参数资料
| 型号： | SC1485ITSTRT |
| 厂商： | Semtech |
| 文件页数： | 12/27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
=
f SW _ VBAT ( MAX )
( V BAT ( MAX ) ON _ VBAT ( MAX ) ) Hz
I INDUCTOR ( MIN ) OUT ( MAX ) +
= I
A ( MIN )
( ERR ? ERR DC ) ? 2
R ESR _ ST ( MAX ) = Ohms
V OUT
? t
t ON is generated by a one-shot comparator that samples
V BAT via R tON , converting this to a current. This current is
used to charge an internal 3.3pF capacitor to V OUT . The
equations above reflect this along with any internal
components or delays that influence t ON . For our example
we select R tON = 1M ? :
t ON_VBAT(MIN) = 563ns and t ON_VBAT(MAX) = 255ns
f SW_VBAT(MIN) = 266kHz and f SW_VBAT(MAX) = 235kHz
Now that we know t ON we can calculate suitable values
for the inductor. To do this we select an acceptable
inductor ripple current. The calculations below assume
50% of I OUT which will give us a starting place.
From this we can calculate the minimum inductor
current rating for normal operation:
I RIPPLE _ VBAT ( MAX )
2
For our example:
I INDUCTOR(MIN) = 7.1A (MIN)
Next we will calculate the maximum output capacitor
equivalent series resistance (ESR). This is determined by
calculating the remaining static and transient tolerance
allowances. Then the maximum ESR is the smaller of the
calculated static ESR (R ESR_ST(MAX) ) and transient ESR
(R ESR_TR(MAX) ):
ST
I RIPPLE _ VBAT ( MAX )
L VBAT ( MIN ) = ( V BAT ( MIN ) ? V OUT ) ?
and
t ON _ VBAT ( MIN )
( 0 . 5 ? I OUT )
H
Where ERR ST is the static output tolerance and ERR DC is
the DC error. The DC error will be 1% plus the tolerance
of the feedback resistors, thus 2% total for 1%
feedback resistors.
L VBAT ( MAX ) = ( V BAT ( MAX ) ? V OUT ) ?
For our example:
t ON _ VBAT ( MAX )
( 0 . 5 ? I OUT )
H
For our example:
ERR ST = 48mV and ERR DC = 24mV, therefore
R ESR_ST(MAX) = 22m ?
L VBAT(MIN) = 1.3μH and L VBAT(MAX) = 1.6μH
I RIPPLE _ VBAT ( MIN ) = ( V BAT ( MIN ) ? V OUT ) ?
A P ? P
I RIPPLE _ VBAT ( MAX ) = ( V BAT ( MAX ) ? V OUT ) ?
A P ? P
( ERR TR ? ERR DC )
R ESR _ TR ( MAX ) = Ohms
? ? I OUT
? ?
+
?
?
We will select an inductor value of 2.2μH to reduce the
ripple current, which can be calculated as follows:
t ON _ VBAT ( MIN )
L
and
t ON _ VBAT ( MAX )
L
For our example:
I RIPPLE_VBAT(MIN) = 1.74A P-P and I RIPPLE_VBAT(MAX) = 2.18A P-P
? I RIPPLE _ VBAT ( MAX ) ?
2
Where ERR TR is the transient output tolerance. Note that
this calculation assumes that the worst case load
transient is full load. For half of full load, divide the I OUT
term by 2.
For our example:
ERR TR = 96mV and ERR DC = 24mV, therefore
R ESR_TR(MAX) = 10.2m ? for a full 6A load transient
? 2005 Semtech Corp.
12
www.semtech.com
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