ADP3309
Rev. C | Page 10 of 12
APPLICATION INFORMATION
CAPACITOR SELECTION: anyCAP
Output Capacitors: As with any micropower device, output
transient response is a function of the output capacitance. The
ADP3309 is stable with a wide range of capacitor values, types,
and ESR (anyCAP). A capacitor as low as 0.47 μF is all that is
needed for stability. However, larger capacitors can be used if
high output current surges are anticipated. The ADP3309 is
stable with extremely low ESR capacitors (ESR ≈ 0), such as
multilayer ceramic capacitors (MLCC) or OSCON.
Input Bypass Capacitor: An input bypass capacitor is not
required. However, for applications where the input source is
high impedance or far from the input pin, a bypass capacitor is
recommended. Connecting a 0.47 μF capacitor from the input
pin (Pin 1) to ground reduces the circuit’s sensitivity to PC
board layout. If a bigger output capacitor is used, the input
capacitor must be 1 μF minimum.
THERMAL OVERLOAD PROTECTION
The ADP3309 is protected against damage due to excessive
power dissipation by its thermal overload protection circuit,
which limits the die temperature to a maximum of 165°C.
Under extreme conditions (that is, high ambient temperature
and power dissipation) where die temperature starts to rise
above 165°C, the output current is reduced until the die
temperature has dropped to a safe level. The output current is
restored when the die temperature is reduced.
Current and thermal limit protections are intended to protect
the device against accidental overload conditions. For normal
operation, device power dissipation should be externally limited
so that junction temperatures do not exceed 125°C.
CALCULATING JUNCTION TEMPERATURE
Device power dissipation is calculated as follows:
P
D
= (V
IN
– V
OUT
) I
LOAD
+ (V
IN
) I
GND
where:
I
LOAD
is the load current.
I
GND
is the ground current.
V
IN
is the input voltage.
V
OUT
is the output voltage.
Assuming I
LOAD
= 100 mA, I
GND
= 2 mA, V
IN
= 5.0 V, and
V
OUT
= 3.3 V, device power dissipation is
P
D
= (5.0 − 3.3) 100 mA + 5.0 × 2 mA = 180 mW
ΔT = T
J
– T
A
= P
D
× θ
JA
= 0.18 × 190 = 34.2°C
With a maximum junction temperature of 125°C, this yields a
maximum ambient temperature of ~90°C.
PRINTED CIRCUIT BOARD LAYOUT
CONSIDERATION
Surface-mount components rely on the conductive traces or
pads to transfer heat away from the device. Appropriate PC
board layout techniques should be used to remove heat from
the immediate vicinity of the package.
The following general guidelines will be helpful when designing
a board layout:
1. PC board traces with larger cross section areas remove
more heat. For optimum results, use PC boards with
thicker copper and/or wider traces.
2. Increase the surface area exposed to open air so heat can be
removed by convection or forced air flow.
3. Do not use solder mask or silk screen on the heat
dissipating traces because it increases the junction to
ambient thermal resistance of the package.
SHUTDOWN MODE
Applying a TTL high signal to the shutdown pin or tying it to
the input pin turns the output on. Pulling the shutdown pin
down to a TTL low signal or tying it to ground turns the output
off. In shutdown mode, quiescent current is reduced to less
than 1 μA.
ERROR FLAG DROPOUT DETECTOR
The ADP3309 maintains its output voltage over a wide range of
load, input voltage, and temperature conditions. If the output is
about to lose regulation, for example, by reducing the supply
voltage below the combined regulated output and dropout
voltages, the
ERR
pin will be activated. The
ERR
output is an
open collector that will be driven low.
Once set, the
ERR
or flag’s hysteresis keeps the output low until
a small margin of operating range is restored either by raising
the supply voltage or reducing the load.
