LM2731XMF/HALF National Semiconductor, LM2731XMF/HALF Datasheet - Page 13

• Manufacturer Part Number: LM2731XMF/HALF
• Manufacturer: National Semiconductor

Specifications of LM2731XMF/HALF

• Lead Free Status / Rohs Status: Supplier Unconfirmed

CALCULATING LOAD CURRENT
As shown in the figure which depicts inductor current, the load
current is related to the average inductor current by the rela-
tion:
Where "DC" is the duty cycle of the application. The switch
current can be found by:
Inductor ripple current is dependent on inductance, duty cy-
cle, input voltage and frequency:
combining all terms, we can develop an expression which al-
lows 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 various input and output
Switch Current Limit vs Duty Cycle - "X"
Switch Current Limit vs Duty Cycle - "Y"
I
RIPPLE
I
I
SW
LOAD
= I
= DC x (V
= I
IND
IND
(AVG) + ½ (I
(AVG) x (1 - DC)
IN
-V
SW
) / (f x L)
RIPPLE
)
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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:
DESIGN PARAMETERS V
The value of the FET "ON" voltage (referred to as V
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
the internal N-FET has less gate voltage in this input voltage
range (see Typical performance Characteristics curves).
Above V
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:
IN
= 5V, the FET gate voltage is internally clamped to
Max. Load Current (typ) vs V
Max. Load Current (typ) vs V
P
(SW)
= DC x I
IND
SW
(AVE)
AND I
IN
2
SW
values below 5V, since
x R
DS
IN
IN
(ON)
- "X"
- "Y"
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in the