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| 型号： | MCP1612T-ADJI/MF |
| 厂商： | Microchip Technology |
| 文件页数： | 12/22页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ 1A 8DFN |
| 标准包装： | 3,300 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.8 V ~ 5 V |
| 输入电压： | 2.7 V ~ 5.5 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 1.4MHz |
| 电流 - 输出： | 1A |
| 同步整流器： | 是 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-VDFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 8-DFN-EP（3x3） |
MCP1612
5.2.2
BUCK INDUCTOR
The value of the buck inductor is chosen to be 3.3 μH.
V OUT
There are many requirements that need to be satisfied
when selecting the buck inductor. The application,
physical size, current rating, resistance, mounting
method, supplier, temperature range, minimum
inductance and cost all need to be considered.
Many suppliers specify the maximum peak current that
an inductor can handle before magnetic saturation
occurs. The peak current is equal to the maximum DC
output current, plus one-half the peak-to-peak AC
ripple current.
When the P-channel MOSFET is on, the current in the
buck inductor is ramped up. The voltage across the
inductor, the inductance and the MOSFET on-time are
required to determine the peak-to-peak ripple current.
When operating in Continuous Current mode, the on-
time of the P-channel MOSFET is determined by
multiplying the duty cycle by the switching period. The
following equation can be used to determine the duty
cycle.
EQUATION 5-2:
DutyCycle = -------------
V IN
The AC ripple current is controlled by the size of the
buck inductor. The value of the inductor will therefore
need to be raised so that the converter operates in
Continuous Conduction mode. Calculation of the buck
inductor current rating follows.
V IN = 3.3V
V OUT = 1.2V
F SW = 1.4 MHz
I OUT(MAX) = 1A
T ON = (1.2V/3.3V) x (1/1.4 MHz)
T ON = 260 ns
V L = (3.3V – 1.2V) = 2.1V
Δ I L = (2.1V/3.3 μH) x 260 ns
Δ I L = 165 mA
I L(PEAK) = I OUT(MAX) + 1/2 Δ I L
I L(PEAK) = 1A + (165 mA)/2
I L(PEAK) = 1.08A
The inductor selected must have an inductance of
3.3 μH at a peak current rating of 1.08A. The DC resis-
tance of the inductor should be as low as is feasibly
possible. Extremely low DC resistance inductors are
available, though a trade-off between size and cost
should be considered.
The on-time is then defined as follows.
5.2.3
OUTPUT CAPACITOR
EQUATION 5-3:
The output capacitor is used to filter the inductor AC
= DutyCycle × ----------
Where:
T ON
1
F SW
ripple current and provide storage for load transients.
The size and Equivalent Series Resistance (ESR) of
the output capacitor determines the amount of ripple
voltage present at the output of the converter. When
Δ I L
V L = L × --------
F SW = switching frequency
The AC ripple current in the inductor can be calculated
by the following relationship.
EQUATION 5-4:
Δ t
Solving for Δ I L yields:
selecting the output capacitor, a design trade-off has to
be made between the acceptable ripple voltage and the
size/cost of the output capacitor. Ceramic capacitors
have very low ESR, but increase in cost with higher
values. Tantalum and electrolytic capacitors are
relatively inexpensive in higher values, but they also
have a much higher ESR.
The amount of capacitance needed to obtain the
desired ripple voltage is calculated by using the
following relationship.
Δ I L = ------ × Δ t
Δ V C
I C = C × -----------
EQUATION 5-5:
V L
L
EQUATION 5-6:
Δ t
Where:
V L = voltage across the inductor
(V IN – V OUT )
Δ t = on-time of the P-channel MOSFET
DS21921B-page 12
? 2005 Microchip Technology Inc.
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