DS9179A-07 March 2007
www.richtek.com
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