1A Ultra Low Dropout Linear Regulator
APPLICATION INFORMATION
TJ3940
Output Capacitor
The TJ3940 requires a proper output capacitance to maintain stability and improve transient response
over current. The ESR of the output capacitor within the limits of 0.5Ω to 10Ω is required. A minimum
capacitance value of 22μF of tantalum or aluminum electrolytic capacitor is recommended. In a case of
ceramic capacitor, a minimum capacitance value of 10μF is required and additional resistance of
minimum 1Ω should be added with the output capacitor in series to maintain its minimum ESR. The
resistance and capacitance have to be varied upon the load current.
Maximum Output Current Capability
The TJ3940 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 TJ3940. Under all possible
conditions, the junction temperature must be within the range specified under operating conditions.
The temperatures over the device are given by:
T
C
= T
A
+ P
D
X
θ
CA
/ T
J
= T
C
+ P
D
X
θ
JC
/ T
J
= T
A
+ P
D
X
θ
JA
where T
J
is the junction temperature, T
C
is the case temperature, T
A
is the ambient temperature, P
D
is the
total power dissipation of the device,
θ
CA
is the thermal resistance of case-to-ambient,
θ
JC
is the thermal
resistance of junction-to-case, and
θ
JA
is the thermal resistance of junction to ambient.
The total power dissipation of the device is given by:
P
D
= P
IN
– P
OUT
= (V
IN
X I
IN
)–(V
OUT
X I
OUT
)
= (V
IN
X (I
OUT
+I
GND
)) – (V
OUT
X I
OUT
) = (V
IN
- V
OUT
) X I
OUT
+ V
IN
X I
GND
where I
GND
is the operating ground current of the device which is specified at the Electrical Characteristics.
The maximum allowable temperature rise (T
Rmax
) depends on the maximum ambient temperature (T
Amax
)
of the application, and the maximum allowable junction temperature (T
Jmax
):
T
Rmax
= T
Jmax
– T
Amax
The maximum allowable value for junction-to-ambient thermal resistance,
θ
JA
, can be calculated using the
formula:
θ
JA
= T
Rmax
/ P
D
= (T
Jmax
– T
Amax
) / P
D
TJ3940 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
θ
JA
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
θ
JA
falls near or below these limits, a heat sink or proper area of
copper plane is required.
Jan. 2011 – Rev.1.0
-6-
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
