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参数资料
| 型号： | LTC3728IG |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A DUAL SWITCHING CONTROLLER, 590 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封装： | 0.209 INCH, PLASTIC, SSOP-28 |
| 文件页数： | 21/36页 |
| 文件大小： | 1661K |
| 代理商： | LTC3728IG |
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LTC3728
28
3728fg
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 using the INTVCC pin generating
1V for 300kHz operation. The minimum inductance for
30% ripple current is:
IL =
VOUT
(f)(L)
1–
VOUT
VIN
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:
tON(MIN) =
VOUT
VIN(MAX)f
=
1.8V
22V(300kHz)
= 273ns
The RSENSE resistor value can be calculated by using the
maximum current sense voltage specication with some
accommodation for tolerances:
RSENSE
60mV
5.84A
0.01
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 specied input
current.
R1(MAX) = 24k
0.8V
2.4V – VOUT
= 24K
0.8V
2.4V – 1.8V
= 32k
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 Siliconix Si4412DY results
in RDS(ON) = 0.042Ω, CRSS = 100pF. At maximum input
voltage with T(estimated) = 50°C:
PMAIN =
1.8V
22V
5
()2 1+(0.005)(50°C–25°C)
[]
0.042
()+1.7 22V
()2 5A
() 100pF
() 300kHz
(
= 220mW
A short-circuit to ground will result in a folded back cur-
rent of:
ISC =
25mV
0.01
+
1
2
200ns(22V)
3.3
μH
= 3.2A
with a typical value of RDS(ON) and δ = (0.005/°C)(20) =
0.1. The resulting power dissipated in the bottom MOSFET
is:
PSYNC =
22V – 1.8V
22V
3.2A
()2 1.1
() 0.042
()
= 434mW
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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