ltc3728legn-trpbf Linear Technology Corporation, ltc3728legn-trpbf Datasheet - Page 28

• Manufacturer Part Number: ltc3728legn-trpbf
• Description: Dual, 550khz, 2-phase Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3728
Design Example
As a design example for one channel, assume V
12V(nominal), V
and 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 using the INTV
1V 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 cur-
rent, or 5.84A, for the 3.3μH value. Increasing the ripple
current will also help ensure that the minimum on-time
of 100ns 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 resis-
tive divider will need to be sized to not only set the output
voltage but also to absorb the SENSE pins specifi ed input
current.
28
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
5.84A
60mV
:
V
IN(MAX)
V
1–
IN
OUT
2.4V – V
= 22V(max), V
2.4V – 1.8V
V
V
OUT
0.01
IN
f
0.8V
0.8V
=
22V(300kHz)
OUT
1.8V
= 32k
OUT
= 1.8V, I
CC
= 273ns
pin generating
MAX
= 5A,
IN
=
Choosing 1% resistors; R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
The power dissipation on the top side MOSFET can be
easily estimated. Choosing a Siliconix Si4412DY results
in; R
voltage with T(estimated) = 50°C:
A short-circuit to ground will result in a folded back cur-
rent of:
with a typical value of R
0.1. 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
is chosen for an RMS current rating of at least 3A at
MAIN
ORIPPLE
SYNC
DS(ON)
=
0.01
25mV
=
= 220mW
=
= 434mW
(
0.042
1.8V
22V
22V – 1.8V
= 0.042Ω, C
= R
22V
ESR
+
( )
5
2
1
)
(ΔI
2
+ 1.7 22V
200ns(22V)
[
1+ (0.005)(50°C – 25°C)
L
(
) = 0.02Ω(1.67A) = 33mV
3.3μH
RSS
DS(ON)
3.2A
(
= 100pF . At maximum input
)
2
)
and δ = (0.005/°C)(20) =
2
( )
1.1
( )
5A
= 3.2A
(
0.042
(
100pF
)
)
(
]
300kHz
P–P
OUT
3728fd
is