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参数资料
| 型号： | LM2734XMKX/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 2.5 A SWITCHING REGULATOR, 1900 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封装： | TSOT-23, 6 PIN |
| 文件页数： | 24/24页 |
| 文件大小： | 466K |
| 代理商： | LM2734XMKX/NOPB |
20102348
FIGURE 5. Boost Voltage Supplied from the Shunt Zener
on V
IN
ENABLE PIN / SHUTDOWN MODE
The LM2734 has a shutdown mode that is controlled by the
enable pin (EN). When a logic low voltage is applied to EN,
the part is in shutdown mode and its quiescent current drops
to typically 30nA. Switch leakage adds another 40nA from the
input supply. The voltage at this pin should never exceed
V
IN + 0.3V.
SOFT-START
This function forces V
OUT to increase at a controlled rate dur-
ing start up. During soft-start, the error amplifier’s reference
voltage ramps from 0V to its nominal value of 0.8V in approx-
imately 200s. This forces the regulator output to ramp up in
a more linear and controlled fashion, which helps reduce in-
rush current. Under some circumstances at start-up, an out-
put voltage overshoot may still be observed. This may be due
to a large output load applied during start up. Large amounts
of output external capacitance can also increase output volt-
age overshoot. A simple solution is to add a feed forward
capacitor with a value between 470pf and 1000pf across the
top feedback resistor (R1). See Figure 7 for further detail.
OUTPUT OVERVOLTAGE PROTECTION
The overvoltage comparator compares the FB pin voltage to
a voltage that is 10% higher than the internal reference Vref.
Once the FB pin voltage goes 10% above the internal refer-
ence, the internal NMOS control switch is turned off, which
allows the output voltage to decrease toward regulation.
UNDERVOLTAGE LOCKOUT
Undervoltage lockout (UVLO) prevents the LM2734 from op-
erating until the input voltage exceeds 2.74V(typ).
The UVLO threshold has approximately 440mV of hysteresis,
so the part will operate until V
IN drops below 2.3V(typ). Hys-
teresis prevents the part from turning off during power up if
V
IN is non-monotonic.
CURRENT LIMIT
The LM2734 uses cycle-by-cycle current limiting to protect
the output switch. During each switching cycle, a current limit
comparator detects if the output switch current exceeds 1.7A
(typ), and turns off the switch until the next switching cycle
begins.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off
the output switch when the IC junction temperature exceeds
165°C. After thermal shutdown occurs, the output switch
doesn’t turn on until the junction temperature drops to ap-
proximately 150°C.
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (V
O) to input voltage (VIN):
The catch diode (D1) forward voltage drop and the voltage
drop across the internal NMOS must be included to calculate
a more accurate duty cycle. Calculate D by using the following
formula:
V
SW can be approximated by:
V
SW = IO x RDS(ON)
The diode forward drop (V
D) can range from 0.3V to 0.7V de-
pending on the quality of the diode. The lower V
D is, the higher
the operating efficiency of the converter.
The inductor value determines the output ripple current. Low-
er inductor values decrease the size of the inductor, but
increase the output ripple current. An increase in the inductor
value will decrease the output ripple current. The ratio of ripple
current (
Δi
L) to output current (IO) is optimized when it is set
between 0.3 and 0.4 at 1A. The ratio r is defined as:
One must also ensure that the minimum current limit (1.2A)
is not exceeded, so the peak current in the inductor must be
calculated. The peak current (I
LPK) in the inductor is calculated
by:
I
LPK = IO + ΔIL/2
If r = 0.5 at an output of 1A, the peak current in the inductor
will be 1.25A. The minimum guaranteed current limit over all
operating conditions is 1.2A. One can either reduce r to 0.4
resulting in a 1.2A peak current, or make the engineering
judgement that 50mA over will be safe enough with a 1.7A
typical current limit and 6 sigma limits. When the designed
maximum output current is reduced, the ratio r can be in-
creased. At a current of 0.1A, r can be made as high as 0.9.
The ripple ratio can be increased at lighter loads because the
net ripple is actually quite low, and if r remains constant the
inductor value can be made quite large. An equation empiri-
cally developed for the maximum ripple ratio at any current
below 2A is:
r = 0.387 x I
OUT
-0.3667
Note that this is just a guideline.
The LM2734 operates at frequencies allowing the use of ce-
ramic output capacitors without compromising transient re-
sponse. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. See the output
9
www.national.com
LM2734
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM2734XQMK | 制造商:Texas Instruments 功能描述:DC DC REG BUCK 1A 1.6MHZ 6TSOT 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT, Primary Input Voltage:20V, No. of Outputs:1, 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT, Primary Input Voltage:20V, No. of Outputs:1, Output Voltage:18V, Output Current:1A, Voltage Regulator Case Style:SOT-23, No. of Pins:6, Operating Temperature Min:-40C, Operating Temperature , RoHS Compliant: Yes |
| LM2734XQMK/NOPB | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
| LM2734XQMKE | 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT |
| LM2734XQMKE/NOPB | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
| LM2734XQMKX/NOPB | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
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