Preliminary
RT9022
Application Information
Enable
Thermal Consideration
The RT9022 goes into sleep mode when the Enable pin is
in a logic low condition. During this condition, the pass
transistor, error amplifier, and bandgap are turned off,
reducing the supply current to 1μA. The Enable input is a
CMOS logic and cannot be left floating.
Thermal protection limits power dissipation in the RT9022.
When the operating junction temperature exceeds 135°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 10°C.
For continuous operation, do not exceed absolute
maximum operation junction temperature 125°C. The
power dissipation definition in device is :
Output capacitor
The RT9022 is specifically designed to use ceramic output
capacitors as low as 2.2μF. Ceramic capacitors below 10μF
offer significant cost and space saving, along with high
frequency noise filtering. The RT9022 doesn't rely on a
zero, which is generated by output capacitor ESR. So,
the output capacitor ESR is not sensitive and very low
ESR is allowed.
PD = (VIN − VOUT) x IOUT + VIN x IQ
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 :
Input capacitor
PD(MAX) = ( TJ(MAX) − TA ) / θJA
A 1μF input capacitor or greater located as close as
possible to the IC is recommended. Larger input capacitor
value with lower ESR provides better power supply noise
rejection and line transient response. Larger load current
requires larger capacitor value.
Where TJ(MAX) is the maximum operation junction
temperature 100°C, TAis the ambient temperature and the
θJA is the junction to ambient thermal resistance.
For recommended operating conditions specification of
the RT9022, where TJ(MAX) is the maximum junction
temperature of the die (100°C) and TA is the maximum
ambient temperature. The junction to ambient thermal
resistance ( θJA is layout dependent) for T/SOT-23-6
package is 250°C/W, and SC-70-6 package is 333°C/W
on standard JEDEC 51-3 thermal test board. The
maximum power dissipation at TA= 25°C can be calculated
by following formula :
PSRR
The power supply rejection ratio (PSRR) is defined as the
gain from the input to output divided by the gain from the
supply to the output. The PSRR is found to be
⎛
⎞
ΔGain Error
ΔSupply
PSRR = 20×log
⎜
⎝
⎟
⎠
Note that when heavy load is measured, Δsupply will cause
Δtemperature. And Δtemperature will cause Δoutput
voltage. So the temperature effect must be taken into
consideration.
PD(MAX) = (100°C − 25°C) / 250°C/W = 300mW for
T/SOT-23-6 package
PD(MAX) = (100°C − 25°C) / 333°C/W = 225mW for
SC-70-6 package
Current Limit
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal
resistance θJA. For the RT9022 packages, the Figure 1 of
de-rating curves allows the designer to see the effect of
rising ambient temperature on the maximum power
allowed.
The RT9022 contains an independent current limiter, which
monitors and controls the pass transistor's gate voltage
to limit the output current to 80mA typically. It protects
the part even directly short the output to GND.
DS9022-00 September 2007
www.richtek.com
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