1A Ultra Low Dropout Linear Regulator
APPLICATION INFORMATION
TJ4940
Maximum Output Current Capability
The TJ4940 can deliver a continuous current of 1A over the full operating junction temperature range.
However, the output current is limited by the restriction of power dissipation which differs from packages.
A heat sink may be required depending on the maximum power dissipation and maximum ambient
temperature of application. With respect to the applied package, the maximum output current of 1A may
be still undeliverable due to the restriction of the power dissipation of TJ4940. Under all possible
conditions, the junction temperature must be within the range specified under operating conditions.
The temperatures over the device are given by:
TC = TA + PD X θCA
/ TJ = TC + PD X θJC / TJ = TA + PD X θJA
where TJ is the junction temperature, TC is the case temperature, TA is the ambient temperature, PD is the
total power dissipation of the device, θCA is the thermal resistance of case-to-ambient, θJC is the thermal
resistance of junction-to-case, and θJA is the thermal resistance of junction to ambient.
The total power dissipation of the device is given by:
PD = PIN – POUT = (VIN X IIN)–(VOUT X IOUT
)
= (VIN X (IOUT+IGND)) – (VOUT X IOUT) = (VIN - VOUT) X IOUT + VIN X IGND
where IGND is the operating ground current of the device which is specified at the Electrical Characteristics.
The maximum allowable temperature rise (TRmax) depends on the maximum ambient temperature (TAmax
)
of the application, and the maximum allowable junction temperature (TJmax):
TRmax = TJmax – TAmax
The maximum allowable value for junction-to-ambient thermal resistance, θJA, can be calculated using the
formula:
θ
JA = TRmax / PD = (TJmax – TAmax) / PD
TJ4940 is available in SOT-223, and TO-252 packages. The thermal resistance depends on amount of
copper area or heat sink, and on air flow. If the maximum allowable value of θJA calculated above is as
described in Table 1, no heat sink is needed since the package can dissipate enough heat to satisfy these
requirements. If the value for allowable θJA falls near or below these limits, a heat sink or proper area of
copper plane is required.
Jul. 2011 – Preliminary
6/8
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
