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| 型号： | LT3688EUF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 15/28页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 0.8A DL 24QFN |
| 标准包装： | 91 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 2 |
| 输出电压： | 0.8 V ~ 32.4 V |
| 输入电压： | 3.8 V ~ 36 V |
| PWM 型： | 电流模式，Burst Mode? |
| 频率 - 开关： | 500kHz ~ 2.1MHz |
| 电流 - 输出： | 800mA |
| 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： | 24-WFQFN 裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 24-QFN（4x4） |
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LT3688
APPLICATIONS INFORMATION
Table 2. Inductor Vendors
When the switch is off, the potential across the induc-
VENDOR
Murata
PART SERIES
LQH55D
TYPE
Open
URL
www.murata.com
tor is the output voltage plus the catch diode drop. This
gives the peak-to-peak ripple current in the inductor
TDK
Toko
SLF7045
SLF10145
DC62CB
Shielded
Shielded
Shielded
www.component.tdk.com
www.toko.com
ΔI L =
( 1 – DC ) ( V OUT + V F )
L?f
D63CB
D75C
D75F
Shielded
Shielded
Open
where f is the switching frequency of the LT3688 and L is the
value of the inductor. The peak inductor and switch current is
Sumida
CR54
CDRH74
CDRH6D38
Open
Shielded
Shielded
www.sumida.com
I SW(PK) = I L(PK) = I OUT +
ΔI L
2
I OUT(MAX) = I LIM –
CR75 Open
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current,
and reduces the output voltage ripple. If your load is lower
than the maximum load current, then you can relax the
value of the inductor and operate with higher ripple current.
This allows you to use a physically smaller inductor, or
To maintain output regulation, this peak current must be
less than the LT3688’s switch current limit I LIM . I LIM is at
least 1.25A for at low duty cycles and decreases linearly
to 0.9A at DC = 0.9. The maximum output current is a
function of the chosen inductor value:
ΔI L
2
one with a lower DCR resulting in higher ef?ciency. Be
aware that if the inductance differs from the simple rule
above, then the maximum load current will depend on
= 1.25A ? ( 1 – 0.3DC ) –
ΔI L
2
L MIN = ( V OUT + V F ) ?
input voltage. In addition, low inductance may result in
discontinuous mode operation, which further reduces
maximum load current. Discontinuous operation occurs
when I OUT is less than ΔI L / 2. For details of maximum
output current and discontinuous mode operation, see
Linear Technology’s Application Note AN44. Finally, for
duty cycles greater than 50% (V OUT /V IN > 0.5), a minimum
inductance is required to avoid sub-harmonic oscillations:
1.2MHz
f SW
where V F is the voltage drop of the catch diode (~0.4V),
f SW is in MHz, and L MIN is in μH.
The current in the inductor is a triangle wave with an average
value equal to the load current. The peak switch current
is equal to the output current plus half the peak-to-peak
inductor ripple current. The LT3688 limits its switch cur-
rent in order to protect itself and the system from overload
faults. Therefore, the maximum output current that the
LT3688 will deliver depends on the switch current limit,
the inductor value, and the input and output voltages.
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors, and
choose one to meet cost or space goals. Then use these
equations to check that the LT3688 will be able to deliver
the required output current. Note again that these equations
assume that the inductor current is continuous.
Input Capacitor
Bypass the input of the LT3688 circuit with a ceramic
capacitor of an X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 2.2μF to 4.7μF ceramic capacitor
is adequate to bypass the LT3688 and will easily handle
the ripple current. Note that larger input capacitance
is required when a lower switching frequency is used.
If the input power source has high impedance, or there
is signi?cant inductance due to long wires or cables,
3688f
15
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