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參數(shù)資料
| 型號： | LTC3728LZEUH |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 3 A DUAL SWITCHING CONTROLLER, 590 kHz SWITCHING FREQ-MAX, PQCC28 |
| 封裝： | 5 X 5 MM, PLASTIC, MO-220WHHD, QFN-28 |
| 文件頁數(shù)： | 18/32頁 |
| 文件大?。?/td> | 467K |
| 代理商： | LTC3728LZEUH |
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LTC3728LZ
25
3728lzf
APPLICATIONS INFORMATION
Design Example
As a design example for one channel, assume VIN =
12V(nominal), VIN = 22V(max), VOUT = 1.8V, IMAX = 5A,
and f = 300kHz.
The inductance value is chosen 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 INTVCC pin, generating
0.7V for 300kHz operation. The minimum inductance for
30% ripple current is:
I
V
fL
V
L
OUT
IN
=
()( )
–
1
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 VIN:
t
V
Vf
V
kHz
ns
ON MIN
OUT
IN MAX
()
.
()
==
=
18
22 300
273
The RSENSE resistor value can be calculated by using the
maximum current sense voltage specication with some
accommodation for tolerances:
R
mV
A
SENSE
≤≈
60
584
001
.
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 pin’s specied input
current.
Rk
V
VV
k
V
VV
k
MAX
OUT
124
08
24
08
24
18
32
()
.
.–
.
.– .
=
=
=
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 Fairchild FDS6982S dual MOSFET
results in: RDS(ON) = 0.035Ω/0.022Ω, CMILLER = 215pF. At
maximum input voltage with T(estimated) = 50°C:
P
V
CC
V
A
pF
kHz
mW
MAIN
=
() +° °
[]
()+()
()( )
+
()=
18
22
5
1
0 005 50
25
0 035
22
5
2
4
215
1
52 3
1
23
300
332
2
.
( .
)(
–
)
.
–.
.
A short-circuit to ground will result in a folded back cur-
rent of:
I
mV
ns
V
H
A
SC
=
=
25
001
1
2
120
22
33
21
.
–
()
.
with a typical value of RDS(ON) and δ = (0.005/°C)(20) =
0.1. The resulting power dissipated in the bottom MOSFET
is:
P
VV
V
A
mW
SYNC
=
() (
)
()
=
22
1 8
22
2 1
1 125 0 022
100
2
–.
..
.
which is less than under full-load conditions.
CIN is chosen for an RMS current rating of at least 3A at
temperature assuming only this channel is on. COUT is
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:
VORIPPLE = RESR (ΔIL) = 0.02Ω(1.67A) = 33mVP–P
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