参数资料
| 型号： | A8670EESTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 18/28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
Thermal Considerations
For a given set of conditions, the junction temperature of the
A8670 can be estimated by carrying out a few calculations. This
is important to ensure an adequate safety margin with respect to
the maximum junction temperature (150°C) to enhance reliabil-
R DS(on)LS(TJ) = R DS(on)LS(25C) 1 +
= 45 × 10 –3 1+
T J – 25
200
125 – 25
200
(30)
ity. This exercise also helps to understand the overall efficiency
of the regulator.
The general approach is to work out what thermal impedance
(R θ J-A ) is required to maintain the junction temperature at a given
level, for a particular power dissipation. It should be noted that
this process is usually iterative to achieve the optimum solution.
= 0.0675 Ω
where R DS(on)LS(25C) is the R DS(on)LS value that can be found from
the Electrical Characteristics table in this datasheet.
c. Estimate the duty cycle (D) by applying equation 3 (t on ):
The following steps can be used as a guideline for determining a
suitable thermal solution. First, estimate the maximum ambient
D = t on × f SW
(31)
temperature (T A ) of the application. Second, define the maximum
junction temperature (T J ). Note that the absolute maximum is
150°C. Third, determine the worst case power dissipation. This
will typically occur at maximum load and minimum V IN .
=
=
V OUT + ( R DS(on)LS + DCR L ) I OUT
V IN + ( R DS(on)LS – R DS(on)HS ) I OUT
1.2 + (0.068 + 0.02 ) 2 1
12 + (0.068 – 0.3 ) 2 500 10 3
1
f SW
500 10 3
f SW
Design Example
Assuming: input voltage (V IN ) = 12 V, output voltage (V OUT ) =
1.2 V, maximum load (I OUT ) = 2 A, switching frequency (f SW )
= 500 kHz, target junction temperature (T J ) ≤ 125oC, maximum
ambient temperature (T A ) = 105°C, and inductive resistance
(DCR L ) = 20 m Ω .
1. The main power loss contributors are calculated separately:
? Switch static losses
a. Estimate the R DS(on) of the high-side switch at the maximum
target junction temperature:
= 0.12
d. The high side static loss can be determined:
P staticHI = I 2OUT × D × R DS(on)HS(TJ)
= 2 2 × 0.12 × 0.3
= 0.144 W
e. The low side static loss can be determined:
P staticLO = I 2OUT × 1 – D × R DS(on)LS(TJ)
(32)
(33)
R DS(on)HS(TJ) = R DS(on)HS(25C) 1 +
T J – 25
200
(29)
= 2 2 × (1 – 0.12) × 0.068
= 0.239 W
= 200 × 10 –3
1+
125 – 25
200
? Switching losses The combined turn on and turn off losses for
both switches are calculated as:
= 0.3 Ω
× I OUT × 6 ×10 –9 × f SW × 2
P switch =
=
× 2 × 6 ×10 –9 × 500 ×10 3 × 2
where R DS(on)HS(25C) is the R DS(on)HS value that can be found
from the Electrical Characteristics table in this datasheet.
b. Estimate the R DS(on) of the low side switch at the given junc-
tion temperature:
V IN
2
12
2
= 0.072 W
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
(34)
17
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