Datasheet

I
LOAD
(max) = (1 - DC) x (I
SW
(max) - DC (V
IN
- V
SW
))
2fL
0 20 40 60 80 100
0
200
400
600
800
1000
1200
1400
1600
SWITCH CURRENT LIMIT (mA)
DUTY CYCLE (%) = [1 - EFF*(VIN/VOUT))]
V
IN
= 5V
V
IN
= 3.3V
V
IN
= 2.7V
LM27313
SNVS487D –DECEMBER 2006–REVISED APRIL 2013 www.ti.com
MAXIMUM SWITCH CURRENT
The maximum FET switch current available before the current limiter cuts in is dependent on duty cycle of the
application. This is illustrated in Figure 15 below which shows typical values of switch current as a function of
effective (actual) duty cycle:
Figure 15. Switch Current Limit vs Duty Cycle
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. (9)
The switch current can be found by:
I
SW
= I
IND(AVG)
+ ½ (I
RIPPLE
) (10)
Inductor ripple current is dependent on inductance, duty cycle, input voltage and frequency:
I
RIPPLE
= DC x (V
IN
- V
SW
) / (f
SW
x L) (11)
Combining all terms, we can develop an expression which allows the maximum available load current to be
calculated:
(12)
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 and displayed the maximum load current available for a typical device in graph
form:
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