Datasheet

2 3 4 5 6 7 8
V
IN
(V)
0
200
400
600
800
1000
1200
MAX LOAD CURRENT (mA)
V
OUT
= 5V
V
OUT
= 8V
V
OUT
= 10V
V
OUT
= 12V
V
IN
(V)
MAX LOAD CURRENT (mA)
0
200
400
600
800
1000
1200
2 3 4 5 6 7 8 9 10 11
V
OUT
= 5V
V
OUT
= 8V
V
OUT
= 10V
V
OUT
= 12V
V
OUT
= 18V
I
LOAD
(max) = (1 - DC) x (I
SW
(max) - DC (V
IN
- V
SW
))
2fL
TI Confidential - NDA Restrictions
LM2731
SNVS217F –MAY 2004–REVISED NOVEMBER 2012 www.ti.com
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) (7)
Where "DC" is the duty cycle of the application. The switch current can be found by:
I
SW
= I
IND
(AVG) + ½ (I
RIPPLE
) (8)
Inductor ripple current is dependent on inductance, duty cycle, input voltage and frequency:
I
RIPPLE
= DC x (V
IN
-V
SW
) / (f x L) (9)
combining all terms, we can develop an expression which allows the maximum available load current to be
calculated:
(10)
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 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:
Figure 33. Max. Load Current (typ) vs V
IN
- "X"
Figure 34. Max. Load Current (typ) vs V
IN
- "Y"
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