14
LTC1624
Boost Converters: Power MOSFET Selection
One external N-channel power MOSFET must be selected
for use with the LTC1624 for the switch. In boost applica-
tions the source of the power MOSFET is grounded along
with the SW pin. The peak-to-peak gate drive levels are set
by the INTV
CC
voltage. The gate drive voltage is equal to
approximately 5V for V
IN
> 5.6V and a logic level MOSFET
can be used. At V
IN
voltages below 5V the gate drive
voltage is equal to V
IN
– 0.6V and a sublogic level MOSFET
should be used.
Selection criteria for the power MOSFET include the “ON”
resistance R
DS(ON)
, reverse transfer capacitance C
RSS
,
input voltage and maximum output current. When the
LTC1624 is operating in continuous mode the duty cycle
for the MOSFET is given by:
Main Switch Duty Cycle = 1−
+
V
VV
IN
OUT D
The MOSFET power dissipation at maximum output cur-
rent is given by:
APPLICATIONS INFORMATION
WUU
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Boost Converter: Inductor Selection
For most applications the inductor will fall in the range of
10µH to 100µH. Higher values reduce the input ripple
voltage and reduce core loss. Lower inductor values are
chosen to reduce physical size.
The input current of the boost converter is calculated at full
load current. Peak inductor current can be significantly
higher than output current, especially with smaller induc-
tors and lighter loads. The following formula assumes
continuous mode operation and calculates maximum peak
inductor current at minimum V
IN
:
II
V
V
I
L PEAK OUT MAX
OUT
IN MIN
L MAX
() ()
()
()
=
+
∆
2
The ripple current in the inductor (∆I
L
) is typically 20% to
30% of the peak inductor current occuring at V
IN(MIN)
and
I
OUT(MAX)
.
∆I
VV V V
kHz L V V
L
IN OUT D IN
OUT D
P-P
()
=
+−
()
()()
+
()
200
with ∆I
L(MAX)
= ∆I
L(P-P)
at V
IN
= V
IN(MIN)
.
Remember boost converters are not short-circuit pro-
tected, and that under output short conditions, inductor
current is limited only by the available current of the input
supply, I
OUT(OVERLOAD)
. Specify the maximum inductor
current to safely handle the greater of I
L(PEAK)
or
I
OUT(OVERLOAD)
. Make sure the inductor’s saturation cur-
rent rating (current when inductance begins to fall)
exceeds the maximum current rating set by R
SENSE
.
Boost Converter: R
SENSE
Selection for Maximum
Output Current
R
SENSE
is chosen based on the required output current.
Remember the LTC1624 current comparator has a maxi-
mum threshold of 160mV/R
SENSE
. The current compara-
tor threshold sets the peak of the inductor current, yielding
a maximum average output current I
OUT(MAX)
equal to
I
L(PEAK)
less half the peak-to-peak ripple current (∆I
L
),
divided by the output-input voltage ratio (see equation for
I
L(PEAK)
)
.
PI
V
VV
R
k V I C kHz
where I I
VV
V
MAIN IN MAX
IN MIN
OUT D
DS ON
OUT IN MAX RSS
IN MAX OUT MAX
OUT D
IN MIN
=
−
+
+
()
+
()
()( )
=
+
()
()
()
()
() ()
()
2
185
11
200
δ
.
δ is the temperature dependency of R
DS(ON)
and k is a
constant inversely related to the gate drive current.
MOSFETs have I
2
R losses, plus the P
MAIN
equation
includes an additional term for transition losses that are
highest at high output voltages. For V
OUT
< 20V the high
current efficiency generally improves with larger MOSFETs,
while for V
OUT
> 20V the transition losses rapidly increase
to the point that the use of a higher R
DS(ON)
device with
lower C
RSS
actual provides higher efficiency. For addi-
tional information refer to Step-Down Converter: Power
MOSFET Selection in the Applications Information
section.
