参数资料
| 型号： | SC4508AMLTRT |
| 厂商： | Semtech |
| 文件页数： | 12/21页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST INV PWM 12-MLPQ |
| 标准包装： | 1 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1.5MHz |
| 占空比： | 97% |
| 电源电压： | 2.7 V ~ 15 V |
| 降压： | 是 |
| 升压： | 是 |
| 回扫： | 无 |
| 反相： | 是 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 12-VQFN 裸露焊盘 |
| 包装： | 标准包装 |
| 产品目录页面： | 1358 (CN2011-ZH PDF) |
| 其它名称： | SC4508AMLDKR |
SC4508A
POWER MANAGEMENT
t f =
θ ja ≤
Application Information (Cont.)
( Q gs 2 + Q gd )R gt
V gsp
Only a portion of the total losses (P g = Q g V cc f s ) is dissipated
in the MOSFET package. Here Qg is the total gate charge
specified in the datasheet. The power dissipated within
the MOSFET package is:
(T j,max , usually 125 o C) is not exceeded under the worst-
case condition. The equivalent thermal impedance from
junction to ambient ( θ ja ) should satisfy:
T j,max ? T a ,max
P loss
θ ja depends on the die to substrate bonding, packaging
material, the thermal contact surface, thermal compound
property, the available effective heat sink area and the
air flow condition (free or forced convection). Actual
P tg =
R g
R gt
Q g V cc f s
temperature measurement of the prototype should be
carried out to verify the thermal design.
The total power loss of the top switch is then:
P t = P tc +P ts +P tg
If the input supply of the power converter varies over a
wide range, then it will be necessary to weigh the relative
importance of conduction and switching losses. This is
because conduction losses are inversely proportional to
the input voltage. Switching loss however increases with
the input voltage. The total power loss of MOSFET should
be calculated and compared for high-line and low-line
cases. The worst case is then used for thermal design.
Freewheeling Diode Selection
The Schottky diode is recommended as freewheeling
diode in the both Buck and Buck-Boost applications. The
diode conducts during the off-time. The diode voltage
and current ratings are selected based on the peak
reverse voltage, the peak current and average power
dissipation. The following could be used to determine the
diode reversed voltage:
Overload Protection and Hiccup
During start-up, the capacitor from the SS/EN pin to
ground functions as a soft-start capacitor. After the
converter starts and enters regulation, the same
capacitor operates as an overload shutoff timing
capacitor. As the load current increases, the cycle-by-
cycle current-limit comparator will first limit the inductor
current. If the over-current persists for more than 32
consecutive switching cycles, the controller will shut off
the MOSFETs. Meanwhile an internal 12mA current
source discharges the soft-start capacitor C SS connected
to the SS/EN pin.
When the capacitor is discharged to 0.5V, a 10 μ A current
source recharges the SS/EN capacitor to 0.9V and driver
stage is enabled. Then a 20uA current source continues
to charge the soft-start capacitor. As the soft-start
capacitor reaches 1.4V, VREF will start to follow the soft-
start capacitor voltage until VREF=0.5V. If overload
persists, the controller will shut down the converter when
? I L
V D ( REV ) = V IN , I D ( PEAK ) = I O +
V ? V O
V IN + V D
V IN + V O + V D ? I L
2
V D ( REV ) = V IN + V O , I D ( PEAK ) = I O (
for Buck ? Boost
, I D ( AVG ) = I O IN
) +
V IN 2
for Buck
, I D ( AVG ) = I O
the soft-start capacitor voltage exceeds 1.4V. The
converter will repeatedly start and shut off until it is no
longer overloaded. This hiccup mode of overload
protection is a form of foldback current limiting. The
following calculations estimate the average inductor
current when the converter output is shorted to the
ground.
t ssr 1 = C SS
The most stressful condition for the diode occurs when
the output is shorted. Under this condition, due to the
V OUT = 0, the diode conducts at close to 100% duty cycle.
Therefore, attention should be paid to the thermal
condition when laying out a board.
Once the power losses (P loss ) for the MOSFET and
freewheeling diode are known, thermal and package
design at component and system level should be done
to verify that the maximum die junction temperature
a) The time taken to charge the capacitor from 0.5V to
0.9V
( 0 . 9 ? 0 . 5 ) V
10 uA
If C SS = 0.1 μ F, t ssr1 is calculated as 4ms.
b) The time to charge the capacitor from 0.9V to 1.4V
(driver is enabled but output duty cycle is 0)
? 2007 Semtech Corp.
12
www.semtech.com
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| SC4509SKTRT | 功能描述:IC LED DRVR WT/OLED BCKLT SOT236 RoHS:是 类别:集成电路 (IC) >> PMIC - LED 驱动器 系列:- 标准包装:60 系列:- 恒定电流:- 恒定电压:- 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 |
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