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| 型号： | LT1767EMS8#TR-3.3 |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 1500 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | PLASTIC, MSOP-8 |
| 文件页数： | 16/16页 |
| 文件大小： | 232K |
| 代理商： | LT1767EMS8#TR-3.3 |
9
LT1767/LT1767-1.8/
LT1767-2.5/LT1767-3.3/LT1767-5
APPLICATIONS INFORMATION
WU
U
CATCH DIODE
The suggested catch diode (D1) is a 1N5818 Schottky, or
its Motorola equivalent, MBR130. It is rated at 1A average
forward current and 30V reverse voltage. Typical forward
voltage is 0.5V at 1A. The diode conducts current only
during switch off time. Peak reverse voltage is equal to
regulator input voltage. Average forward current in normal
operation can be calculated from:
I
IV
V
D AVG
OUT
IN
OUT
IN
( )=
()
This formula will not yield values higher than 1A with
maximum load current of 1.3A unless the ratio of input to
output voltage exceeds 4.3:1. The only reason to consider
a larger diode is the worst-case condition of a high input
voltage and shorted output. With a shorted condition,
diode current will increase to a typical value of 2A, deter-
mined by peak switch current limit. This is safe for short
periods of time, but it would be prudent to check with the
diode manufacturer if continuous operation under these
conditions must be tolerated.
BOOST PIN
For most applications, the boost components are a 0.1
F
capacitor and a 1N914 or 1N4148 diode. The anode is
typically connected to the regulated output voltage to
generate a voltage approximately VOUT above VIN to drive
the output stage. The output driver requires at least 2.7V
of headroom throughout the on period to keep the switch
fully saturated. However, the output stage discharges the
boost capacitor during the on time. If the output voltage is
less than 3.3V, it is recommended that an alternate boost
supply is used. The boost diode can be connected to the
input, although, care must be taken to prevent the 2x VIN
boost voltage from exceeding the BOOST pin absolute
maximum rating. The additional voltage across the switch
driver also increases power loss, reducing efficiency. If
available, an independent supply can be used with a local
bypass capacitor.
A 0.1
F boost capacitor is recommended for most appli-
cations. Almost any type of film or ceramic capacitor is
suitable, but the ESR should be <1
to ensure it can be
omit this step. Powdered iron cores are forgiving
because they saturate softly, whereas ferrite cores
saturate abruptly. Other core materials fall somewhere
in between.
II
VV
V
Lf V
PEAK
OUT
IN
OUT
IN
=+
()
()( )( )
2
VIN = Maximum input voltage
f = Switching frequency, 1.25MHz
3. Decide if the design can tolerate an “open” core geom-
etry like a rod or barrel, which have high magnetic field
radiation, or whether it needs a closed core like a toroid
to prevent EMI problems. This is a tough decision
because the rods or barrels are temptingly cheap and
small and there are no helpful guidelines to calculate
when the magnetic field radiation will be a problem.
4. After making an initial choice, consider the secondary
things like output voltage ripple, second sourcing, etc.
Use the experts in the Linear Technology’s applica-
tions department if you feel uncertain about the final
choice. They have experience with a wide range of
inductor types and can tell you about the latest devel-
opments in low profile, surface mounting, etc.
Table 3
PART NUMBER
VALUE (uH)
IRMS (Amps) DCR () HEIGHT (mm)
Coiltronics
TP1-2R2
2.2
1.3
0.188
1.8
TP2-2R2
2.2
1.5
0.111
2.2
TP3-4R7
4.7
1.5
0.181
2.2
TP4- 100
10
1.5
0.146
3.0
Murata
LQH1C1R0M04
1.0
0.51
0.28
1.8
LQH3C1R0M24
1.0
0.06
2.0
LQH3C2R2M24
2.2
0.79
0.1
2.0
LQH4C1R5M04
1.5
1
0.09
2.6
Sumida
CD73- 100
10
1.44
0.080
3.5
CDRH4D18-2R2
2.2
1.32
0.058
1.8
CDRH5D18-6R2
6.2
1.4
0.071
1.8
CDRH5D28-100
10
1.3
0.048
2.8
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