PAM2304
3MHz, 1A Step-Down DC-DC Converter
Application Information
The basic PAM2304 application circuit is shown
in Page 1. External component selection is
determined by the load requirement, selecting L
first and then Cin and Cout.
far exceeds the IRIPPLE(P-P) requirement. The
output ripple △Vout is determined by:
1
VVOUT @VI
L
ESR+
8fCOUT
Inductor Selection
Where f = operating frequency, COUT =output
capacitance and ΔIL = ripple current in the
inductor. For a fixed output voltage, the output
ripple is highest at maximum input voltage since
ΔIL increases with input voltage.
For most applications, the value of the inductor
will fall in the range of 1µH. 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
VIN or Vout also increases the ripple current as
shown in equation 1. A reasonable starting point
for setting ripple current is △IL = 400mA (40% of
1A).
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.
1
V
OUT
DIL =
V
OUT 1-
(1)
f
L
V
IN
( )( )
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-resistance inductor.
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.
CIN and COUT Selection
Thermal consideration
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:
Thermal protection limits power dissipation in
the PAM2304. 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
r e s u m e s o p e r a t i o n a f t e r t h e j u n c t i o n
temperature drops below 120°C.
2
VOUT
V
(
IN - VOUT 1
)
CIN required IRMS @ IOMAX
V
IN
F or c o n ti n u ou s o pe ra ti o n , t he j un c t i on
temperature should be maintained below 125°C.
The power dissipation is defined as:
This formula has a maximum at VIN =2Vout,
where IR M S =IO U T /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.
VORDSONH+(V -VO )R
IN
P =IO
DSONL + tSWFSIO +IQ
V
2
(
)
D
IN
V
IN
IQ is the step-down converter quiescent current.
The term tsw is used to estimate the full load
step-down converter switching losses.
The selection of Cout is driven by the required
effective series resistance (ESR).
For the condition where the step-down converter
is in dropout at 100% duty cycle, the total device
dissipation reduces to:
Typically, once the ESR requirement for Cout
has been met, the RMS current rating generally
Power Analog Microelectronics, Inc
www.poweranalog.com
11/2011 Rev1.1
8
PAM [ POWER ANALOG MICOELECTRONICS ]
