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| 型号： | LTC1779ES6#TR |
| 厂商： | Linear Technology |
| 文件页数： | 8/12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
APPLICATIO S I FOR ATIO
I D = ? IN OUT ? I OUT
? V OUT RIPPLE ? ESR +
≈ I
?
8 fC OUT ?
Under normal load conditions, the average current con-
ducted by the diode is:
? V ? V ?
? V IN + V D ?
The allowable forward voltage drop in the diode is calcu-
lated from the maximum short-circuit current as:
The selection of C OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering.
The output ripple ( ? V OUT ) is approximated by:
? 1 ?
?
V F ≈
P D
I CL ( MAX )
where f is the operating frequency, C OUT is the output
capacitance and I RIPPLE is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
where P D is the allowable power dissipation and will be
determined by efficiency and/or thermal requirements.
A fast switching diode must also be used to optimize
efficiency. Schottky diodes are a good choice for low
forward drop and fast switching times. Remember to keep
lead length short and observe proper grounding (see
Board Layout Checklist) to avoid ringing and increased
dissipation.
C IN and C OUT Selection
In continuous mode, the source current of the internal
P-channel MOSFET is a square wave of duty cycle
(V OUT + V D )/(V IN + V D ). To prevent large voltage tran-
sients, a low ESR input capacitor sized for the maximum
RMS current must be used. The maximum RMS capaci-
tor current is given by:
since ? I L increases with input voltage.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR (size)
product of any aluminum electrolytic at a somewhat
higher price. Once the ESR requirement for C OUT has been
met, the RMS current rating generally far exceeds the
I RIPPLE(P-P) requirement.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. In the case of tantalum, it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
C IN Required I RMS ≈ I MAX
[ V OUT ( V IN ? V OUT ) ] 1 / 2
V IN
TPS, AVX TPSV and KEMET T510 series of surface mount
tantalum, available in case heights ranging from 2mm to
4mm. Other capacitor types include Sanyo OS-CON,
Nichicon PL series and Panasonic SP.
This formula has a maximum at V IN = 2V OUT , where I RMS
= I OUT /2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet the
size or height requirements in the design. Due to the high
operating frequency of the LTC1779, ceramic capacitors
can also be used for C IN . Always consult the manufacturer
if there is any question.
8
Low Supply Operation
Although the LTC1779 can function down to approxi-
mately 2.0V, the maximum allowable output current is
reduced when V IN decreases below 3V. Figure 3 shows the
amount of change as the supply is reduced down to 2V.
Also shown in Figure 4 is the effect of V IN on V REF as V IN
goes below 2.3V.
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