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參數(shù)資料
| 型號： | MIC3833BN |
| 廠商： | MICREL INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Current-Fed PWM Controllers |
| 中文描述： | 1 A SWITCHING CONTROLLER, 500 kHz SWITCHING FREQ-MAX, PDIP16 |
| 封裝： | PLASTIC, DIP-16 |
| 文件頁數(shù)： | 7/10頁 |
| 文件大?。?/td> | 100K |
| 代理商： | MIC3833BN |
April 1998
4-141
MIC3832/3833
Functional Description
Refer to the block diagram and Figure 5.
The MIC3832 and MIC3833 are self-contained controllers,
with a voltage reference, voltage-mode error amplifier; cur-
rent-mode, maximum duty cycle, overcurrent, and shutdown
comparators; and an undervoltage lockout circuit. Three
control loops are provided: voltage-mode through the error
amplifier, current-mode through the PWM comparator, and
overcurrent through the shutdown comparator. Three totem-
pole outputs provide up to 1A peak synchronized drive to
external FETs for current-fed push-pull or bridge transformer
applications.
Undervoltage Lockout (UVLO)
Undervoltage lockout (UVLO) requires that the input voltage
rise above 15.9V (MIC3832) or 8.3V (MIC3833) before the
startup circuit is energized. Once operating, the controller will
not shut down until the supply drops to 9.8V (MIC3832) or
7.8V (MIC3833). There is an internal 22V zener between V
DD
and ground for overvoltage clamping. Zener current should
be limited to less than 20mA.
Voltage Reference (REF)
The reference consists of a 5V bandgap reference internally
trimmed to 2% accuracy. It provides an internal reference
and can be used to supply up to 25mA to external circuits.
Oscillator (R
T
/C
T
)
The oscillator stage performs two functions. First, it provides
a linear sawtooth waveform which is fed to the PWM com-
parator in voltage-mode control. Second, it toggles the flip-
flop which provides the Q and Q outputs. The oscillator
frequency is configured using an external timing resistor and
capacitor. A nominal voltage of 3.6V appears on the R
T
pin;
the resulting current is then mirrored through the C
T
pin which
charges the timing capacitor and generates the linear ramp.
It is important to select an appropriate capacitor. At high
frequencies effective series resistance, effective series in-
ductance, dielectric loss and dielectric absorption all affect
frequency stability and accuracy. RF capacitors such as
silver mica, glass, polystyrene, or COG ceramics are recom-
mended. High K ceramics should not be used.
Front-Edge Blanking
This feature provides a fixed delay time prior to current
sensing becoming active. This prevents the overcurrent
sensing function from being falsely tripped by initial system
transients. Timing is set to a nominal 140ns.
Error Amplifier (EA)
The error amplifier is an opamp with a low impedance output
that is used to sense output conditions and provide a dc
output based on those conditions to the PWM comparator.
The output of this stage is brought out to allow tailoring of the
closed loop gain or frequency response. The open loop gain
of this stage is typically 95dB. The inputs are diode clamped
to each other.
Micrel
PWM Comparator
A sawtooth waveform is compared to the output of the error
amplifier. The sawtooth is generally the oscillator waveform
on C
T
in voltage-mode control systems. In current-mode
control systems, it is often the inductor current waveform.
Both systems result in a square wave output which, after
being NOR’ed with the MDC output (see below), is used to
drive the main (PWM) output stage.
PWM Latch and Output
The PWM latch is reset by an oscillator rising cycle, turning
the PWM output on if SHDN or UVLO are inactive. The PWM
comparator trips when the CMR rising ramp voltage exceeds
the error amplifier output voltage, setting the PWM latch and
terminating the PWM output, after a minimum time set by the
front edge blanking one-shot. If the output voltage is below
the setpoint, the PWM cycle is terminated at a maximum duty
cycle set by the voltage on the MDC/SS pin. If the output
voltage is above the setpoint, the error amplifier output is low,
and the PWM cycle terminates after the minimum set by the
front edge blanking one-shot. The PWM output is designed
to source and sink 1A peak into 1,000 pF loads. The output
is disabled when SHDN is enabled.
Push-Pull Outputs (Q and Q)
Two push-pull outputs are provided, with their leading edge
synchronized to alternating PWM rising ramp initiation. The
two outputs are 180
°
out of phase, with a slight (50ns typ.)
overlap. The push-pull outputs are designed to source and
sink 1A peak into 1,000 pF loads. This peak current was
chosen to provide the designer with the option of using
bipolar, MOSFET, or IGBT switching elements. To minimize
ringing on the output waveform, the series inductance seen
by the drivers should be as low as possible. This can be
accomplished by keeping the distance between the MIC3832/
3 and the switching elements as short as possible, or by using
carbon composition resistors in series with the FET gates.
The Q and Q outputs have a small overlap with no dead time.
While advantageous to current-fed topologies, other topolo-
gies may require slight modification to accommodate this
overlap. The outputs are disabled when SHDN is enabled.
Maximum Duty Cycle (MDC)
This feature, which uses the same pin as soft start (MDC/SS),
provides another method of limiting duty cycle. The voltage
seen by this pin determines the maximum duty cycle that can
be obtained from the PWM output. As this feature can vary
by as much as 15% over temperature, it is not recommended
that it be used in place of a well designed feedback loop.
The voltage on MDC/SS, the inverting input of the MDC
comparator, is compared to the voltage on CMR, with internal
front edge blanking.
For voltage-mode operation, refer to the graph, “Typical
Characteristics: Voltage-Mode Max. Duty Cycle vs. MDC
Voltage.” Voltage-mode operation requires the timing ca-
pacitor ramp, from C
T
, be connected directly to CMR.
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相關(guān)代理商/技術(shù)參數(shù) |
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| MIC3833BWM | 制造商:MICREL 制造商全稱:Micrel Semiconductor 功能描述:Current-Fed PWM Controllers |
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| MIC3838BMM | 功能描述:IC REG CTRLR ISO PWM 10-MSOP RoHS:否 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:4,000 系列:- PWM 型:電壓模式 輸出數(shù):1 頻率 - 最大:1.5MHz 占空比:66.7% 電源電壓:4.75 V ~ 5.25 V 降壓:是 升壓:無 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 85°C 封裝/外殼:40-VFQFN 裸露焊盤 包裝:帶卷 (TR) |
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