TK712xx
DEFINITION AND EXPLANATION OF TECHNICAL TERMS
LINE REGULATION (LINE REG)
PACKAGE POWER DISSIPATION (PD)
Line regulation is the ability of the regulator to maintain a
constant output voltage as the input voltage changes.
This is the power dissipation level at which the thermal
sensor is activated. The IC contains an internal thermal
sensorwhichmonitorsthejunctiontemperature. Whenthe
junction temperature exceeds the monitor threshold of
150 °C, the IC is shut down. The junction temperature
rises as the difference between the input power (VIN x IIN)
and the output power (VOUT x IOUT) increases. The rate of
temperature rise is greatly affected by the mounting pad
configuration on the PCB, the board material, and the
ambient temperature. When the IC mounting has good
thermal conductivity, the junction temperature will be low
even if the power dissipation is great. When mounted on
the mounting pad, the power dissipation of the SOT-25 is
increased to 350 mW. For operation at ambient
temperatures over 25 °C, the power dissipation of the
SOT-25 device should be derated at 2.8 mW/°C. To
determine the power dissipation for shutdown when
mounted, attach the device on the actual PCB and
deliberately increase the output current (or raise the input
voltage) until the thermal protection circuit is activated.
Calculatethepowerdissipationofthedevicebysubtracting
the output power from the input power. These
measurements should allow for the ambient temperature
ofthePCB. ThevalueobtainedfromPD /(150 °C-TA)isthe
derating factor. The PCB mounting pad should provide
maximum thermal conductivity in order to maintain low
device temperatures. As a general rule, the lower the
temperature, the better the reliability of the device. The
thermalresistancewhenmountedisexpressedasfollows:
LOAD REGULATION (LOAD REG)
Load regulation is the ability of the regulator to maintain a
constant output voltage as the load current changes. It is
a pulsed measurement to minimize temperature effects.
The load regulation is specified an output current step
condition of 1 mA to 60 mA.
QUIESCENT CURRENT (IQ)
The quiescent current is the current which flows through
thegroundterminalundernoloadconditions(IOUT =0mA).
GROUND CURRENT (IGND
)
Ground current is the current which flows through the
ground pin(s). It is defined as IIN - IOUT, excluding ICONT
.
DROPOUT VOLTAGE (VDROP
)
Thisisameasureof howwelltheregulatorperformsasthe
input voltage decreases. The smaller the number, the
further the input voltage can decrease before regulation
problems occur. Nominal output voltage is first measured
when VIN = VOUT + 1 at a chosen load current. When the
output voltage has dropped 100 mV from the nominal, VIN
- VO is the dropout voltage. This voltage is affected by load
current and junction temperature.
Tj = 0jA x PD + TA
OUTPUT NOISE VOLTAGE
This is the effective AC voltage that occurs on the output
voltage under the condition where the input noise is low
and with a given load, filter capacitor, and frequency
range.
For Toko ICs, the internal limit for junction temperature is
150 °C. If the ambient temperature (TA) is 25 °C, then:
150 °C = 0jA x PD + 25 °C
0jA = 125 °C / PD
THERMAL PROTECTION
This is an internal feature which turns the regulator off
when the junction temperature rises above 150 °C. After
the regulator turns off, the temperature drops and the
regulator output turns back on. Under certain conditions,
theoutputwaveformmayappeartobeanoscillationasthe
output turns off and on and back again in succession.
PD is the value when the thermal sensor is activated. A
simple way to determine PD is to calculate VIN x IIN when
the output side is shorted. Input current gradually falls as
temperature rises. You should use the value when thermal
equilibrium is reached.
January 1999 TOKO, Inc.
Page 15
TOKO [ TOKO, INC ]
