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參數資料
| 型號： | MC33365 |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | Isolated Flyback Switching Regulator with 9V Output |
| 中文描述： | 隔離反激式開關穩壓9V輸出 |
| 文件頁數： | 8/12頁 |
| 文件大小： | 233K |
| 代理商： | MC33365 |
MC33365
8
MOTOROLA ANALOG IC DEVICE DATA
OPERATING DESCRIPTION
Introduction
The MC33365 represents a new higher level of integration
by providing all the active high voltage power, control, and
protection circuitry required for implementation of a flyback or
forward converter on a single monolithic chip. This device is
designed for direct operation from a rectified 240 Vac line
source and requires a minimum number of external
components to implement a complete converter. A
description of each of the functional blocks is given below,
and the representative block and timing diagrams are shown
in Figures 16 and 17.
Oscillator and Current Mirror
The oscillator frequency is controlled by the values
selected for the timing components RT
and CT. Resistor R
T
programs the oscillator charge/discharge current via the
Current Mirror 4
I
output, Figure 3. Capacitor CT is charged
and discharged by an equal magnitude internal current
source and sink. This generates a symmetrical 50 percent
duty cycle waveform at Pin 7, with a peak and valley
threshold of 2.6 V and 0.6 V respectively. During the
discharge of C
T
, the oscillator generates an internal blanking
pulse that holds the inverting input of the AND gate Driver
high. This causes the Power Switch gate drive to be held in a
low state, thus producing a well controlled amount of output
deadtime. The amount of deadtime is relatively constant with
respect to the oscillator frequency when operating below
1.0 MHz. The maximum Power Switch duty cycle at Pin 16
can be modified from the internal 50% limit by providing an
additional charge or discharge current path to CT, Figure 18.
In order to increase the maximum duty cycle, a discharge
current resistor R
D
is connected from Pin 7 to ground. To
decrease the maximum duty cycle, a charge current resistor
RC is connected from Pin 7 to the Regulator Output. Figure 4
shows an obtainable range of maximum output duty cycle
versus the ratio of either RC
or RD with respect to RT.
Figure 18. Maximum Duty Cycle Modification
PWM
Current
Mirror
Regulator Output
1.0
R
C
8
6
2.25 I
I
T
T
4 I
Oscillator
Comparator
RD
RC
7
Current
Limit
Reference
Blanking
Pulse
The formula for the charge/discharge current along with
the oscillator frequency are given below. The frequency
formula is a first order approximation and is accurate for CT
values greater than 500 pF. For smaller values of CT, refer to
Figure 1. Note that resistor RT also programs the Current
Limit Comparator threshold.
Ichg dscg
5.4
RT
f
Ichg dscg
4CT
PWM Comparator and Latch
The pulse width modulator consists of a comparator with
the oscillator ramp voltage applied to the non–inverting input,
while the error amplifier output is applied into the inverting
input. The Oscillator applies a set pulse to the PWM Latch
while CT is discharging, and upon reaching the valley
voltage, Power Switch conduction is initiated. When CT
charges to a voltage that exceeds the error amplifier output,
the PWM Latch is reset, thus terminating Power Switch
conduction for the duration of the oscillator ramp–up period.
This PWM Comparator/Latch combination prevents multiple
output pulses during a given oscillator clock cycle. The timing
diagram shown in Figure 17 illustrates the Power Switch duty
cycle behavior versus the Compensation voltage.
Current Limit Comparator and Power Switch
The MC33365 uses cycle–by–cycle current limiting as a
means of protecting the output switch transistor from
overstress. Each on–cycle is treated as a separate situation.
Current limiting is implemented by monitoring the output
switch current buildup during conduction, and upon sensing
an overcurrent condition, immediately turning off the switch
for the duration of the oscillator ramp–up period.
The Power Switch is constructed as a SenseFET allowing
a virtually lossless method of monitoring the drain current. It
consists of a total of 1462 cells, of which 36 are connected to
a 8.1
ground–referenced sense resistor. The Current
Sense Comparator detects if the voltage across the sense
resistor exceeds the reference level that is present at the
inverting input. If exceeded, the comparator quickly resets
the PWM Latch, thus protecting the Power Switch. The
current limit reference level is generated by the 2.25
I
output
of the Current Mirror. This current causes a reference voltage
to appear across the 405
resistor. This voltage level, as
well as the Oscillator charge/discharge current are both set
by resistor RT. Therefore when selecting the values for RT
and CT, RT must be chosen first to set the Power Switch peak
drain current, while CT is chosen second to set the desired
Oscillator frequency. A graph of the Power Switch peak drain
current versus RT is shown in Figure 2 with the related
formula below.
Ipk
8.8
RT
1000
– 1.077
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相關代理商/技術參數 |
參數描述 |
|---|---|
| MC33368D | 功能描述:功率因數校正 IC High Voltage PFC RoHS:否 制造商:Fairchild Semiconductor 開關頻率:300 KHz 最大功率耗散: 最大工作溫度:+ 125 C 安裝風格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Reel |
| MC33368DG | 功能描述:功率因數校正 IC High Voltage PFC w/Critical Mode RoHS:否 制造商:Fairchild Semiconductor 開關頻率:300 KHz 最大功率耗散: 最大工作溫度:+ 125 C 安裝風格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Reel |
| MC33368DG | 制造商:ON Semiconductor 功能描述:DC/DC Converter (DC-DC) / Switching Regu |
| MC33368DR2 | 功能描述:功率因數校正 IC High Voltage PFC RoHS:否 制造商:Fairchild Semiconductor 開關頻率:300 KHz 最大功率耗散: 最大工作溫度:+ 125 C 安裝風格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Reel |
| MC33368DR2G | 功能描述:功率因數校正 IC High Voltage PFC w/Critical Mode RoHS:否 制造商:Fairchild Semiconductor 開關頻率:300 KHz 最大功率耗散: 最大工作溫度:+ 125 C 安裝風格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Reel |
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