Pam2307, Application information – Diodes PAM2307 User Manual
Page 9
PAM2307
Document number: DSxxxxx Rev. 2 - 1
9 of 14
www.diodes.com
September 2012
© Diodes Incorporated
PAM2307
A Product Line of
Diodes Incorporated
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 C
IN
and C
OUT
.
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 V
OUT
also increases
the ripple current as shown in equation 3A reasonable starting point for setting ripple current is ΔI
L
= 1.2A (40% of 3A).
( )( )
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
=
Δ
V
V
1
V
L
f
1
I
IN
OUT
OUT
L
Equation (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 (3A + 1.2A). For better efficiency, choose a low DC-resistance inductor.
V
O
1.2V 1.5V 1.8V 2.5V 3.3V
L 1.2µH 1.5µH 2.2µH 2.2µH 2.2µH
C
IN
and C
OUT
Selection
In continuous mode, the source current of the top MOSFET is a square wave of duty cycle V
OUT
/V
IN
. 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:
(
)
[
]
V
V
V
V
I
I
required
C
IN
2
/
1
OUT
IN
OUT
OMAX
RMS
IN
−
≅
This formula has a maximum at V
IN
= 2V
OUT
, 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 C
OUT
is driven by the required effective series resistance (ESR).
Typically, once the ESR requirement for C
OUT
has been met, the RMS current rating generally far exceeds the I
RIPPLE
(P-P) requirement. The
output ripple ΔV
OUT
is determined by:
(
)
C
f
8
/
1
ESR
I
V
OUT
L
OUT
+
Δ
≈
Δ
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 formul ations. 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 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:
(
)
(
)
V
I
I
F
t
V
R
V
V
R
V
I
P
IN
Q
O
S
SW
IN
L
)
ON
(
DS
O
IN
H
)
ON
(
DS
O
2
O
D
+
+
−
+
=
I
Q
is the step-down converter quiescent current. The term tsw is used to estimate the full load step-down converter switching losses.