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参数资料
| 型号： | MIC3838YMM TR |
| 厂商： | Micrel Inc |
| 文件页数： | 9/11页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR ISO PWM 10-MSOP |
| 标准包装： | 2,500 |
| PWM 型： | 电流/电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 占空比： | 50% |
| 电源电压： | 2 V ~ 14 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 是 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽） |
| 包装： | 带卷 (TR) |
| 其它名称： | MIC3838YMMTR MIC3838YMMTR-ND |
MIC3838/3839
Soft Start
The soft start feature helps reduce surge currents at the
power supply input source. An internal current source and
capacitor ramp up from 0V to near V DD at a typical rate of
1V/ms. The soft start feature limits the output voltage of the
error amplifier at the COMP pin. As the soft start voltage rises,
it allows the COMP pin voltage to rise, which in turn allows the
duty cycle of the output drivers to increase. The internal soft
start voltage is discharged and remains discharged during
the following conditions:
1. The V DD voltage drops below the turn-off
threshold.
2. The voltage on the CS pin exceeds the overcur-
rent comparator threshold.
Once the internal soft start discharge FET is turned on, it
cannot be turned off until the internal soft start voltage drops
down below 0.5V. This insures a clean restart.
Oscillator
The oscillator operates at twice the switching frequency of
either OUTA or OUTB. The oscillator generates a sawtooth
waveform on the RC pin. The rising edge of the waveform is
controlled by the external resistor/capacitor combination.
The fall time is set by the on-resistance of the discharge FET
(see Figure 3). The fall time sets the delay (dead time)
between the turn-off of one output driver and the turn-on of the
other driver. A toggle flip-flop insures that drive signals to
OUTA and OUTB are alternated and therefore insures a
maximum duty cycle of less than 50% for each output driver.
Graphs of component values vs. oscillator frequency and
dead time are shown in the typical characteristic section of
this specification.
V DD
4
RC
Micrel
Graphs of oscillator frequency and dead time vs component
values are shown in the Typical Characteristic section of this
specification. The recommended range of timing resistors
and capacitors is 7k ? to 200k ? and 100pF to 1000pF. To
minimize oscillator noise and insure a stable waveform the
following layout rules should be followed:
1. The higher impedance of capacitor values less
than 100pF may causes the oscillator circuit to
become more susceptible to noise. Parasitic pin
and etch trace capacitances become a larger
part of the total RC capacitance and may
influence the desired switching frequency.
2. The circuit board etch between the timing
resistor, capacitor, RC pin and ground must be
kept as short as possible to minimize noise
pickup and insure a stable oscillator waveform.
3. The ground lead of the capacitor must be routed
close to the ground lead of the MIC3838/9.
Current Sensing and Overcurrent Protection
The features are:
? Peak current limit
? Overcurrent limit
? Internal current sense discharge
? Front edge blanking
In current mode control, a PWM comparator uses the inductor
current signal and the error amplifier signal to determine the
operating duty cycle. In the MIC3838/9 the signal at the CS
pin is level shifted up before it reaches the PWM comparator
as shown in Figure 1. This allows operation of the error
amplifier and PWM comparator in a linear region.
There are two current limit thresholds in the MIC3838/9; peak
current limit and overcurrent limit. The normal operating
voltage at the I LIM pin is designed less than these thresholds.
A pulse-by-pulse current limit occurs when the inductor
VDD
2
0.2V
S
R
Q
OSCILLATOR
OUTPUT
current signal at the I LIM pin exceeds the peak current limit
threshold. The on-time is terminated for the remainder of the
switching cycle, regardless of whether OUTA or OUTB is
active.
If the signal at the I LIM pin goes past the peak threshold and
exceeds the overcurrent limit threshold, the overcurrent limit
F OSCILLATOR =
Figure 3. Oscillator
The voltage source to the resistor/capacitor timing compo-
nents is V DD . The internal turn-off comparator threshold in the
oscillator circuit is V DD /2. This allows the oscillator to track
changes in V DD and minimize frequency variations in the
oscillator. The oscillator frequency can be roughly approxi-
mated using the following formula:
1.41
R × C
Where: frequency is in Hz
Resistance is in ohms
Capacitance is in Farads.
comparator forces the soft start node to discharge and
initiates a soft start reset.
An internal FET discharges the RAMP and I LIM pins at the end
of the oscillator charge time. The FET turns on when the
voltage on the RC pin reaches the upper threshold (V DD /2)
and remains on for the duration of the RC pin discharge time
and for typically 100ns after the start of the next on-time
period. The 100ns period at the beginning of the on-time
implements a front edge blanking feature that prevents false
triggering of the PWM comparator due to noise spikes on the
leading edge of the current turn-on signal. The front edge
blanking also sets the minimum on-time for OUTA and OUTB.
The timing diagram for the RAMP pin is shown in Figure 4.
April 2005
9
MIC3838/3839
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相关代理商/技术参数 |
参数描述 |
|---|---|
| MIC3839 | 制造商:MICREL 制造商全称:Micrel Semiconductor 功能描述:Flexible Push-Pull PWM Controller |
| MIC3839BMM | 功能描述:IC REG CTRLR ISO PWM 10-MSOP RoHS:否 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 标准包装:4,000 系列:- PWM 型:电压模式 输出数:1 频率 - 最大:1.5MHz 占空比:66.7% 电源电压:4.75 V ~ 5.25 V 降压:是 升压:无 回扫:无 反相:无 倍增器:无 除法器:无 Cuk:无 隔离:无 工作温度:-40°C ~ 85°C 封装/外壳:40-VFQFN 裸露焊盘 包装:带卷 (TR) |
| MIC3839BMM TR | 功能描述:IC REG CTRLR ISO PWM 10-MSOP RoHS:否 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 标准包装:4,000 系列:- PWM 型:电压模式 输出数:1 频率 - 最大:1.5MHz 占空比:66.7% 电源电压:4.75 V ~ 5.25 V 降压:是 升压:无 回扫:无 反相:无 倍增器:无 除法器:无 Cuk:无 隔离:无 工作温度:-40°C ~ 85°C 封装/外壳:40-VFQFN 裸露焊盘 包装:带卷 (TR) |
| MIC3839YMM | 功能描述:DC/DC 开关控制器 Push-Pull DC/DC Controller (Lead Free) RoHS:否 制造商:Texas Instruments 输入电压:6 V to 100 V 开关频率: 输出电压:1.215 V to 80 V 输出电流:3.5 A 输出端数量:1 最大工作温度:+ 125 C 安装风格: 封装 / 箱体:CPAK |
| MIC3839YMM TR | 功能描述:DC/DC 开关控制器 Push-Pull DC/DC Controller (Lead Free) RoHS:否 制造商:Texas Instruments 输入电压:6 V to 100 V 开关频率: 输出电压:1.215 V to 80 V 输出电流:3.5 A 输出端数量:1 最大工作温度:+ 125 C 安装风格: 封装 / 箱体:CPAK |
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