APL5312
Application Information (Cont.)
Noise, PSRR, and Load-Transient Response
(Cont.)
techniques (see the Figure Output Noise vs. BP
Capacitance).
Shutdown
The APL5312 has an active high enable function. Force
SHDN high (>1.6V) enables the V
OUT
, SHDN low (<0.4V)
disables the V
OUT
. Enter the shutdown mode, it also
causes the output voltage to discharge through a 500Ω
resistance to ground. In shutdown mode, the
quiescent current can reduce to 0.1uA. The SHDN
pin cannot be floating, a floating SHDN pin may cause
an indeterminate state on the output. If it is no use,
connect to V
IN
for normal operation.
Input-Output (Dropout) Voltage
The minimum input-output voltage differential (dropout)
determines the lowest usable supply voltage. The
dropout voltage is a function of drain-to-source on
resistance multiplied by the load current.
Current Limit
APL5312 includes a current-limit circuitry for linear
regulator. The current limit protection, which sense
the current flows the P-channel MOSFET, and con-
trols the output voltage. The point where limiting oc-
curs is I
OUT
= 500mA . The output can be shorted to
ground for an indefinite amount of time without damaging to
the part.
Thermal Protection
Thermal protection limits total power dissipation in the
APL5312. When the junction temperature exceeds
T
J
= +160
°
C, the thermal sensor generate a logic sig-
nal to turn off the pass element and let IC to cool.
When the IC’ junction temperature cools by 20
°
C,
s
the thermal sensor will turn the pass element on again,
resulting in a pulsed output during continuous thermal
Copyright
©
ANPEC Electronics Corp.
Rev. A.1 - Sep., 2005
10
protection. Thermal protection is designed to protect
the IC in the event of fault conditions.
Operating Region and Power Dissipation
The thermal resistance of the case and circuit board,
ambient and junction air temperature, and the rate of
air flow all control the APL5312’ maximum power
s
dissipation. The power dissipation across the device
is P = I
OUT
(V
IN
-V
OUT
). The maximum power dissipation
is:
P
MAX
= (T
J
- T
A
) / (θ
JC
+
θ
CA
)
θ
JA
=
θ
JC
+
θ
CA
where T
J
- T
A
is the temperature difference between the
junction and ambient air.
θ
JC
is the thermal resistance of the package,
θ
CA
is the
thermal resistance through the printed circuit board,
copper traces, and other materials to the surrounding
air,
θ
JA
= is the thermal resistance between Junction
and ambient air. For continual operation, do not exceed
the absolute maximum junction Temperature rating of
T
J
= 125°C.
For example:
The SOT23-5 package has maximum power dissipation
300mW at T
A
= 55°C, relatively 225mW at SC70-5
package (see the Figure Maximum Power Dissipation
vs. Ambient Temperature).
V
IN
= 5V, I
OUT
= 250mA, V
OUT
= 3.3V,
P
D
= (5-3.3)V x 150mA = 255mW
According the power dissipation issue, we should adapt
the SOT23-5 package. It could reduce the thermal
resistance to maintain the IC longer life.
The GND pin provides an electrical connection to
ground and channeling heat away. The printed circuit
board (PCB) forms a heat sink and dissipates most of
the heat into ambient air.
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