LTC3729EG#TR Linear Technology, LTC3729EG#TR Datasheet - Page 22

• Manufacturer Part Number: LTC3729EG#TR
• Description: IC SW REG SYNC STEP-DOWN 28-SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3729EG#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 5 V
• Current - Output: 5A
• Frequency - Switching: 1.1MHz
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LTC3729EGTR

APPLICATIONS INFORMATION
LTC3729
Design Example (Using Two Phases)
As a design example, assume V
(max), V
The inductance value is chosen first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the PLLFLTR
pin to a resistive divider using the INTV
1V for 300kHz operation. The minimum inductance for
30% ripple current is:
A 2µH inductor will produce 20% ripple current. The peak
inductor current will be the maximum DC value plus one
half the ripple current, or 11.5A. The minimum on‑time
occurs at maximum V
The R
maximum current sense voltage specification with some
accomodation for tolerances:
Choosing 1% resistors: R1 = 16.5k and R2 = 13.2k yields
an output voltage of 1.80V.
22
L ≥
R
t
ON MIN
SENSE
≥
≥ 1.35µH
SENSE
(
(
V
f ∆I
300kHz
( )
OUT
OUT
)
=
resistors value can be calculated by using the
=
= 1.8V, I

 
11.5A
50mV
1−
V
V
OUT
)
1.8V
IN
(
V
30%
f
V
OUT
IN
MAX
=
≈ 0.005Ω
(
IN
)

 
5.5V
(
:
= 20A, T
10A
1.8V
)
)
(
IN
300kHz

 
1−
= 5V (nominal), V
A
= 70°C and f = 300kHz.
1.8V
5.5V
)
CC
= 1.1µs

 
pin to generate
IN
= 5.5V
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
R
voltage with T
temperature:
The worst‑case power disipated by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction
temperature rise is:
A short‑circuit to ground will result in a folded back
current of:
The worst‑case power disipated by the synchronous
MOSFET under short‑circuit conditions at elevated ambi‑
ent temperature and estimated 50°C junction temperature
rise is:
DS(ON)
P
P
I
P
SC
MAIN
SYNC
SYNC
=
0.005Ω
= 0.013Ω, C
=
=
= 1.29W
=
= 360mW
25mV
0.013Ω + 1.7 5.5V
(
1.8V
5.5V
5.5V − 1.8V
5.5V − 1.8V
310kHz
j
(estimated) = 110°C at an elevated ambient
5.5V
5.5V
( )
+
10
2
1
)
= 0.61W



2
RSS
200ns 5.5V
 
1+ 0.005
(
(
(
10A
5.28A
= 300pF . At maximum input
(
2µH
(
)
2
)
)
2
(
2
1.48
(
(
)
10A
1.48
)
(
110°C − 25°C

 = 5.28A

)
)
(
(
0.013Ω
)
300pF
(
0.013Ω
)
)
)
)
 
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