LTC1624
U
W U U
APPLICATIONS INFORMATION
Step-Down Converter: Power MOSFET Selection
characteristics. The constant k = 2.5 can be used to
estimate the contributions of the two terms in the PMAIN
dissipation equation.
One external N-channel power MOSFET must be selected
for use with the LTC1624 for the top (main) switch.
Step-Down Converter: Output Diode Selection (D1)
The peak-to-peak gate drive levels are set by the INTVCC
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
(VIN < 5V) sublogic level threshold MOSFETs should be
used. PaycloseattentiontotheBVDSS specificationforthe
MOSFETs as well; many of the logic level MOSFETs are
limited to 30V or less.
The Schottky diode D1 shown in Figure 1 conducts during
theoff-time. Itisimportanttoadequatelyspecifythediode
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 (VOUT = 0V). Under this condition, the diode
must safely handle ISC(PK) at close to 100% duty cycle.
Under normal load conditions, the average current con-
ducted by the diode is simply:
Selection criteria for the power MOSFET include the “ON”
resistance RDS(ON), reverse transfer capacitance CRSS
,
input voltage and maximum output current. When the
LTC1624 is operating in continuous mode the duty cycle
for the top MOSFET is given by:
V
V
OUT
−
IN
I
=I
DIODE AVG
LOAD AVG
(
)
(
)
V + V
IN
D
V
+ V
D
OUT
Remember to keep lead lengths short and observe proper
grounding (see Board Layout Checklist) to avoid ringing
and increased dissipation.
Main Switch Duty Cycle =
V + V
IN
D
The MOSFET power dissipation at maximum output
current is given by:
The forward voltage drop allowable in the diode is calcu-
lated from the maximum short-circuit current as:
2
) (
V
+ V
D
OUT
P
V + V
IN D
P
=
I
1+ δ R
+
D
(
)
MAIN
MAX
DS ON
(
)
V ≈
V + V
D
D
IN
I
V
IN
SC AVG
(
)
1.85
)
k V
(
I
C
f
(
)(
)( )
IN
MAX RSS
where PD is the allowable diode power dissipation and will
be determined by efficiency and/or thermal requirements
(see Efficiency Considerations).
where δ is the temperature dependency of RDS(ON) and k
is a constant inversely related to the gate drive current.
MOSFETs have I2R losses, plus the PMAIN equation
includes an additional term for transition losses that are
highest at high output voltages. For VIN < 20V the high
currentefficiencygenerallyimproveswithlargerMOSFETs,
whileforVIN >20Vthetransitionlossesrapidlyincreaseto
the point that the use of a higher RDS(ON) device with lower
CRSS 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%.
Step-Down Converter: CIN and COUT Selection
In continuous mode the source current of the top
N-channel MOSFET is a square wave of approximate duty
cycle VOUT/VIN. 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:
1/2
]
V
V − V
OUT
(
)
OUT IN
[
C Required I
≈I
Theterm(1+δ)isgenerallygivenforaMOSFETintheform
of a normalized RDS(ON) vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltageMOSFETs.CRSSisusuallyspecifiedintheMOSFET
IN
RMS MAX
V
IN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
8
Linear [ Linear ]
