PAM2325 View Datasheet(PDF) - Diodes Incorporated.
Part Name
Description
Manufacturer
PAM2325 Datasheet PDF : 12 Pages
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A Product Line of
Diodes Incorporated
PAM2325
Application Information
The basic PAM2325 application circuit is shown on Page 1. External component selection is determined by the load requirement, selecting L first
and then CIN and COUT.
Inductor Selection
For most applications, the value of the inductor will fall in the range of 1μH to 3.3μH. Its value is chosen based on the desired ripple current.
Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the ripple
current as shown in equation 3.5A reasonable starting point for setting ripple current is ΔIL = 1.4A (40% of 3.5A).
ΔIL
=
(f
1
)(L)
VOUT
⎜⎜⎝⎛1
−
VOUT
VIN
⎟⎟⎠⎞
(1)
The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation.
Thus, a 4.2A rated inductor should be enough for most applications (3.5A + 0.7A). For better efficiency, choose a low DC-resistance inductor.
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low
ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by:
[ ( )] CINrequiredIRMS ≅ IOMAX
VOUT
VIN − VOUT
VIN
1/ 2
This formula has a maximum at VIN =2VOUT, where IRMS = IOUT/2. This simple worst -case condition is commonly used for design because even
significant deviations do not offer much relief. Note that the capacitor manufacturer's ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Consult the
manufacturer if there is any question.
The selection of Cout is driven by the required effective series resistance (ESR).
Typically, once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE (P-P) requirement. The
output ripple ΔVOUT is determined by:
ΔVOUT ≈ ΔIL (ESR + 1/ 8fCOUT )
Where f = operating frequency, COUT =output capacitance and ΔIL = ripple current in the inductor. For a fixed output voltage, the output ripple is
highest at maximum input voltage since ΔIL increases with input voltage.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and
low ESR make them ideal for switching regulator applications. Using ceramic capacitors can achieve very low output ripple and small circuit size.
When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best
temperature and voltage characteristics of all the ceramics for a given value and size.
Thermal Consideration
Thermal protection limits power dissipation in the PAM2325. When the junction temperature exceeds +150°C, the OTP (Over Temperature
Protection) starts the thermal shutdown and turns the pass transistor off. The pass transistor resumes operation after the junction temperature
drops below 120°C.
For continuous operation, the junction temperature should be maintained below 125°C. The power dissipation is defined as:
( ) ( ) PD = IO2
VO RDS(ON)H +
VIN − VO
VIN
RDS(ON)L
+
tSW FS IO + IQ
VIN
IQ is the step-down converter quiescent current. The term tsw is used to estimate the full load step-down converter switching losses.
For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to:
PD = IO 2 RDS(ON)H + IQ VIN
PAM2325
Document number: DSxxxxx Rev. 2 - 0
8 of 12
www.diodes.com
January 2013
© Diodes Incorporated
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