PAM2307
3A Low Noise Step-Down DC-DC Converter
Application Information
The basic PAM2307 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 2.7µH. Its value is
chosen based on the desired ripple current and
efficiency. 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 3A
reasonable starting point for setting ripple
current is
△
I
L
= 1.2A (40% of 3A).
D
I
L
=
1
V
OUT
V
OUT
1-
(
f
)(
L
)
V
IN
(1)
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
RIPPLE
(P-P) requirement. The
output ripple
△
Vout is determined by:
△Vout≈△I
L
(ESR+1/8fCout)
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
formul ations. These dielectrics have the best
temperature and voltage charac teristics of all
the ceramics for a given value and size.
Thermal consideration
Thermal protection limits power dissipation in
the PAM2307. 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:
+
(
t
SW
F
S
I
O
+I
Q
)
V
IN
V
IN
I
Q
is the step-down converter quiescent current.
The term tsw is used to estimate the f ull load
step-down converter switching losses.
P
D
=I
O
2
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 (3A + 1.2A). For better
efficiency, choose a low DC-resis tance inductor.
Vo
1.2V
1.5V
1.8V
2.5V
3.3V
L
1.2µ
H
1.5µH
2.2µH
2.2µH
2.2µ
H
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:
OUT
(
V
IN
-
V
O UT
)
2
V
C
IN
required I
RMS
@
I
OMAX
V
IN
1
This formula has a maximum at V
IN
=2Vout,
w h e r e I
R M S
= I
O U T
/ 2 . T h i s s i m p l e w o r s t - c a s e
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 advis able 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.
V
O
R
DSONH
+
(
V
IN
-V
O
)
R
DSONL
Power Analog Microelectronics
,
Inc
www.poweranalog.com
03/2011 Rev2.1
9
PAM [ POWER ANALOG MICOELECTRONICS ]
