ADP3338
APPLICATION INFORMATION
CAPACITOR SELECTION
CALCULATING POWER DISSIPATION
Output Capacitor
Device power dissipation is calculated as
The stability and transient response of the LDO is a function of
the output capacitor. The ADP3338 is stable with a wide range
of capacitor values, types, and ESR (anyCAP). A capacitor as
low as 1 µF is the only requirement for stability. A higher ca-
pacitance may be necessary if high output current surges are
anticipated, or if the output capacitor cannot be located near the
output and ground pins. The ADP3338 is stable with extremely
low ESR capacitors (ESR ≈ ±) such as multilayer ceramic capacitors
(MLCC) or OSCON. Note that the effective capacitance of some
capacitor types falls below the minimum over temperature or
with dc voltage.
PD = (VIN – VOUT) × ILOAD + (VIN × IGND
)
Where ILOAD and IGND are load current and ground current, and
IN and VOUT are the input and output voltages, respectively.
Assuming the worst-case operating conditions are ILOAD = 1.± A,
IGND = 1± mA, VIN = 3.3 V, and VOUT = 2.5 V, the device power
dissipation is
V
PD = (3.3 V – 2.5 V) × 1±±± mA + (3.3 V × 1± mA) = 833 mW
So, for a junction temperature of 125°C and a maximum
ambient temperature of 85°C, the required thermal resistance
from junction to ambient is
Input Capacitor
An input bypass capacitor is not strictly required, but is recom-
mended in any application involving long input wires or high
source impedance. Connecting a 1 µF capacitor from the input
to ground reduces the sensitivity of the circuit to PC board
layout and input transients. If a larger output capacitor is
necessary, a larger value input capacitor is recommended.
125°C − 85°C
θJA
=
= ꢁ8°C/W
±.833 W
PRINTED CIRCUIT BOARD LAYOUT
CONSIDERATIONS
The thermal resistance, θJA, of the SOT-223 is determined by the
sum of the junction-to-case and the case-to-ambient thermal
resistances. The junction-to-case thermal resistance, θJC, is
determined by the package design and is specified at 26.8°C/W.
However, the case-to-ambient thermal resistance is determined
by the printed circuit board design.
OUTPUT CURRENT LIMIT
The ADP3338 is short-circuit protected by limiting the pass
transistor’s base drive current. The maximum output current is
limited to approximately 2 A (see Figure 16).
THERMAL OVERLOAD PROTECTION
As shown in Figure 22, the amount of copper to which the
ADP3338 is mounted affects thermal performance. When
mounted to the minimal pads of 2 oz. copper, as shown in
Figure 22 (a), θJA is 126.6°C/W. Adding a small copper pad
under the ADP3338, as shown in Figure 22 (b), reduces the θJA to
1±2.9°C/W. Increasing the copper pad to one square inch, as
shown in Figure 22 (c), reduces the θJA even further to 52.8°C/W.
The ADP3338 is protected against damage due to excessive
power dissipation by its thermal overload protection circuit.
Thermal protection limits the die temperature to a maximum of
16±°C. Under extreme conditions, such as high ambient
temperature and power dissipation where the die temperature
starts to rise above 16±°C, the output current is reduced until
the die temperature has dropped to a safe level.
Current and thermal limit protections are intended to protect
the device against accidental overload conditions. For normal
operation, externally limit the power dissipation of the device
so the junction temperature does not exceed 15±°C.
a
b
c
Figure 22. PCB Layouts
Rev. B | Page 10 of 16
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