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参数资料
| 型号： | LTC1909-8EG#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 19/32页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 28-SSOP |
| 标准包装： | 2,000 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 200kHz |
| 占空比： | 90% |
| 电源电压： | 4 V ~ 36 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 28-SSOP（0.209"，5.30mm 宽） |
| 包装： | 带卷 (TR) |
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LTC1909-8
APPLICATIO S I FOR ATIO
f =
V OUT
V VON R ON ( 10 pF )
[ Hz ]
Inductor Selection
Given the desired input and output voltages, the inductor
? I L = ? OUT ? ? 1 ? OUT ?
Toholdfrequencyconstantduringoutputvoltagechanges,
tie the V ON pin to V OUT . The V ON pin has internal clamps
that limit its input to the one-shot timer. If the pin is tied
below 0.7V, the input to the one-shot is clamped at 0.7V.
Similarly, if the pin is tied above 2.4V, the input is clamped
at 2.4V.
Because the voltage at the I ON pin is about 0.7V, the
current into this pin is not exactly inversely proportional to
V IN , especially in applications with lower input voltages.
To correct for this error, an additional resistor R ON2
connected from the I ON pin to the 5V INTV CC supply will
further stabilize the frequency.
value and operating frequency determine the ripple
current:
? V ? ? V ?
? fL ? ? V IN ?
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
R ON 2 =
5 V
0 . 7 V
R ON
that is about 40% of I OUT(MAX) . The largest ripple current
occurs at the highest V IN . To guarantee that ripple current
does not exceed a specified maximum, the inductance
L = ? ? ? 1 ? V ?
Changes in the load current magnitude will also cause
frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the I TH pin to the V ON pin and V OUT . The values
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the I TH pin to the V ON pin as
shown in Figure 3a. Place capacitance on the V ON pin to
filter out the I TH variations at the switching frequency. The
resistor load on I TH reduces the DC gain of the error amp
and degrades load regulation, which can be avoided by
using the PNP emitter follower of Figure 3b.
R VON1
30k
should be chosen according to:
? V OUT ? ? V OUT ?
? f ? I L ( MAX ) ? ? IN ( MAX ) ?
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool M μ ? cores. A variety of inductors designed for high
current, low voltage applications are available from manu-
facturers such as Sumida, Panasonic, Coiltronics, Coil-
craft and Toko.
Kool M μ is a registered trademark of Magnetics, Inc.
R VON1
3k
V OUT
R VON2
100k
C VON
0.01 μ F
R C
V ON
LTC1909-8
V OUT
INTV CC
10k
R VON2
10k
C VON
0.01 μ F
R C
V ON
LTC1909-8
C C
(3a)
I TH
2N5087
C C
(3b)
I TH
19098 F03
Figure 3. Correcting Frequency Shift with Load Current Changes
19098f
19
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