参数资料
型号: LTC1871EMS#TR-1
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO10
封装: PLASTIC, MSOP-10
文件页数: 14/36页
文件大小: 354K
代理商: LTC1871EMS#TR-1
21
LTC1871-1
18711fa
The inductor ripple current is:
Δ ==
=
I
D
A
L
O MAX
MAX
χ
.
–.
.
()
1
04
7
10 39
46
And so the inductor value is:
L
V
If
D
V
A
kHz
H
IN MIN
L
MAX
=
Δ
==
μ
()
.
33
4 6
300
039
0 93
The component chosen is a 1
μH inductor made by
Sumida (part number CEP125-H 1ROMH) which has a
saturation current of greater than 20A.
5. With the input voltage to the IC bootstrapped to the
output of the power supply (5V), a logic-level MOSFET
can be used. Because the duty cycle is 39%, the
maximum SENSE pin threshold voltage is reduced from
its low duty cycle typical value of 150mV to approxi-
mately 140mV. Assuming a MOSFET junction tempera-
ture of 125
°C, the room temperature MOSFET RDS(ON)
should be less than:
RV
D
I
V
A
m
DS ON
SENSE MAX
MAX
O MAX
T
()
(
)
()
.
–.
.
.
.
+
=
+
=
Ω
1
2
0 140
10 39
1
04
2
71 5
68
χ
ρ
The MOSFET used was the Fairchild FDS7760A, which
has a maximum RDS(ON) of 8mΩ at 4.5V VGS, a BVDSS
of greater than 30V, and a gate charge of 37nC at 5V
VGS.
6. The diode for this design must handle a maximum DC
output current of 10A and be rated for a minimum
reverse voltage of VOUT, or 5V. A 25A, 15V diode from
On Semiconductor (MBRB2515L) was chosen for its
high power dissipation capability.
7. The output capacitor usually consists of a high valued
bulk C connected in parallel with a lower valued, low
ESR ceramic. Based on a maximum output ripple
voltage of 1%, or 50mV, the bulk C needs to be great-
er than:
C
I
Vf
A
V
kHz
F
OUT
OUT MAX
OUT
=
μ
()
.
.
001
7
0 01 5
300
466
The RMS ripple current rating for this capacitor needs
to exceed:
II
VV
V
A
VV
V
A
RMS COUT
O MAX
OIN MIN
IN MIN
()
(
)
()
–.
.
=
7
53 3
33
5
To satisfy this high RMS current demand, four 150
μF
Panasonic capacitors (EEFUEOJ151R) are required.
In parallel with these bulk capacitors, two 22
μF, low
ESR (X5R) Taiyo Yuden ceramic capacitors
(JMK325BJ226MM) are added for HF noise reduction.
Check the output ripple with a single oscilloscope
probe connected directly across the output capacitor
terminals, where the HF switching currents flow.
8. The choice of an input capacitor for a boost converter
depends on the impedance of the source supply and the
amount of input ripple the converter will safely tolerate.
For this particular design and lab setup a 100
μF Sanyo
Poscap (6TPC 100M), in parallel with two 22
μF Taiyo
Yuden ceramic capacitors (JMK325BJ226MM) is re-
quired (the input and return lead lengths are kept to a
few inches, but the peak input current is close to 20A!).
As with the output node, check the input ripple with a
single oscilloscope probe connected across the input
capacitor terminals.
APPLICATIO S I FOR ATIO
WU
UU
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