參數(shù)資料
型號(hào): LM2731YMF
廠商: NATIONAL SEMICONDUCTOR CORP
元件分類: 穩(wěn)壓器
英文描述: 0.6/1.6 MHz Boost Converters With 22V Internal FET Switch in SOT-23
中文描述: 2 A SWITCHING REGULATOR, 800 kHz SWITCHING FREQ-MAX, PDSO5
封裝: SOT-23, 5 PIN
文件頁數(shù): 14/16頁
文件大?。?/td> 528K
代理商: LM2731YMF
Application Hints
(Continued)
20059150
Switch Current Limit vs Duty Cycle - "X"
20059151
Switch Current Limit vs Duty Cycle - "Y"
CALCULATING LOAD CURRENT
As shown in the figure which depicts inductor current, the
load current is related to the average inductor current by the
relation:
I
LOAD
= I
IND
(AVG) x (1 - DC)
Where "DC" is the duty cycle of the application. The switch
current can be found by:
I
SW
= I
IND
(AVG) +
1
2
(I
RIPPLE
)
Inductor ripple current is dependent on inductance, duty
cycle, input voltage and frequency:
I
RIPPLE
= DC x (V
IN
-V
SW
) / (f x L)
combining all terms, we can develop an expression which
allows the maximum available load current to be calculated:
The equation shown to calculate maximum load current
takes into account the losses in the inductor or turn-OFF
switching losses of the FET and diode. For actual load
current in typical applications, we took bench data for vari-
ous input and output voltages for both the "X" and "Y"
versions of the LM2731 and displayed the maximum load
current available for a typical device in graph form:
20059148
Max. Load Current (typ) vs V
IN
- "X"
20059149
Max. Load Current (typ) vs V
IN
- "Y"
DESIGN PARAMETERS V
SW
AND I
SW
The value of the FET "ON" voltage (referred to as V
in the
equations) is dependent on load current. A good approxima-
tion can be obtained by multiplying the "ON Resistance" of
the FET times the average inductor current.
FET on resistance increases at V
values below 5V, since
the internal N-FET has less gate voltage in this input voltage
range (see Typical performance Characteristics curves).
Above V
IN
= 5V, the FET gate voltage is internally clamped
to 5V.
The maximum peak switch current the device can deliver is
dependent on duty cycle. For higher duty cycles, see Typical
performance Characteristics curves.
THERMAL CONSIDERATIONS
At higher duty cycles, the increased ON time of the FET
means the maximum output current will be determined by
power dissipation within the LM2731 FET switch. The switch
power dissipation from ON-state conduction is calculated by:
L
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