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| 型号： | LT3430IFE-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 12/28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
Reduced Inductor Value and Discontinuous Mode
If the smallest inductor value is of most importance to a
converter design, in order to reduce inductor size/cost,
discontinuous mode may yield the smallest inductor solu-
I OUT(MAX)
Discontinuous =
Mode
I OUT(MAX)
3 2 ? (200 ? 10 3 )(4. 7 ? 10 ? 6 )(15)
2 ( 5 + 0 . 52 )( 15 – 5 – 0 . 52 )
= 1.21A
tion. The maximum output load current in discontinuous
mode, however, must be calculated and is de?ned later
in this section.
Discontinuous mode is entered when the output load
current is less than one-half of the inductor ripple current
(I LP-P ). In this mode, inductor current falls to zero before
the next switch turn on (see Figure 8). Buck converters
will be in discontinuous mode for output load current
given by:
Discontinuous Mode
What has been shown here is that if high inductor ripple
current and discontinuous mode operation can be tolerated,
small inductor values can be used. If a higher output load
current is required, the inductor value must be increased.
If I OUT(MAX) no longer meets the discontinuous mode
criteria, use the I OUT(MAX) equation for continuous mode;
the LT3430/LT3430-1 are designed to operate well in both
modes of operation, allowing a large range of inductor
I OUT
Discontinuous Mode
<
(V OUT + V F )(V IN – V OUT – V F )
( 2 )( V IN )( f )( L )
values to be used.
Short-Circuit Considerations
The inductor value in a buck converter is usually chosen
large enough to keep inductor ripple current (I LP-P ) low;
this is done to minimize output ripple voltage and maximize
output load current. In the case of large inductor values,
as seen in the equation above, discontinuous mode will
be associated with “light loads.”
When choosing small inductor values, however, discontinu-
ous mode will occur at much higher output load currents.
The limit to the smallest inductor value that can be chosen
is set by the LT3430/LT3430-1 peak switch current (I P ) and
the maximum output load current required, given by:
The LT3430/LT3430-1 are current mode controllers. They
use the V C node voltage as an input to a current compara-
tor which turns off the output switch on a cycle-by-cycle
basis as this peak current is reached. The internal clamp on
the V C node, nominally 2V, then acts as an output switch
peak current limit. This action becomes the switch current
limit speci?cation. The maximum available output power
is then determined by the switch current limit.
A potential controllability problem could occur under
short-circuit conditions. If the power supply output is
short circuited, the feedback ampli?er responds to the
I OUT(MAX)
Discontinuous Mode
I P 2
=
( 2 )( I LP-P )
=<
I LP-P
2
low output voltage by raising the control voltage, V C , to its
peak current limit value. Ideally, the output switch would
be turned on, and then turned off as its current exceeded
the value indicated by V C . However, there is ?nite response
time involved in both the current comparator and turnoff
of the output switch. These result in a minimum on time
I P ( f ? L ? V IN )
=
2 ( V OUT
2
+ V F )( V IN – V OUT – V F )
t ON(MIN) . When combined with the large ratio of V IN to
(V F + I ? R), the diode forward voltage plus inductor I ? R
voltage drop, the potential exists for a loss of control.
Example: For V IN = 15V, V OUT = 5V, V F = 0.52V, f = 200kHz
and L = 4.7μH.
Expressed mathematically the requirement to maintain
control is:
f ? t ON ≤
V F + I ? R
V IN
34301fa
12
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