参数资料
| 型号： | A8670EESTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 8/28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
Functional Description
Basic Operation
At the beginning of a switching cycle, the high-side switch is
turned on for a duration determined by the current flowing into
TON. The magnitude of current is determined by the value of the
input voltage and the value of the on-time resistor (RTON, R1 in
the Typical Applications section circuit diagrams).
During the on-time period, the current builds up through the
inductor at a rate determined by the voltage developed across it
and the inductance value. When the on-time period elapses, the
ence bias current will increase, affecting the regulation. In addi-
tion, high value resistors are more prone to noise pick-up effects
which may affect performance. As some kind of compromise, it
is recommended that R6 be in the region of 10 k Ω .
Switch On-Time and Switching Frequency
The switching frequency of the converter is selected by choosing
the appropriate on-time. The on-time can be estimated to a first
order by using the following formula:
output of an RS latch resets, turning off the high-side switch.
After a small dead-time delay, the low-side switch is turned on.
t on =
V OUT
V IN
1
f SW
(2)
The current through the inductor decays at a rate determined
by the output voltage and the inductance value. The current is
sensed through the low-side switch and is compared to the cur-
rent demand signal . The current demand signal is generated by
comparing the output voltage (stepped down to the FB pin) with
an accurate reference voltage.
When the current through the low-side switch drops to the current
demand level, the low-side switch is turned off. After a further
dead-time delay, the high-side switch is turned on again, and the
process is repeated.
where:
V OUT is the output voltage,
f SW is the switching frequency, and
V IN is the nominal input voltage.
To factor-in the effects of resistive voltage drops in the converter
circuit, the following formula can be used to produce a more
accurate estimate of what the on-time has to be for a required
switching frequency:
Output Voltage Selection
The output voltage (V OUT ) of the converter is set by selecting the
appropriate feedback resistors using the following formula:
t on =
where:
V OUT + ( R DS(on)LS + DCR L ) I OUT
V IN + ( R DS(on)LS – R DS(on)HS ) I OUT
1
f SW
(3)
where:
V OUT = V FB
R 5
R 6
+ 1 + I FB
R 5 R 6
R 5 + R 6
(1)
R DS(on)LS is the low-side MOSFET on-resistance,
R DS(on)HS is the high-side MOSFET resistance, and
DCR L is the inductive resistance.
V FB is the reference voltage,
R5 and R6 are as shown in the Typical Applications section
circuit diagrams, and
I FB is the reference bias current.
It is important to consider the tolerance of the feedback resistors,
because they directly affect the overall setpoint accuracy of the
output voltage.
It is also important to consider the actual resistor values selected
and consider the trade-offs. High value resistors will minimize
the shunt current flowing through the feedback network, enhanc-
ing efficiency. However, the offset error produced by the refer-
The switching frequency will vary slightly as the resistive voltage
drops in the circuit change, either due to temperature effects or to
input voltage variations.
Note that when selecting the switching frequency, care should
be taken to ensure the converter does not operate near either the
minimum on-time (50 ns) or the minimum off-time (350 ns).
Minimum on-times will typically occur in combinations of
maximum input voltage, minimum output voltage with minimum
load, and maximum switching frequency. Minimum off-times
will typically occur in combinations of minimum input voltage,
maximum output voltage with maximum load, and maximum
switching frequency.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
7
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