12
LTC3737
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APPLICATIO S I FOR ATIO
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The typical LTC3737 application circuit is shown in Figure
1. External component selection for each of the LTC3737’s
controllers is driven by the load requirement and begins
with the selection of the inductor (L) and the power
MOSFET M1. Next, the output diode D1 is selected. Finally
C
IN
and C
OUT
are chosen.
Power MOSFET Selection
An external P-channel MOSFET must be selected for use
with each channel of the LTC3737. The main selection
criteria for the power MOSFET are the breakdown voltage
V
BR(DSS)
, threshold voltage V
GS(TH)
, on-resistance
R
DS(ON)
, reverse transfer capacitance C
RSS
and the total
gate charge Q
G
.
The gate drive voltage is the input supply voltage. Since the
LTC3737 is designed for operation down to low input
voltages, a sublogic level MOSFET (R
DS(ON)
guaranteed at
V
GS
= 2.5V) is required for applications that work close to
this voltage. When these MOSFETs are used, make sure
that the input supply to the LTC3737 is less than the abso-
lute maximum MOSFET V
GS
rating, which is typically 8V.
The P-channel MOSFET’s on-resistance is chosen based
on the required load current. The maximum average
output load current, I
OUT(MAX)
,
is equal to the peak induc-
tor current minus half the peak-to-peak ripple current,
I
RIPPLE
. The LTC3737’s current comparator monitors the
drain-to-source voltage, V
DS
, of the P-channel MOSFET,
which is sensed between the SENSE
+
and SW pins. The
peak inductor current is limited by the current threshold,
set by the voltage on the I
TH
pin, of the current comparator.
The voltage on the I
TH
pin is internally clamped, which
limits the maximum current sense threshold ∆V
SENSE(MAX)
to approximately 125mV when IPRG is floating (85mV
when IPRG is tied low; 204mV when IPRG is tied high).
The output current that the LTC3737 can provide is given
by:
I
V
R
I
OUT MAX
SENSE MAX
DS ON
RIPPLE
()
()
()
–=
∆
2
where I
RIPPLE
is the inductor peak-to-peak ripple current
(see Inductor Value Calculation).
A reasonable starting point is setting ripple current I
RIPPLE
to be 40% of I
OUT(MAX)
. Rearranging the above equation
yields:
R
V
I
DS ON MAX
SENSE MAX
OUT MAX
()( )
()
()
•=
∆
5
6
for Duty Cycle < 20%
However, for operation above 20% duty cycle, slope
compensation has to be taken into consideration to select
the appropriate value of R
DS(ON)
to provide the required
amount of load current:
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX
()( )
()
()
••=
∆
5
6
where SF is a scale factor whose value is obtained from the
curve in Figure 2.
These must be further derated to take into account the
significant variation in on-resistance with temperature.
The following equation is a good guide for determining the
required R
DS(ON)MAX
at 25°C (manufacturer’s specifica-
tion), allowing some margin for variations in the LTC3737
and external component values:
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX T
()( )
()
()
•.• •
•
=
∆
5
6
09
ρ
The ρ
T
is a normalizing term accounting for the temperature
variation in on-resistance, which is typically about 0.4%/°C,
as shown in Figure 5. Junction to case temperature T
JC
is
Figure 5. R
DS(ON)
vs Temperature
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
3737 F05
0.5
0
0
50
100
2.0
