Datasheet

I
LOAD
(max) = (1 - DC) x (I
SW
(max) - DC (V
IN
- V
SW
))
2fL
20 30 40 50 60 70 80 90 100
DUTY CYCLE (%) = [1 - EFF*(V
IN
/ V
OUT
)]
0
500
1000
1500
2000
2500
3000
SW CURRENT LIMIT (mA)
V
IN
= 5V
V
IN
= 3.3V
V
IN
= 2.7V
V
IN
= 3V
LMR62014
www.ti.com SNVS735B –OCTOBER 2011–REVISED APRIL 2013
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 the graphs below which show typical values of switch current as a function of
effective (actual) duty cycle:
Figure 21. Switch Current Limit vs Duty Cycle
CALCULATING LOAD CURRENT
As shown in Figure 20 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. (7)
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 that displayed the maximum load current available for a typical device in graph
form:
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