RT9198/A
Applications Information
Like any low-dropout regulator, the external capacitors used
for the RT9198/A must be carefully selected for regulator
stability and performance. Using a capacitor whose value
is > 1μF on the RT9198/A input and the amount of
capacitance can be increased without limit. The input
capacitor must be located a distance of not more than
0.5 inch from the input pin of the IC and returned to a
clean analog ground. Any good quality ceramic or tantalum
can be used for this capacitor. The capacitor with larger
value and lower ESR (equivalent series resistance) provides
better PSRR and line-transient response.
The output capacitor must meet both requirements for
minimum amount of capacitance and ESR in all LDOs
application. The RT9198/A is designed specifically to work
with low ESR ceramic output capacitor in space-saving
and performance consideration. Using a ceramic capacitor
whose value is at least 1μF with ESR is more than 20mΩ
on the RT9198/A output ensures stability. The RT9198/A
still works well with output capacitor of other types due to
the wide stable ESR range. Figure 1. shows the curves of
allowable ESR range as a function of load current for various
output capacitor values. Output capacitor of larger
capacitance can reduce noise and improve load transient
response, stability, and PSRR. The output capacitor should
be located with in 0.5 inch from the V
OUT
pin of the
RT9198/A and returned to a clean analog ground.
Enable Function
The RT9198/A features an LDO regulator enable/disable
function.
To assure the RT9198 LDO regulator will switch on, the
EN turn on control level must be greater than 1.2V. The
RT9198 LDO regulator will go into shutdown mode when
the voltage on the EN pin falls below 0.4V. For RT9198A,
the EN turn on control level must be lower than 0.4V to
assure the RT9198A LDO regulator switch on. The
RT9198A LDO regulator will go into shutdown mode when
the voltage on the EN pin is higher than 1.2 V. The RT9198A
equips a quick-discharge function to protect the system.
When the regulator is turned off by EN pin, the internal
MOSFET between V
OUT
and GND will be turned on to
discharge output voltage quickly. If the enable function is
not needed in a specific application, it may be tied to
GND/VIN to keep the LDO regulator in a continuously on
state.
Thermal Considerations
Thermal protection limits power dissipation in RT9198/A.
When the operating junction temperature exceeds 165°C,
the OTP circuit starts the thermal shutdown function and
turns the pass element off. The pass element turns on
again after the junction temperature cools by 30°C.
For continuous operation, do not exceed absolute
maximum operatiog junction temperature 125°C. The
power dissipation definition in device is shown as following
formula :
Instable
Region of Stable C
OUT
ESR vs. Load Current
100.00
100
10
10.00
P
D
= (V
IN
−
V
OUT
) x I
OUT
+ V
IN
x I
Q
C
OUT
= 1uF
1.00
1
0.10
Stable
The maximum power dissipation depends on the thermal
resistance of IC package, PCB layout, the rate of
surroundings airflow and temperature difference between
junction to ambient. The maximum power dissipation can
be calculated by following formula :
P
D(MAX)
= ( T
J(MAX)
- T
A
) /
θ
JA
C
OUT
ESR (Ω)
0.01
0.00
RT9198/A-15xU5
C
OUT
= 1μF, X7R
0
50
100
Simulation Verify
150
200
250
300
Load Current (mA)
Where T
J(MAX)
is the maximum operating junction
temperature 125°C, T
A
is the ambient temperature and the
θ
JA
is the junction to ambient thermal resistance.
Figure 1
DS9198/A-12 May 2008
Instable
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RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
