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| 型号： | LT3430IFE-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 13/28页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 3A 16TSSOP |
| 标准包装： | 95 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 1.2 V ~ 54 V |
| 输入电压： | 5.5 V ~ 60 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 100kHz |
| 电流 - 输出： | 3A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 16-TSSOP-EP |
| 产品目录页面： | 1331 (CN2011-ZH PDF) |
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LT3430/LT3430-1
APPLICATIONS INFORMATION
where:
f = switching frequency
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
t ON = switch minimum on time
V F = diode forward voltage
V IN = Input voltage
I ? R = inductor I ? R voltage drop
If this condition is not observed, the current will not be
limited at I PK , but will cycle-by-cycle ratchet up to some
E Case Size
AVX TPS, Sprague 593D
D Case Size
AVX TPS, Sprague 593D
C Case Size
AVX TPS
ESR (Max., Ω )
0.1 to 0.3
0.1 to 0.3
0.2 (typ)
Ripple Current (A)
0.7 to 1.1
0.7 to 1.1
0.5 (typ)
higher value. Using the nominal LT3430/LT3430-1 clock
frequencies of 200KHz/100kHz, a V IN of 40V and a (V F +
I ? R) of say 0.7V, the maximum t ON to maintain control
would be approximately 90ns for the LT3430 and 180ns
for the LT3430-1, unacceptably short times.
The solution to this dilemma is to slow down the oscil-
lator when the FB pin voltage is abnormally low thereby
indicating some sort of short-circuit condition. Oscillator
frequency is unaffected until FB voltage drops to about
2/3 of its normal value. Below this point the oscillator
frequency decreases roughly linearly down to a limit of
about 40kHz. (30kHz for LT3430-1) This lower oscillator
frequency during short-circuit conditions can then maintain
control with the effective minimum on time.
It is recommended that for [V IN /(V OUT + V F )] ratios >
10, a soft-start circuit should be used for the LT3430 to
control the output capacitor charge rate during start-up
or during recovery from an output short circuit, thereby
adding additional control over peak inductor current. See
Buck Converter with Adjustable Soft-Start later in this
data sheet.
from 22μF to greater than 500μF work well, but you cannot
cheat mother nature on ESR. If you ?nd a tiny 22μF solid
tantalum capacitor, it will have high ESR, and output ripple
voltage will be terrible. Table 3 shows some typical solid
tantalum surface mount capacitors.
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents. This
is historically true, and type TPS capacitors are specially
tested for surge capability, but surge ruggedness is not
a critical issue with the output capacitor. Solid tantalum
capacitors fail during very high turn-on surges, which
do not occur at the output of regulators. High discharge
surges, such as when the regulator output is dead shorted,
do not harm the capacitors.
Unlike the input capacitor, RMS ripple current in the output
capacitor is normally low enough that ripple current rating
is not an issue. The current waveform is triangular with
a typical value of 250mA RMS . The formula to calculate
this is:
Output capacitor ripple current (RMS):
OUTPUT CAPACITOR
The output capacitor is normally chosen by its effective
I RIPPLE ( RMS ) =
0.29 ( V OUT ) ( V IN ? V OUT )
( L )( f )( V IN )
series resistance (ESR), because this is what determines
output ripple voltage. To get low ESR takes volume , so
physically smaller capacitors have high ESR. The ESR
range for typical LT3430 applications is 0.05 Ω to 0.2 Ω .
A typical output capacitor is an AVX type TPS, 100μF at
10V, with a guaranteed ESR less than 0.1 Ω (The LT3430-1
will typically use two of these capacitors in parallel). This
is a “D” size surface mount solid tantalum capacitor. TPS
capacitors are specially constructed and tested for low
ESR, so they give the lowest ESR for a given volume. The
value in microfarads is not particularly critical, and values
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available. They are generally chosen for their
good high frequency operation, small size and very low
ESR (effective series resistance). Their low ESR reduces
output ripple voltage but also removes a useful zero in the
loop frequency response, common to tantalum capaci-
tors. To compensate for this, a resistor R C can be placed
in series with the V C compensation capacitor C C . Care
must be taken however, since this resistor sets the high
34301fa
13
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