LTC3736-2 Linear Technology, LTC3736-2 Datasheet - Page 14

• Manufacturer Part Number: LTC3736-2
• Description: Synchronous Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3736-2
The typical LTC3736-2 application circuit is shown in
Figure 13. External component selection for each of the
LTC3736-2’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 LTC3736-2’s two controllers requires two
external 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
threshold voltage V
transfer capacitance C
total gate charge Q
The gate drive voltage is the input supply voltage. Since
the LTC3736-2 is designed for operation down to low
input voltages, a sublogic level MOSFET (R
guaranteed at V
that work close to this voltage. When these MOSFETs are
used, make sure that the input supply to the LTC3736-2
is less than the absolute maximum MOSFET V
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
current minus half the peak-to-peak ripple current I
The LTC3736-2’s current comparator monitors the drain-
to-source voltage V
sensed between the SENSE
inductor current is limited by the current threshold, set by
the voltage on the I
voltage on the I
the maximum current sense threshold ∆V
approximately 240mV when IPRG is floating (167mV
when IPRG is tied low; 345mV when IPRG is tied high).
The output current that the LTC3736-2 can provide is
given by:
14
I
OUT MAX
(
)
=
TH
∆
GS
V
pin is internally clamped, which limits
SENSE MAX
R
G
U
GS(TH)
DS
TH
= 2.5V) is required for applications
OUT(MAX)
.
DS ON
pin of the current comparator. The
of the P-channel MOSFET, which is
RSS
(
(
, on-resistance R
U
)
, turn-off delay t
is equal to the peak inductor
+
)
–
and SW pins. The peak
I
RIPPLE
W
2
DS(ON)
D(OFF)
SENSE(MAX)
U
GS
, reverse
BR(DSS)
and the
RIPPLE
DS(ON)
rating,
to
,
.
A reasonable starting point is setting ripple current I
to be 40% of I
yields:
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
amount of load current:
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 determin-
ing the required R
specification), allowing some margin for variations in
the LTC3736-2 and external component values:
The ρ
ture variation in on-resistance, which is typically about
0.4%/°C, as shown in Figure 4. Junction to case tempera-
ture T
mum ambient temperature of 70°C, using ρ
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 LTC3736-2 is operating in continuous
mode, the duty cycles for the MOSFETs are:
R
R
R
Bottom N-Channel Duty Cycle =
Top P-Channel Duty Cycle =
DS ON MAX
DS ON MAX
DS ON MAX
T
JC
(
(
(
is a normalizing term accounting for the tempera-
is about 10°C in most applications. For a maxi-
)(
)(
)(
OUT(MAX)
)
)
)
=
=
=
5
6
5
6
5
6
DS(ON)MAX
•
•
• . •
SF
0 9
. Rearranging the above equation
∆
V
I
OUT MAX
SENSE MAX
•
DS(ON)
∆
SF
(
V
I
OUT MAX
SENSE MAX
at 25°C (manufacturer’s
•
(
I
w w w . D a t a S h e e t 4 U . c
∆
to provide the required
OUT MAX
(
)
V
V
V
OUT
SENSE MAX
(
)
IN
(
V
)
IN
)
(
)
–
V
•
IN
V
80°C
ρ
OUT
T
)
~ 1.3 in
RIPPLE
37362fa