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參數(shù)資料
| 型號： | LM2734 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Thin SOT23 1A Load Step-Down DC-DC Regulator |
| 中文描述： | 薄型SOT23 1A負(fù)載降壓DC - DC穩(wěn)壓 |
| 文件頁數(shù)： | 9/22頁 |
| 文件大小： | 459K |
| 代理商： | LM2734 |
Application Information
(Continued)
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
during start up. During soft-start, the error amplifier’s refer-
ence voltage ramps from 0V to its nominal value of 0.8V in
approximately 200μs. This forces the regulator output to
ramp up in a more linear and controlled fashion, which helps
reduce inrush current.
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
operating until the input voltage exceeds 2.74V(typ).
The UVLO threshold has approximately 440mV of hyster-
esis, so the part will operate until V
drops below 2.3V(typ).
Hysteresis 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 ex-
ceeds 165C. After thermal shutdown occurs, the output
switch doesn’t turn on until the junction temperature drops to
approximately 150C.
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (V
O
) to input voltage (V
IN
):
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 follow-
ing formula:
V
SW
can be approximated by:
V
SW
= I
O
x R
DS(ON)
The diode forward drop (V
) can range from 0.3V to 0.7V
depending 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.
Lower 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 (
i
) to output current (I
) 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 calcu-
lated by:
I
LPK
= I
O
+
I
L
/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
empirically 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.
20102348
FIGURE 5. Boost Voltage Supplied from the Shunt
Zener on V
IN
L
www.national.com
9
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| LM27341 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:2 MHz 1.5A/2A Wide Input Range Step-Down DC-DC Regulator with Frequency Synchronization |
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| LM27341MYX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:2 MHz 1.5A/2A Wide Input Range Step-Down DC-DC Regulator with Frequency Synchronization |
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