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| 型号： | LT3487EDD#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 8/16页 |
| 文件大小： | 0K |
| 描述： | IC REG BOOST INV ADJ 10DFN |
| 标准包装： | 121 |
| 类型： | 升压（升压），反相 |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 1.23 V ~ 28 V |
| 输入电压： | 2.3 V ~ 16 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 2MHz |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 10-WFDFN 裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 10-DFN（3x3） |
| 产品目录页面： | 1331 (CN2011-ZH PDF) |
LT3487
APPLICATIO S I FOR ATIO
Operation
The LT3487 uses a constant frequency, current mode con-
trol scheme to provide excellent line and load regulation.
Operation can be best understood by referring to the Block
Diagram in Figure 1. At the start of each oscillator cycle,
the SR latch X1 is set, which turns on the power switch
Q1. A voltage proportional to the switch current is added
to a stabilizing ramp and the resulting sum is fed into the
positive terminal of the PWM comparator A2. When this
voltage exceeds the level at the negative input of A2, the
SR latch X1 is reset, turning off the power switch Q1. The
level at the negative input of A2 is set by the error ampli?er
A1, and is simply an ampli?ed version of the difference
between the feedback voltage and the reference voltage
of 1.23V. In this manner, the error ampli?er sets the cor-
rect peak current level to keep the output in regulation.
If the error ampli?er’s output increases, more current is
delivered to the output; if it decreases, less current is
delivered. The second channel is an inverting converter.
The basic operation is the same as the positive channel.
long as CAP is typically 1.2V and worst-case 1.6V (cold)
above the V BAT voltage. The drive current for the output
disconnect PNP is returned to the V BAT pin. This allows
the pass transistor to turn off when the CAP voltage
falls to less than 1.2V above V BAT . The V BAT pin allows
applications in which the power (inductors L1 and L2)
and internal control circuitry (V IN pin) are powered from
different sources.
Inductor Selection
A 10μH inductor is recommended for the LT3487 boost
channel. The inverting channel can use uncoupled 15μH
inductors, or coupled 10μH inductors. Small size and
high ef?ciency are the major concerns for most LT3487
applications. Inductors with low core losses and small
DCR (copper wire resistance) at 2MHz are good choices
for LT3487 applications. The inductor DCR should be on
the order of half of the switch on-resistance for its chan-
nel. Some inductors in this category with small size are
listed in Table 1.
The SR latch X2 is also set at the start of each oscillator
cycle. The power switch Q2 is turned on at the same time
Table 1. Recommended Inductors
CURRENT
as Q1. Q2 turns off based on its own feedback loop, which
consists of error ampli?er A3 and PWM comparator A4.
The reference voltage of this negative channel is ground.
Voltage clamps on V CP and V CN (not shown) enforce current
PART NUMBER
DB318C-A997AS-
100M
CDRH3D18-100
INDUCTANCE DCR
(μH) ( Ω )
10 0.18
10 0.205
RATING
(mA) MANUFACTURER
580 Toko
www. tokoam.com
900 Sumida
limit. Switching waveforms with typical load conditions
are shown in Figure 2.
The PNP Q3 is used as an output disconnect pass transistor.
Q3 disconnects the load from the input during shutdown.
CDRH2D18HP-100
CDRH3D23-100
CDRH2D18/HP-150
CDRH3D18-150
CDRH3D23-150
10
10
15
15
15
0.245
0.117
0.345
0.301
0.191
850
850
700
750
700
www.sumida.com
The anti-sat driver keeps Q3 at the edge of saturation as
V SWP
20V/DIV
I LI
100mA/DIV
V SWN
20V/DIV
I SWN
100mA/DIV
Capacitor Selection
The small size of ceramic capacitors makes them suitable
for LT3487 applications. X5R and X7R types of ceramic
capacitors are recommended because they retain their
capacitance over wider voltage and temperature ranges
than other types such as Y5V or Z5U. A 1μF input capaci-
tor is suf?cient for most LT3487 applications. The output
capacitors required for stability depend on the application.
V IN = 3.6V
V POS = 15V, 25mA
V NEG = –8V, 50mA
200ns/DIV
3487 F02
For the typical Li-Ion to +15V, –8V application, the positive
channel requires a 4.7μF output capacitor and the negative
Figure 2. Switching Waveforms
channel requires at least 10μF of capacitance.
3487f
8
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