LTC1624
APPLICATIONS INFORMATION
Step-Down Converter: Power MOSFET Selection
One external N-channel power MOSFET must be selected
for use with the LTC1624 for the top (main) switch.
The peak-to-peak gate drive levels are set by the INTV
CC
voltage. This voltage is typically 5V. Consequently, logic
level threshold MOSFETs must be used in most LTC1624
applications. If low input voltage operation is expected
(V
IN
< 5V) sublogic level threshold MOSFETs should be
used. Pay close attention to the BV
DSS
specification for the
MOSFETs as well; many of the logic level MOSFETs are
limited to 30V or less.
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 top MOSFET is given by:
characteristics. The constant k = 2.5 can be used to
estimate the contributions of the two terms in the P
MAIN
dissipation equation.
Step-Down Converter: Output Diode Selection (D1)
The Schottky diode D1 shown in Figure 1 conducts during
the off-time. It is important to adequately specify the diode
peak current and average power dissipation so as not to
exceed the diode ratings.
The most stressful condition for the output diode is under
short circuit (V
OUT
= 0V). Under this condition, the diode
must safely handle I
SC(PK)
at close to 100% duty cycle.
Under normal load conditions, the average current con-
ducted by the diode is simply:
I
DIODE AVG
=
I
LOAD AVG
V
IN
−
V
OUT
V
+
V
IN
D
Main Switch Duty Cycle =
V
OUT
+
V
D
V
IN
+
V
D
The MOSFET power dissipation at maximum output
current is given by:
2
V
OUT
+
V
D
P
MAIN
=
I
MAX
1
+ δ
R
DS ON
+
V
IN
+
V
D
( )( )
( )
1
.
85
k
(
V
IN
) (
I
MAX
)(
C
RSS
)(
f
)
where
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
IN
< 20V the high
current efficiency generally improves with larger MOSFETs,
while for V
IN
> 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. The diode losses
are greatest at high input voltage or during a short circuit
when the diode duty cycle is nearly 100%.
The term (1+
δ)
is generally given for a MOSFET in the form
of a normalized R
DS(ON)
vs Temperature curve, but
δ
= 0.005/°C can be used as an approximation for low
voltage MOSFETs. C
RSS
is usually specified in the MOSFET
8
U
W
U
U
( )
( )
Remember to keep lead lengths short and observe proper
grounding (see Board Layout Checklist) to avoid ringing
and increased dissipation.
The forward voltage drop allowable in the diode is calcu-
lated from the maximum short-circuit current as:
V
D
≈
V
IN
+
V
D
I
SC AVG
V
IN
P
D
( )
where P
D
is the allowable diode power dissipation and will
be determined by efficiency and/or thermal requirements
(see Efficiency Considerations).
Step-Down Converter: C
IN
and C
OUT
Selection
In continuous mode the source current of the top
N-channel MOSFET is a square wave of approximate duty
cycle V
OUT
/V
IN
. 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:
C
IN
Required I
RMS
≈
I
MAX
[
V
(
V
OUT IN
−
V
OUT
)
]
1
/
2
V
IN
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
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