参数资料
| 型号： | A8670EESTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 9/28页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 2A 20QFN |
| 标准包装： | 1 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 可调至 0.6V |
| 输入电压： | 7 V ~ 16 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 200kHz ~ 1MHz |
| 电流 - 输出： | 2A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 20-WFQFN 裸露焊盘 |
| 包装： | 标准包装 |
| 供应商设备封装： | 20-QFN 裸露焊盘（4x4） |
| 其它名称： | 620-1397-6 |
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A8670
Fixed Frequency, 2 A Synchronous Buck Regulator
With Fault Warnings and Power OK
V IN – V OUT
The t on from either of the above formulae can be used to deter-
mine the TON resistor value, R TON (R1 in Typical Applications
section circuit drawings):
The required (minimum) inductance can be found:
L (min) = D (min)
I ripp
1
f SW
(6)
R TON = ( V IN – 0.67)
t on – 8 ×10 –9
25 ×10 –12
– 500
(4)
Note that the inductor manufacturer tolerances on the inductance
value should be taken into account. This can be as high as ±30%.
Table 1 provides preferred resistor values for a given output
voltage at target switching frequencies of 500 kHz, 700 kHz, and
1 MHz:
Table 1. Recommended RTON Resistor Values
Switching Frequency, f SW
It is recommended that gapped ferrite solutions be used as
opposed to powdered iron solutions. This is because powdered
iron cores exhibit relatively high core losses, especially at higher
switching frequencies. Higher core losses do have a detrimental
impact on the long term reliability of the component.
Inductors are typically specified at two current levels:
500 kHz
V OUT R TON
(V) (k Ω )
700 kHz
V OUT R TON
(V) (k Ω )
1 MHz
V OUT R TON
(V) (k Ω )
? Saturation Current (I sat ) The worst case maximum peak cur-
rent should not exceed the saturation current and indeed some
margin should be allowed. The maximum peak current in an
5.0
3.3
2.5
1.8
374
243
187
137
5.0
3.3
2.5
1.8
267
174
133
95.9
5.0
3.3
2.5
1.8
182
121
90.9
64.9
inductor occurs during an overload condition where the circuit
operates in current limit. The typical valley current limit (I LIM )
is 2.7 A. The peak current through the inductor is effectively the
valley current limit plus the ripple current:
1.5
113
1.5
80.6
1.5
54.9
I sat > I LIM + I ripp
(7)
1.2
1.0
0.8
0.6
90.9
76.8
60.4
44.2
1.2
1.0
0.8
0.6
63.4
52.3
42.2
30.9
1.2
1.0
0.8
0.6
43.2
35.7
28.7
23.2
? Rms Current (I rms ) It is important to understand how the rms
current level is specified in terms of ambient temperature. Some
manufacturers quote an ambient whilst others quote a tempera-
ture that includes a self-temperature rise. For example, if an
inductor is rated for 85°C and includes a self-temperature rise of
25°C at maximum load, then the inductor cannot be safely oper-
Inductor Selection
The main factor in selecting the inductance value is the ripple
current. The ripple current affects the output voltage ripple and
current limit. A reasonable figure of merit for the ripple current
(I ripp ) is 25% of the maximum load. So for a maximum load of
ated beyond an ambient temperature of 60°C at full load.
The rms current through the inductor should not exceed the rat-
ing for the inductor, taking into account the maximum ambient
temperature. The maximum rms current is effectively the valley
current limit (I LIM ) plus half of the ripple current:
2 A, the peak-to-peak ripple current should be 500 mA.
I rms (max) > I LIM + I ripp / 2
(8)
The maximum peak-to-peak ripple current occurs at the maxi-
mum input voltage. To a reasonable approximation, the minimum
duty cycle can be found:
A final consideration in the selection of the inductor is the series
resistance (DCR). A lower DCR will reduce the power loss and
enhance power efficiency. The trade-off in using an inductor with
a relatively low DCR is the physical size is typically larger.
D (min) =
V OUT
V IN (max)
(5)
Recommended inductors include the NR8040 or NR6045 series
manufactured by Taiyo Yuden.
Allegro MicroSystems, LLC
115 Northeast Cutoff
8
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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