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| 型号： | ISL97678IRZ |
| 厂商： | Intersil |
| 文件页数： | 16/18页 |
| 文件大小： | 0K |
| 描述： | IC LED DVR PWM DIMMING 8CH 32QFN |
| 标准包装： | 60 |
| 拓扑： | 线性（LDO），PWM，升压（升压） |
| 输出数： | 8 |
| 内部驱动器： | 是 |
| 类型 - 主要： | 背光 |
| 类型 - 次要： | RGB，白色 LED |
| 频率： | 500kHz ~ 1.5MHz |
| 电源电压： | 4.75 V ~ 26 V |
| 输出电压： | 45V |
| 安装类型： | * |
| 封装/外壳： | * |
| 供应商设备封装： | * |
| 包装： | * |
| 工作温度： | -40°C ~ 85°C |
ISL97678
Components Selections
According to the inductor Voltage-Second Balance
principle, the change of inductor current during the
switching regulator On-time is equal to the change of
inductor current during the switching regulator Off-time.
Since the voltage across an inductor is as shown in
Equation 6:
offer small size and a lower value of temperature and
voltage coefficient compared to other ceramic capacitors.
It is recommended that an input capacitor of at least
10μF be used. Ensure the voltage rating of the input
capacitor is suitable to handle the full supply range.
Inductor
V L = L × Δ I L ? Δ t
and Δ I L @ On = Δ I L @ Off, therefore:
( V I – 0 ) ? L × D × t S = ( V O – V D – V I ) ? L × ( 1 – D ) × t S
(EQ. 6)
(EQ. 7)
The selection of the inductor should be based on its
maximum and saturation current (I SAT ) characteristics,
power dissipation (DCR), EMI susceptibility (shielded vs
unshielded), and size. Inductor type and value influence
many key parameters, including ripple current, current
limit, efficiency, transient performance and stability.
V O ? V I = 1 ? ( 1 – D )
D = ( V O – V I ) ? V O
where D is the switching duty cycle defined by the
turn-on time over the switching periods. V D is Schottky
diode forward voltage that can be neglected for
approximation.
Rearranging the terms without accounting for V D gives
the boost ratio and duty cycle respectively as Equations 8
and 9:
(EQ. 8)
(EQ. 9)
Input Capacitor
Switching regulators require input capacitors to deliver
peak charging current and to reduce the impedance of
the input supply. This reduces interaction between the
regulator and input supply, thereby improving system
stability. The high switching frequency of the loop causes
almost all ripple current to flow in the input capacitor,
which must be rated accordingly.
A capacitor with low internal series resistance should be
chosen to minimize heating effects and improve system
efficiency, such as X5R or X7R ceramic capacitors, which
16
The inductor’s maximum current capability must be
adequate enough to handle the peak current at the worst
case condition. Additionally, if an inductor core is chosen
with too low a current rating, saturation in the core will
cause the effective inductor value to fall, leading to an
increase in peak to average current level, poor efficiency
and overheating in the core. The series resistance, DCR,
within the inductor causes conduction loss and heat
dissipation. A shielded inductor is usually more suitable
for EMI susceptible applications, such as LED
backlighting.
The peak current can be derived from the voltage across
the inductor during the Off-period, as expressed in
Equation 10:
IL peak = ( V O × I O ) ? ( 85% × V I ) + 1 ? 2 [ V I × ( V O – V I ) ? ( L × V O × f SW )
(EQ. 10)
The choice of 85% is just an average term for the
efficiency approximation. The first term is the average
current, which is inversely proportional to the input
voltage. The second term is the inductor current change,
which is inversely proportional to L and f SW . As a result,
for a given switching.
FN6998.1
November 5, 2009
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