LTC3736EUF#TR Linear Technology, LTC3736EUF#TR Datasheet - Page 14

• Manufacturer Part Number: LTC3736EUF#TR
• Description: IC CTRLR SW SYNC DUAL 2PH 24QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3736EUF#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 9.8 V
• Current - Output: 1A
• Frequency - Switching: 550kHz ~ 750kHz
• Voltage - Input: 2.75 ~ 9.8 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
LTC3736
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
voltage V
capacitance C
charge Q
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
V
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
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’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 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:
14
GS
I
OUT MAX
= 2.5V) is required for applications that work close to
(
G
GS(TH)
.
)
=
RSS
TH
, on-resistance R
∆
DS
, turn-off delay t
V
pin is internally clamped, which limits
SENSE MAX
R
U
TH
OUT(MAX)
of the P-channel MOSFET, which is
DS ON
pin of the current comparator. The
(
(
U
GS
)
rating, which is typically 8V.
is equal to the peak inductor
+
)
–
and SW pins. The peak
I
DS(ON)
D(OFF)
RIPPLE
W
2
DS(ON)
BR(DSS)
and the total gate
, reverse transfer
guaranteed at
SENSE(MAX)
, threshold
U
RIPPLE
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 determining the
required R
tion), allowing some margin for variations in the LTC3736
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 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-
)(
)(
)(
DS(ON)MAX
OUT(MAX)
)
)
)
=
=
=
6
6
6
5
5
5
•
•
• . •
at 25°C (manufacturer’s specifica-
SF
0 9
∆
. Rearranging the above equation
V
I
OUT MAX
SENSE MAX
•
DS(ON)
∆
SF
(
V
I
OUT MAX
SENSE MAX
•
(
I
∆
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
3736fa