PAM2301
800mA Step-Down DC-DC Converters
Application Information
The basic PAM2301 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
= 320mA (40% of 800mA).
æ
V
OUT
ö
V
OUT
ç
1-
D
I
L
=
÷
V
IN
ø
(
f
)(
L
)
è
1
Typically, once the ESR requirement for Cout
has been met, the RMS current rating generally
far exceeds the I
RIPPLE
(P-P) requirement. The
output ripple
△
Vout is determined by:
1
ö
æ
V
V
OUT
@
V
I
L
ç
ESR+
÷
8fC
OUT
ø
è
Where f = operating frequency, C
OUT
=output
capacitance and Δ I
L
= ripple current in the
inductor. For a fixed output voltage, the output
ripple is highest at maximum input voltage since
Δ I
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 charac teristics of all
the ceramics for a given value and size.
Setting the Output Voltage
The internal reference is 0.6V (Typical). The
output voltage is calculated as below:
(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 1120mA rated inductor should be enough
for most applications (800mA + 320mA). For
better efficiency, choose a low DC-resis tance
inductor.
C
IN
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:
é
V
OUT
(
V
IN
-
V
OUT
)
ù
û
C
IN
required I
RMS
@
I
OMAX
ë
V
IN
1
2
R
1
ö
æ
V
O
=
0.6
´ ç
1
+
÷
è
R
2
ø
The output voltage is given by Table 1.
Table 1:
Resistor selection for output voltage
setting
Vo
1.2V
1.5V
1.8V
2.5V
3.3V
R1
100k
150k
200k
380k
540k
R2
100k
100k
100k
120k
120k
This formula has a maximum at VIN =2Vout,
where I
R M S
=Iout/2. This simple worst-case
condition is com monly 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 manufac turer if there is any question.
The selection of Cout is driven by the required
effective series resistance (ESR).
Power Analog Microelectronics
,
Inc
www.poweranalog.com
09/2008 Rev 1.3
10
PAM [ POWER ANALOG MICOELECTRONICS ]
