LTC3728LEGN Linear Technology, LTC3728LEGN Datasheet - Page 28

• Manufacturer Part Number: LTC3728LEGN
• Description: IC REG SW DUAL 2PH STPDWN 28SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3728LEGN

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 7 V
• Current - Output: 3A
• Frequency - Switching: 250kHz ~ 550kHz
• Voltage - Input: 4.5 ~ 28 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: -

APPLICATIONS INFORMATION
LTC3728L/LTC3728LX
Design Example
As a design example for one channel, assume V
(nominal), V
f = 300kHz.
The inductance value is chosen fi rst 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 from the INTV
0.7V for 300kHz operation. The minimum inductance for
30% ripple current is:
A 4.7μH inductor will produce 23% ripple current and a
3.3μH will result in 33%. The peak inductor current will be
the maximum DC value plus one half the ripple current, or
5.84A, for the 3.3μH value. Increasing the ripple current
will also help ensure that the minimum on-time of 200ns
is not violated. The minimum on-time occurs at maximum
V
The R
maximum current sense voltage specifi cation with some
accommodation for tolerances:
Since the output voltage is below 2.4V, the output resistive
divider will need to be sized to not only set the output
voltage but also to absorb the SENSE pin’s specifi ed
input current.
28
IN
R
R1
t
:
ON(MIN)
SENSE
I
L
(MAX)
SENSE
=
(f)(L)
V
OUT
= 24k
= 24k
=
resistor value can be calculated by using the
IN
60mV
5.84A
V
IN(MAX)
= 22V(max), V
V
1–
OUT
2.4V – V
2.4V – 1.8V
V
V
OUT
0.01
IN
f
0.8V
0.8V
=
22V(300kHz)
OUT
OUT
1.8V
= 32k
= 1.8V, I
CC
= 273ns
pin, generating
MAX
= 5A and
IN
= 12V
Choosing 1% resistors: R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: R
maximum input voltage with T(estimated) = 50°C:
A short-circuit to ground will result in a folded back
current of:
with a typical value of R
The resulting power dissipated in the bottom MOSFET is:
which is less than under full-load conditions.
C
temperature assuming only this channel is on. C
chosen with an ESR of 0.02Ω for low output ripple. The
output ripple in continuous mode will be highest at the
maximum input voltage. The output voltage ripple due to
ESR is approximately:
IN
V
I
P
(
P
SC
0.035
is chosen for an RMS current rating of at least 3A at
MAIN
ORIPPLE
SYNC
5 – 2.3
=
1
0.01
25mV
=
=
= 100mW
1.8V
22V
22V – 1.8V
DS(ON)
+
)
= R
+ 22V
2.3
(
1
22V
ESR
–
( )
5
2
1
= 0.035Ω/0.022Ω, C
(
(ΔI
300kHz
2
)
120ns(22V)
[
2
1+ (0.005)(50°C – 25°C)
DS(ON)
L
(
) = 0.02Ω(1.67A) = 33mV
3.3μH
5A
2.1A
2
)
and δ = (0.005/°C)(20) = 0.1.
= 332mW
)
( )
2
4
(
1.125
(
= 2.1A
215pF
)
MILLER
(
0.022
)
•
= 215pF . At
]
•
)
P–P
OUT
3728lxfe
is