14
LTC3736
3736fa
The typical LTC3736 application circuit is shown in Fig-
ure 13. External component selection for each of the
LTC3736’s controllers is driven by the load requirement
and begins with the selection of the inductor (L) and the
power MOSFETs (MP and MN).
Power MOSFET Selection
Each of the LTC3736’s two controllers requires two exter-
nal power MOSFETs: a P-channel MOSFET for the topside
(main) switch and an N-channel MOSFET for the bottom
(synchronous) switch. Important parameters for the power
MOSFETs are the breakdown voltage V
BR(DSS)
, threshold
voltage V
GS(TH)
, on-resistance R
DS(ON)
, reverse transfer
capacitance C
RSS
, turn-off delay t
D(OFF)
and the total gate
charge Q
G
.
The gate drive voltage is the input supply voltage. Since the
LTC3736 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 LTC3736 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 inductor
current minus half the peak-to-peak ripple current I
RIPPLE
.
The LTC3736’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 128mV when IPRG is floating (86mV when
IPRG is tied low; 213mV when IPRG is tied high).
The output current that the LTC3736 can provide is given
by:
I
V
R
I
OUT MAX
SENSE MAX
DS ON
RIPPLE
(
)
(
)
(
)
–
=
∆
2
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:
R
SF
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 1.
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 LTC3736
and external component values:
R
SF
V
I
DS ON MAX
SENSE MAX
OUT MAX
T
(
)(
)
(
)
(
)
• . •
•
•
=
∆
5
6
0 9
ρ
The
ρ
T
is a normalizing term accounting for the tempera-
ture variation in on-resistance, which is typically about
0.4%/
°
C, as shown in Figure 4. Junction to case tempera-
ture T
JC
is about 10
°
C in most applications. For a maxi-
mum ambient temperature of 70
°
C, using
ρ
80
°
C
~ 1.3 in
the above equation is a reasonable choice.
The power dissipated in the top and bottom MOSFETs
strongly depends on their respective duty cycles and load
current. When the LTC3736 is operating in continuous
mode, the duty cycles for the MOSFETs are:
Top P-Channel Duty Cycle =
V
Bottom N-Channel Duty Cycle =
V
OUT
IN
V
V
V
IN
OUT
IN
–
APPLICATIO S I FOR ATIO
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