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| 型号： | LTC3215EDD#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 9/14页 |
| 文件大小： | 0K |
| 描述： | IC LED DRIVR WHITE BCKLGT 10-DFN |
| 标准包装： | 2,500 |
| 拓扑： | PWM，升压（升压），切换式电容器（充电泵） |
| 输出数： | 1 |
| 内部驱动器： | 是 |
| 类型 - 主要： | 闪灯/白光 |
| 类型 - 次要： | 白色 LED |
| 频率： | 600kHz ~ 1.2MHz |
| 电源电压： | 2.9 V ~ 4.4 V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 10-WFDFN 裸露焊盘 |
| 供应商设备封装： | 10-DFN（3x3） |
| 包装： | 带卷 (TR) |
| 工作温度： | -40°C ~ 85°C |
LTC3215
APPLICATIONS INFORMATION
V IN , CPO Capacitor Selection
The value and type of capacitors used with the LTC3215
determine several important parameters such as regulator
control loop stability, output ripple, charge pump strength
and minimum start-up time.
To reduce noise and ripple, it is recommended that low
equivalent series resistance (ESR) ceramic capacitors be
used for both C VIN and C CPO . Tantalum and aluminum ca-
pacitors are not recommended because of their high ESR.
The value of C CPO directly controls the amount of output
ripple for a given load current. Increasing the size of
C CPO will reduce the output ripple at the expense of higher
start-up current. The peak-to-peak output ripple for 1.5x
mode is approximately given by the expression:
the charge pump is on either the input charging phase or
the output charging phase but will drop to zero during
the clock nonoverlap times. Since the nonoverlap time
is small (~15ns), these missing “notches” will result in
only a small perturbation on the input power supply line.
Note that a higher ESR capacitor such as tantalum will
have higher input noise due to the input current change
times the ESR. Therefore, ceramic capacitors are again
recommended for their exceptional ESR performance. Input
noise can be further reduced by powering the LTC3215
through a very small series inductor as shown in Figure
3. A 10nH inductor will reject the fast current notches,
thereby presenting a nearly constant current load to the
input power supply. For economy, the 10nH inductor can
be fabricated on the PC board with about 1cm (0.4") of
PC board trace.
V RIPPLE(P-P) = I OUT /(3f OSC ? C CPO )
(3)
Where f OSC is the LTC3215’s oscillator frequency (typically
900kHz) and C CPO is the output storage capacitor.
0.1μF
10nH
2.2μF
V IN
LTC3215
Both the style and value of the output capacitor can sig-
nificantly affect the stability of the LTC3215. As shown in
the block diagram, the LTC3215 uses a control loop to
adjust the strength of the charge pump to match the cur-
rent required at the output. The error signal of this loop
is stored directly on the output charge storage capacitor.
The charge storage capacitor also serves as the dominant
pole for the control loop. To prevent ringing or instability,
it is important for the output capacitor to maintain at least
2.2μF of actual capacitance over all conditions.
Likewise, excessive ESR on the output capacitor will tend
to degrade the loop stability of the LTC3215. The closed
loop output resistance of the LTC3215 is designed to be
76mΩ. For a 100mA load current change, the error signal
will change by about 7.6mV. If the output capacitor has
76mΩ or more of ESR, the closed-loop frequency response
will cease to roll off in a simple one-pole fashion and poor
load transient response or instability could result. Multilayer
ceramic chip capacitors typically have exceptional ESR
performance. MLCCs combined with a tight board layout
will yield very good stability. As the value of C CPO controls
the amount of output ripple, the value of C VIN controls the
amount of ripple present at the input pin (V IN ). The input
current to the LTC3215 will be relatively constant while
GND
3215 F03
Figure 3. 10nH Inductor Used for Input Noise Reduction
(Approximately 1cm of Wire)
Flying Capacitor Selection
Warning: Polarized capacitors such as tantalum or alumi-
num should never be used for the flying capacitors since
their voltage can reverse upon start-up of the LTC3215.
Ceramic capacitors should always be used for the flying
capacitors.
The flying capacitors control the strength of the charge
pump. In order to achieve the rated output current it is
necessary to have at least 2.2μF of actual capacitance for
each of the flying capacitors. Capacitors of different ma-
terials lose their capacitance with higher temperature and
voltage at different rates. For example, a ceramic capacitor
made of X7R material will retain most of its capacitance
from – 40 o C to 85 o C whereas a Z5U or Y5V style capacitor
will lose considerable capacitance over that range. Z5U
and Y5V capacitors may also have a very poor voltage
coefficient causing them to lose 60% or more of their
capacitance when the rated voltage is applied. Therefore,
3215fc
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