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參數資料
| 型號： | LT1939IDD#TRPBF |
| 廠商： | Linear Technology |
| 文件頁數： | 15/24頁 |
| 文件大小： | 349K |
| 描述： | IC REG DL BUCK/LINEAR 12DFN |
| 標準包裝： | 2,500 |
| 拓撲： | 降壓（降壓）（1），線性（LDO）（1） |
| 功能： | 任何功能 |
| 輸出數： | 2 |
| 頻率 - 開關： | 500kHz ~ 2.4MHz |
| 電壓/電流 - 輸出 1： | 0.8 V ~ 24.25 V，2A |
| 電壓/電流 - 輸出 2： | 0.8 V ~ 24.25 V，1A |
| 帶 LED 驅動器： | 無 |
| 帶監控器： | 無 |
| 帶序列發生器： | 無 |
| 電源電壓： | 3 V ~ 25 V |
| 工作溫度： | -40°C ~ 125°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 12-WFDFN 裸露焊盤 |
| 供應商設備封裝： | 12-DFN |
| 包裝： | 帶卷 (TR) |
LT1939
15
1939f
The ESRs of most aluminum electrolytics are too large to
deliver low output ripple. Tantalum and newer, lower ESR
organic electrolytic capacitors intended for power supply
use, are suitable and the manufacturers will specify the
ESR. The choice of capacitor value will be based on the
ESR required for low ripple. Because the volume of the
capacitor determines its ESR, both the size and the value
will be larger than a ceramic capacitor that would give you
similar ripple performance. One bene t is that the larger
capacitance may give better transient response for large
changes in load current.
Catch Diode
The diode D1 conducts current only during switch off
time. Use a Schottky diode to limit forward voltage drop to
increase ef ciency. The Schottky diode must have a peak
reverse voltage that is equal to regulator input voltage and
sized for average forward current in normal operation.
Average forward current can be calculated from:
I
D(AVG)
=
I
OUT1
V
I
N
" V
IN
V
OUT1
(
)
The only reason to consider a larger diode is the worst-
case condition of a high input voltage and shorted output.
With a shorted condition, diode current will increase to a
typical value of 3A, determined by the peak switch current
limit of the LT1939. This is safe for short periods of time,
but it would be prudent to check with the diode manu-
facturer if continuous operation under these conditions
can be tolerated.
BST Pin Considerations
The capacitor and diode tied to the BST pin generate
a voltage that is higher than the input voltage. In most
cases a 0.47糉 capacitor and fast switching diode (such
as the CMDSH-3 or FMMD914) will work well. Almost
any type of lm or ceramic capacitor is suitable, but the
ESR should be <1?to ensure it can be fully recharged
during the off time of the switch. The capacitor value can
be approximated by:
C
BST
=
I
OUT1(MAX)
"DC
50" V
OUT1
V
BST(MIN)
(
)
"
f
where I
OUT1(MAX)
is the maximum load current, and
V
BST(MIN)
is the minimum boost voltage to fully saturate
the switch.
Figure 5 shows four ways to arrange the boost circuit.
The BST pin must be more than 2.2V above the SW pin
for full ef ciency.
APPLICATIONS INFORMATION
Figure 5. BST Pin Considerations
V
BST
V
SW
= V
X
V
BST(MAX)
= V
X
V
X
> V
IN
+ 3V
(5d)
(5c)
(5b)
V
BST
V
SW
= V
IN
V
BST(MAX)
= 2 "燰
IN
V
IN
V
IN
D2
D1
D1
D1
D1
(5a)
LT1939
LDRV
BST
SW
V
BST
V
SW
= V
OUT1
V
BST(MAX)
= V
IN
+ V
OUT1
V
BST
V
SW
= V
OUT2
V
BST(MAX)
= V
IN
+ V
OUT2
V
OUT2
e 2.5V
V
OUT1
V
OUT2
C3
V
IN
V
IN
D2
LT1939
LDRV
BST
SW
V
OUT1
V
IN
V
IN
LT1939
LDRV
BST
SW
V
OUT1
C3
V
IN
V
IN
D2
LT1939
LDRV
BST
SW
D2
V
OUT1
C3
1939 F05
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