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| 型号： | LTC1779ES6#TR |
| 厂商： | Linear Technology |
| 文件页数： | 6/12页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 0.25A SOT23-6 |
| 标准包装： | 2,500 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.8 V ~ 9.8 V |
| 输入电压： | 2.5 V ~ 9.8 V |
| PWM 型： | 电流模式，混合 |
| 频率 - 开关： | 100kHz ~ 550kHz |
| 电流 - 输出： | 250mA |
| 同步整流器： | 无 |
| 工作温度： | -65°C ~ 150°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | SOT-23-6 |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | SOT-23-6 |
| 其它名称： | LTC1779ES6TR |
LTC1779
OPERATIO
(Refer to Functional Diagram)
The variable M is the ratio of the total switch current to that
portion of the switch current that flows through R SENSE . M
is a function of both R SENSE and R OUT of the internal power
switch, which in turn, is a strong function of supply
voltage. For values of M refer to Figure 3. In order to
guarantee the desired I PK over the full range of supply
voltage, the minimum value of M, corresponding to the
minimum supply voltage seen in the application, should
be chosen. Note that the selection of R SENSE , and hence
60
55
50
45
40
35
30
25
R SENSE = 18.2 ?
R SENSE = 14 ?
R SENSE = 10 ?
R SENSE = 6.2 ?
R SENSE = 2 ?
R SENSE = 0 ?
the resulting M, is an iterative process. For most applica-
tions, a value of R SENSE between 0 ? and 20 ? will be
chosen.
20
0
1
2
3 4 5 6 7
SUPPLY VOLTAGE (V)
8
9 10
1779 F03
Figure 3. M vs Supply Voltage
APPLICATIO S I FOR ATIO
( V IN ( MAX ) – V OUT ) ? V OUT
V IN ( MAX ) ? ? ? I RIPPLE
The basic LTC1779 application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection of L1 and
R SENSE (= R1). Next, the output diode D1 is selected
followed by C IN (= C1)and C OUT (= C2).
Inductor Value Calculation
current at maximum input voltage. Use the following
equations to calculate L:
I RIPPLE = 0 . 4 ? I OUT ( MAX )
L =
The inductance value has a direct effect on ripple current.
The ripple current, I RIPPLE , decreases with higher induc-
I L ( MAX ) = I OUT ( MAX ) +
I RIPPLE
2
( V IN – V OUT ) V OUT
I RIPPLE =
tance or frequency and increases with higher V IN . The
inductor’s peak-to-peak ripple current is given by:
V IN ? ? ? L
where f is the operating frequency fixed at 550kHz in the
LTC1779.
and then choose an appropriate L and recalculate the
ripple current.
In Burst Mode operation on the LTC1779, the ripple
current is normally set such that the inductor current is
continuous during the burst periods. Therefore, the peak-
to-peak ripple current must not exceed:
A smaller value of L results in higher current ripple and
output voltage ripple as well as greater core losses. Larger
values of L decrease the ripple, but require finding physi-
I RIPPLE ≤
M ( 0 . 030 )
( R SENSE + 2 ? )
cally larger inductors since maximum DC current rating
This implies a minimum inductance of:
f ? ?
decreases significantly as inductance increases within
inductor product types. Generally, by choosing the de-
sired ripple current based on the maximum output cur-
rent, the inductor value can be calculated from the previous
equation. It is typical to choose the inductor so that the
L MIN =
V IN ? V OUT ? V OUT + V D ?
? ?
? M ( 0 . 030 ) ? ? V IN + V D ?
? ( R SENSE + 2 ? ) ?
ripple current is about 40% of the maximum output
6
(Use V IN(MAX) = V IN )
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