GENERAL OVERVIEW
Power MOSFET Selection
Select the Power MOSFET for Voltage rating
BV
DSS
, On resistance R
DS(ON)
, and thermal
resistance Rthja. BV
DSS
should be about twice
as high as V
IN
in order to guard against
switching transients. Recommended MOSFET
voltage rating for V
IN
of 5V, 12V and 24V is
12V, 30V and 40V respectively. R
DS(ON)
has to
be selected such that when operating at peak
current
and
junction
temperature
the
Overcurrent threshold of the SP6126 is not
exceeded. Allowing 50% for temperature
coefficient of R
DS(ON)
and 15% for inductor
current ripple, the following expression can be
used:
where:
Vf is diode forward voltage at
I
OUT
Schottky’s AC losses due to its switching
capacitance are negligible.
Inductor Selection
Select the Inductor for inductance L and
saturation current
I
SAT
. Select an inductor with
I
SAT
higher than the programmed overcurrent.
Calculate inductance from:
300
mV
RDS
(
ON
)
≤
1.5
×
1.15
×
Iout
Within this constraint, selecting MOSFETs with
lower R
DS(ON)
will reduce conduction losses at
the expense of increased switching losses. As
a rule of thumb select the highest R
DS(ON)
MOSFET that meets the above criteria.
Switching losses can be assumed to roughly
equal the conduction losses. A simplified
expression for conduction losses is given by:
Vout
1
1
L
=
(
Vin
−
Vout
)
×
×
×
Vin
f
Irip
where:
V
IN
is converter input voltage
V
OUT
is converter output voltage
f
is switching frequency
I
RIP
is inductor peak-to-peak current ripple
(nominally set to 30% of I
OUT
)
Keep in mind that a higher
I
RIP
results in a
smaller inductor which has the advantages of
small size, low DC equivalent resistance DCR,
high saturation current
I
SAT
and allows the use
of a lower output capacitance to meet a given
step load transient. A higher
I
RIP
, however,
increases the output voltage ripple and
increases the current at which converter enters
Discontinuous Conduction Mode. The output
current at which converter enters DCM is �½ of
I
RIP
. Note that a negative current step load that
drives the converter into DCM will result in a
large output voltage transient. Therefore the
lowest current for a step load should be larger
than �½ of
I
RIP
.
Output Capacitor Selection
Select the output capacitor for voltage rating,
capacitance and Equivalent Series Resistance
(ESR). Nominally the voltage rating is selected
to be twice as large as the output voltage.
Select the capacitance to satisfy the
specification
for
output
voltage
overshoot/undershoot caused by current step
load. A steady-state output current I
OUT
2
corresponds to inductor stored energy of �½ L I
OUT
.
Vout
Pcond
=
Iout
×
RDS
(
ON
)
×
Vin
MOSFET’s junction
estimated from:
temperature
can
be
T
=
(
2
×
Pc
×
Rthja
)
+
Tambient
Schottky Rectifier selection
Select the Schottky for Voltage rating V
R
,
Forward voltage V
f
, and thermal resistance
Rthja. Voltage rating should be selected using
the same guidelines outlined for MOSFET
voltage selection. For a low duty cycle
application such as the circuit shown on first
page, the Schottky is conducting most of the
time and its conduction losses are the largest
component of losses in the converter.
Conduction losses can be estimated from:
Vout
Pc
=
Vf
×
Iout
×
1
−
Vin
Nov07-08
RevG
SP6126: TSOT-6 PFET Buck Controller
2008 Exar Corporation
7
EXAR [ EXAR CORPORATION ]
