Datasheet

LTC1871-1
16
18711fb
APPLICATIONS INFORMATION
for V
SENSE(MAX)
and the R
DS(ON)
of the MOSFET listed in
the manufacturer’s data sheet.
The power dissipated by the MOSFET in a boost converter
is:
P
FET
=
I
O(MAX)
1–D
MAX






2
•R
DS(ON)
•D
MAX
• 
T
+k•V
O
1.85
•
I
O(MAX)
1–D
MAX
( )
•C
RSS
•f
The fi rst term in the equation above represents the I
2
R
losses in the device, and the second term, the switching
losses. The constant, k = 1.7, is an empirical factor inversely
related to the gate drive current and has the dimension
of 1/current.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
T
J
= T
A
+ P
FET
• R
TH(JA)
The R
TH(JA)
to be used in this equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the case to the ambient temperature (R
TH(CA)
). This value
of T
J
can then be compared to the original, assumed value
used in the iterative calculation process.
Boost Converter: Output Diode Selection
To maximize effi ciency, a fast switching diode with low
forward drop and low reverse leakage is desired. The output
diode in a boost converter conducts current during the
switch off-time. The peak reverse voltage that the diode
must withstand is equal to the regulator output voltage.
The average forward current in normal operation is equal
to the output current, and the peak current is equal to the
peak inductor current.
I
D(PEAK)
= I
L(PEAK)
= 1+

2






•
I
O(MAX)
1–D
MAX
The power dissipated by the diode is:
P
D
= I
O(MAX)
• V
D
and the diode junction temperature is:
T
J
= T
A
+ P
D
• R
TH(JA)
The R
TH(JA)
to be used in this equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the board to the ambient temperature in the enclosure.
Remember to keep the diode lead lengths short and to
observe proper switch-node layout (see Board Layout
Checklist) to avoid excessive ringing and increased
dissipation.
Boost Converter: Output Capacitor Selection
Contributions of ESR (equivalent series resistance), ESL
(equivalent series inductance) and the bulk capacitance
must be considered when choosing the correct component
for a given output ripple voltage. The effects of these three
parameters (ESR, ESL and bulk C) on the output voltage
ripple waveform are illustrated in Figure 12e for a typical
boost converter.
The choice of component(s) begins with the maximum
acceptable ripple voltage (expressed as a percentage of
the output voltage), and how this ripple should be divided
between the ESR step and the charging/discharging
Δ V.
For the purpose of simplicity we will choose 2% for the
maximum output ripple, to be divided equally between the
ESR step and the charging/discharging
Δ V. This percent-
age ripple will change, depending on the requirements
of the application, and the equations provided below can
easily be modifi ed.
For a 1% contribution to the total ripple voltage, the ESR
of the output capacitor can be determined using the fol-
lowing equation:
ESR
COUT

0.01• V
O
I
IN(PEAK)
where:
I
IN(PEAK)
= 1+

2






•
I
O(MAX)
1–D
MAX
For the bulk C component, which also contributes 1% to
the total ripple:
C
OUT

I
O(MAX)
0.01• V
O
• f