3-Pin Synchronous Step-Up DC/DC Converter
Application Information
Inductor Selection
To select an inductor for PAM2400 applications,
it should be noted that the inductors current
saturation rating should be larger than the
possible peak inductor current to ensure proper
operation, and it should have low DCR (DC
resistance). Using an inductor with the saturation
current lower than the possible peak indicator
current can cause a dramatic drop in the
inductance and severely decay the maximum
output current levels. An inductor is chosen
based on desired ripple current. Larger
inductors result in lower ripple currents, and
smaller ones higher ripple. A 100uH inductor will
b e a s u i t a b l e c h o i c e f o r m o s t PA M 2 4 0 0
applications. The following equation can also
help to get a good approximate value for the
inductor.
L=V
IN
*D
/(△
I
L
*
f
)
D
(
Duty Cycle)
=
1
-
V
IN
/
V
OUT
ΔI
L
(Ripple Current) in the Inductor, i.e., 20% to
40% of the maximum inductor current(Ip).
F (Switching Frequency)=100K.
Output and Input Capacitor Selection
Input Capacitor
An input capacitor of minimum 22μF is
recommended to reduce input ripple and
switching noise for normal operating conditions.
A larger capacitor with lower ESR (Equivalent
Series Resistance) may be needed if the
application requires very low input ripple.
Usually ceramic capacitors are a good choice for
applications. Note that the input capacitor should
be located as close as possible to the IC.
Output Capacitor
A minimum 47μF output capacitor is
recommended and may-be a larger one is better.
The total output voltage ripple has two
components, the capacitive ripple caused by the
charging and discharging on the output
capacitor, and the ohmic ripple due to the
capacitor's equivalent series resistance (ESR):
V
RIPPLE
=V
RIPPLE(C)
+V
RIPPLE(ESR)
V
RIPPLE(C
)
≈
1
/
2
*(
L
/(
C
(
OUT
)
*((
V
OUT
(
MAX
)
-
V
IN
(
MIN
)
)) ))
*(
I
2PEAK
-
I
2OUT
)
And V
RIPPLE
(
ESR
)
=
I
PEAK
*
R
ESR
(
COUT
)
Where Ipeak is the peak inductor current
.
Multilayer ceramic capacitors are a good choice
as they have extremely low ESR and are
available in small footprints. Capacitance and
ESR variation vs temperature should be
considered for best performance in applications
with wide operating temperature ranges.
Component Power Dissipation
Operating in discontinuous mode, power loss in
the winding resistance of the inductor is
approximately equal to
PDL=2/3(T
ON
/L)*R
D
*((V
OUT
+V
F
)/V
OUT
)*P
OUT
The power dissipated in a switch loss is
PDsw=2/3(T
ON
/L)*R
ON
*((V
OUT
+V
D
-V
IN
)/V
OUT
)*(P
OUT
)
The power disspated in rectifier diode(Internal
Diode) is
PDd=V
D
*I
OUT
Where P
OUT
=V
OUT
*I
OUT
, R
D
=Inductor DCR,
V
D
=Diode drop
PC Board Layout considerations
Careful PC board layout is important for
minimizing ground bounce and noise. Keep the
IC's GND pin and the ground leads of input and
output capacitors as close as possible. In
addition, connect L1 and SW with short and wide
connections.
PAM2400
Power Analog Microelectronics
,
Inc
www.poweranalog.com
12/2007 Rev 1.0
11
PAM [ POWER ANALOG MICOELECTRONICS ]
