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| 型号： | IRU3138CSTRPBF |
| 厂商： | International Rectifier |
| 文件页数： | 5/18页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 14-SOIC |
| 标准包装： | 2,500 |
| PWM 型： | 电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 440kHz |
| 占空比： | 90% |
| 电源电压： | 4.25 V ~ 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： | 0°C ~ 70°C |
| 封装/外壳： | 14-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
IRU3138
THEORY OF OPERATION
Introduction
The IRU3138 is a fixed frequency, voltage mode syn-
chronous controller and consists of a precision refer-
ence voltage, an error amplifier, an internal oscillator, a
PWM comparator, 1A peak gate driver, soft-start and
shutdown circuits (see Block Diagram). The output volt-
age of the synchronous converter is set and controlled
by the output of the error amplifier; this is the amplified
error signal from the sensed output voltage and the refer-
ence voltage. This voltage is compared to a fixed fre-
quency linear sawtooth ramp and generates fixed fre-
quency pulses of variable duty-cycle, which drives the
two N-channel external MOSFETs.The timing of the IC
is provided through an internal oscillator circuit which
uses on-chip capacitor. The oscillation frequency is pro-
grammable between 200KHz to 400KHz by using an
external resistor. Figure 4A shows switching frequency
vs. external resistor.
Soft-Start
The IRU3138 has a programmable soft-start to control
the output voltage rise and limit the current surge at the
start-up. To ensure correct start-up, the soft-start se-
quence initiates when the Vc and Vcc rise above their
threshold (3.5V and 4.25V respectively) and generates
the Power On Reset (POR) signal. Soft-start function
operates by sourcing an internal current to charge an
external capacitor to about 3V. Initially, the soft-start func-
tion clamps the E/A’s output of the PWM converter and
disables the short circuit protection. During the power
The magnitude of this current is inversely proportional to
the voltage at soft-start pin.
The 20 μ A current source starts to charge up the exter-
nal capacitor. In the mean time, the soft-start voltage
ramps up, the current flowing into Fb pin starts to de-
crease linearly and so does the voltage at the positive
pin of feedback UVLO comparator and the voltage nega-
tive input of E/A.
When the soft-start capacitor is around 1V, the current
flowing into the Fb pin is approximately 32 μ A. The volt-
age at the positive input of the E/A is approximately:
32 μ A × 25K = 0.8V
The E/A will start to operate and the output voltage starts
to increase. As the soft-start capacitor voltage contin-
ues to go up, the current flowing into the Fb pin will keep
decreasing. Because the voltage at pin of E/A is regu-
lated to reference voltage 0.8V, the voltage at the Fb is:
V FB = 0.8-25K × (Injected Current)
The feedback voltage increases linearly as the injecting
current goes down. The injecting current drops to zero
when soft-start voltage is around 2V and the output volt-
age goes into steady state.
As shown in Figure 4, the positive pin of feedback UVLO
comparator is always higher than 0.4V, therefore, feed-
back UVLO is not functional during soft-start.
up, the output starts at zero and voltage at Fb is below
0.4V. The feedback UVLO is disabled during this time
by injecting a current (64 μ A) into the Fb. This generates
a voltage about 1.6V (64 μ A × 25K) across the negative
Output of UVLO
POR
3V
? 2V
input of E/A and positive input of the feedback UVLO
comparator (see Fig3).
Soft-Start
0V
? 1V
20uA
3V
Voltage
Current flowing
64uA
SS/SD
64uA
Max
HDrv
into Fb pin
0uA
POR
Comp
25K
Error Amp
LDrv
Voltage at negative input ? 1.6V
of Error Amp and Feedback
UVLO comparator
0.8V
0.8V
25K
0.8V
Fb
Voltage at Fb pin
0V
0.4V
Figure 4 - Theoretical operational waveforms
64uA × 25K=1.6V
When SS=0
Feeback
POR
during soft-start.
UVLO Comp
Figure 3 - Soft-start circuit for IRU3138.
www.irf.com
5
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