PAM2308 Datasheet, PDF (11/15 Pages) Power Analog Micoelectronics – Dual High-Efficiency PWM Step-Down DC-DC Coverter

PAM2308
Dual High-Efficiency PWM Step-Down DC-DC Coverter
Application Information
The basic PAM2308 application circuit is shown
in 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 4.7μH. 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 V_IN or Vout also increases the
ripple current as shown in equation 1. A
reasonable starting point for setting ripple
current is △I_L = 400mA (40% of 1A).
DI_L
=
(f
1
)(L
)
VOUT
æ
çè
1-
VOUT
VIN
ö
÷ø
(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 1.4A rated inductor should be enough for
most applications (1A + 400mA). For better
efficiency, choose a low DC-resis tance inductor.
Vo 1.2V 1.5V 1.8V 2.5V 3.3V
L 2.2μH 2.2μH 2.2μH 4.7μH 4.7μH
C_IN and C_OUT 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:
( )¹
CIN required IRMS @ IOMAX ^{éëVOUT VIN - VOUT ùû 2}
VIN
This formula has a maximum at V_IN =2Vout,
where I_RMS=I_OUT/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 I_RI ( _PPLE P-P) requirement. The
output ripple △Vout is determined by:
VVOUT
@VIL
æçè _ESR+
1
8fCOUT
ö
÷ø
Where f = operating frequency, C_OUT=output
capacitance and ΔI_L = ripple current in the
inductor. For a fixed output voltage, the output
ripple is highest at maximum input voltage since
ΔI_L 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 PAM2308. 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}
V_OR_DSONH +
V_IN -V_O
V_IN
R_{DSONL + tSWFSIO +IQ}
V_IN
I_Q is the step-down converter quiescent current.
The term tsw is used to estimate the full load
step-down converter switching losses.
Power Analog Microelectronics,Inc
www.poweranalog.com
11
07/2008 Rev 1.0