LTC3863
18
3863fa
For more information www.linear.com/3863
applicaTions inForMaTion
also be iteratively determined by the two equations below,
where V
F(IOUT
,
TJ)
is a function of both I
F(AVG)
and junction
temperature T
J
. Note that the thermal resistance, θ
JA(DIODE)
,
given in the data sheet is typical and can be highly layout
dependent. It is therefore important to make sure that the
Schottky diode has adequate heat sinking.
T
J
≅ P
DIODE
• θ
JA(DIODE)
P
DIODE
≅ I
F(AVG)
• V
D(IOUT,TJ)
The Schottky diode forward voltage is a function of both
I
F(AVG)
and T
J
, so several iterations may be required to
satisfy both equations. The Schottky forward voltage,
V
D
, should be taken from the Schottky diode data sheet
curve showing instantaneous forward voltage. The forward
voltage will change as a function of both T
J
and I
F(AVG)
.
The nominal forward voltage will also tend to increase as
the reverse breakdown voltage increases. It is therefore
advantageous to select a Schottky diode appropriate to the
input voltage requirements. The diode reverse breakdown
voltage must meet the following condition:
V
R
> V
IN(MAX)
+ |V
OUT
|
C
IN
and C
OUT
Selection
The input and output capacitance, C
IN
/C
OUT
, are required
to filter the square wave current through the P-channel
MOSFET and diode respectively. Use a low ESR capacitor
sized to handle the maximum RMS current:
I
CIN(RMS)
=I
COUT(RMS)
=I
OUT
•
|V
OUT
|+V
D
V
IN
The formula shows that the RMS current is greater than
the maximum I
OUT
when V
OUT
is greater than V
IN
. Choose
capacitors with higher RMS rating with sufficient margin.
Note that ripple current ratings from capacitor manufac
-
turers are often based on only 2000 hours of life, which
makes it advisable to derate the capacitor.
The selection of C
OUT
is primarily determined by the ESR
required to minimize voltage ripple and load step transients.
The ∆V
OUT
is approximately bounded by:
∆V
OUT
≤I
L(PEAK)
•ESR+
OUT
f • C
OUT
where I
L(PEAK)
is the peak inductor current and it’s given as:
I
L(PEAK)
=
I
OUT
V
IN
+|V
OUT
|+V
D
V
IN
+
V
IN
• |V
OUT
|+V
D
( )
2•L • f • V
IN
+|V
OUT
|+V
D
Since I
L(PEAK)
and D reach their maximum values at mini-
mum V
IN
, the output voltage ripple is highest at minimum
V
IN
and maximum I
OUT
. Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering
and has the necessar
y RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, specialty polymer, aluminum electrolytic
and ceramic capacitors are all available in surface mount
packages. Specialty polymer capacitors offer very low
ESR but have lower specific capacitance than other types.
Tantalum capacitors have the highest specific capacitance,
but it is important to only use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have significantly higher ESR, but
can be used in cost-sensitive applications provided that
consideration is given to ripple current ratings and long-
term reliability. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient and
audible piezoelectric effects.
The high Q of ceramic capacitors with trace inductance
can also lead to significant ringing. When used as input
capacitors, care must be taken to ensure that ringing from
inrush currents and
switching does not pose an overvolt-
age
hazard to the power switch and controller. To
dampen
input voltage transients, add a small 5μF to 40μF aluminum
electrolytic capacitor with an ESR in the range of 0.5Ω to
2Ω. High performance through-hole capacitors may also
be used, but an additional ceramic capacitor in parallel
is recommended to reduce the effect of lead inductance.
