TJ49300
capacitor is recommended. Larger values will help to improve ripple rejection by bypassing the input to the
regulator, further improving the integrity of the output voltage.
Feed Forward Capacitor
The TJ49300 requires a feed forward capacitor to stabilize output in case of complicated transient load condition.
It also performs a soft-start(SS) function on the output voltage. A 10nF is recommended to support its stability
support function and its higher value will increase the time of SS function.
Maximum Output Current Capability
The TJ49300 can deliver a continuous current of 3.0A 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 3.0A may be still
undeliverable due to the restriction of the power dissipation of TJ49300. 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) + (VBIAS X IBIAS)} – (VOUT X IOUT
)
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:
θJA = TRmax / PD = (TJmax – TAmax) / PD
TJ49300 is available in SOP8-P P, TO252, and TO263 packages. The thermal resistance depends on amount of
copper area or heat sink, and on air flow.
If proper cooling solution such as heat sink, copper plane area, air flow is applied, the maximum allowable power
dissipation could be increased. However, if the ambient temperature is increased, the allowable power dissipation
would be decreased.
The θJA could be decreased with respect to the copper plane area. So, the specification of maximum power
dissipation for an application is fixed, the proper copper plane area could be estimated by following graphs.
Wider copper plane area leads lower θJA.
The maximum allowable power dissipation is also influenced by the ambient temperature. With the θJA-Copper
10
February 2011
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
