参数资料
| 型号： | SC1485ITSTRT |
| 厂商： | Semtech |
| 文件页数： | 14/27页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM 28-TSSOP |
| 标准包装： | 1 |
| PWM 型： | 控制器 |
| 输出数： | 2 |
| 占空比： | 90% |
| 电源电压： | 1.8 V ~ 25 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 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： | 28-TSSOP（0.173"，4.40mm 宽） |
| 包装： | 标准包装 |
| 产品目录页面： | 1358 (CN2011-ZH PDF) |
| 其它名称： | SC1485ITSDKR |
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POWER MANAGEMENT
NOT RECOMMENDED FOR NEW DESIGN
SC1485
For our example:
C OUT(MIN) = 610μF.
We will select 440μF, using two 220μF, 25m ?
capacitors in parallel. For smaller load release overshoot,
660μF may be used. Alternatively, one 15m ? or 12m ?
220μF, 330μF or 470μF capacitor may be used (with
the appropriate change to the calculation for C TOP ),
depending upon the load transient requirements.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
Thermal Considerations
The junction temperature of the device may be
calculated as follows:
T J = T A + P D ? θ JA ° C
Where:
T A = ambient temperature (°C)
P D = power dissipation in (W)
θ JA = thermal impedance junction to ambient from
absolute maximum ratings (°C/W)
The power dissipation may be calculated as follows:
I IN ( RMS ) = V OUT ? ( V BAT ( MIN ) ? V OUT ) ?
For our example:
I OUT
V BAT _ MIN
A RMS
P D = 2 ? ( VCCA ? I VCCA + VDDP ? I VDDP )
+ V g ? Q g 1 ? f 1 + VBST ? 1 mA ? D 1
+ V g ? Q g 2 ? f 2 + VBST ? 1 mA ? D 2
W
I VALLEY = I OUT ?
I RIPPLE _ VBAT ( MIN )
R ILIM = ( I VALLEY ? 1 . 2 ) ?
R DS ( ON ) ? 1 . 4
I IN(RMS) = 2.14A RMS
Input capacitors should be selected with sufficient ripple
current rating for this RMS current, for example a 10μF,
1210 size, 25V ceramic capacitor can handle
approximately 3A RMS . Refer to manufacturer ’s data
sheets and derate appropriately.
Finally, we calculate the current limit resistor value. As
described in the current limit section, the current limit
looks at the “valley current”, which is the average output
current minus half the ripple current. We use the
maximum room temperature specification for MOSFET
R DS(ON) at V GS = 4.5V for purposes of this calculation:
A
2
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under
normal operating conditions.
Ohms
10 ? 10 ? 6
For our example:
I VALLEY = 5.13A, R DS(ON) = 9m ? and R ILIM = 7.76k ?
We select the next lowest 1% resistor value: 7.68k ?
Where:
VCCA = chip supply voltage (V)
I VCCA = operating current (A)
VDDP = gate drive supply voltage (V)
I VDDP = gate drive operating current (A)
V g = gate drive voltage, typically 5V (V)
Q gx = FET gate charge, from the FET datasheet (C)
f x = switching frequency (kHz)
VBST = boost pin voltage during t ON (V)
D x = duty cycle
Inserting the following values for VBAT (MIN) condition (since
this is the worst case condition for power dissipation in
the controller) as an example (OUT1 = 1.5V, OUT2 = 1.2V):
T A = 85°C
θ JA = 84°C/W
VCCA = VDDP = 5V
I VCCA = 1100μA (data sheet maximum)
I VDDP = 150μA (data sheet maximum)
V g = 5V
Q gx = 60nC
f 1 = 250kHz
f 2 = 300kHz
VBAT (MIN) = 8V
VBST (MIN) = VBAT (MIN) +VDDP = 13V
D 1(MIN) = 1.5/8 = 0.1875
D 2(MIN) = 1.2/8 = 0.15
? 2005 Semtech Corp.
14
www.semtech.com
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