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參數(shù)資料
| 型號： | LTC1871IMS-1#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | PLASTIC, MSOP-10 |
| 文件頁數(shù)： | 14/36頁 |
| 文件大小： | 523K |
| 代理商： | LTC1871IMS-1#TR |
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LTC1871-1
21
18711fb
APPLICATIONS INFORMATION
The inductor ripple current is:
IL =
IO(MAX)
1– DMAX
= 0.4
7
1– 0.39
= 4.6A
And so the inductor value is:
L
=
VIN(MIN)
IL f
DMAX =
3.3V
4.6A 300kHz
0.39
= 0.93μH
The component chosen is a 1μH 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 maxi-
mum SENSE pin threshold voltage is reduced from its
low duty cycle typical value of 150mV to approximately
140mV. Assuming a MOSFET junction temperature of
125°C, the room temperature MOSFET RDS(ON) should
be less than:
RDS(ON) VSENSE(MAX)
1– DMAX
1
+
2
IO(MAX) T
= 0.140V
1– 0.39
1
+
0.4
2
7A 1.5
= 6.8m
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 greater than:
COUT
IOUT(MAX)
0.01 VOUT f
=
7A
0.01 5V 300kHz
= 466μF
The RMS ripple current rating for this capacitor needs
to exceed:
IRMS(COUT) IO(MAX)
VO –VIN(MIN)
VIN(MIN)
=
7A
5V – 3.3V
3.3V
= 5A
To satisfy this high RMS current demand, four 150μF
Panasonic capacitors (EEFUEOJ151R) are required.
In parallel with these bulk capacitors, two 22μF, low
ESR (X5R) Taiyo Yuden ceramic capacitors (JMK-
325BJ226MM) 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 ow.
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 toler-
ate. For this particular design and lab setup a 100μF
Sanyo Poscap (6TPC 100M), in parallel with two 22μF
Taiyo Yuden ceramic capacitors (JMK325BJ226MM)
is required (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.
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