Datasheet

R
SEN
=
V
SENSE
Vo - Vi
L x f
S
x D
ISW
LIMIT
+
ISW
LIMIT
=
V
SENSE
- (D x(V
SL
+ 'V
SL
))
R
SEN
R
SL
>
40 PA
R
SEN
x (Vo - 2V
IN
)
2 x f
S
x L
- V
SL
R
SEN
<
2 x V
SL
x f
S
x L
Vo - (2 x V
IN
)
R
SEN
=
V
SENSE
- (D x V
SENSE
x V
SL
ratio)
ISW
LIMIT
LM3488
LM3488-Q1
SNVS089M –JULY 2000–REVISED MARCH 2013 www.ti.com
(18)
R
SEN
is part of the current mode control loop and has some influence on control loop stability. Therefore, once
the current limit threshold is set, loop stability must be verified. To verify stability, use the following equation:
(19)
If the selected R
SEN
is greater than this value, additional slope compensation must be added to ensure stability,
as described in CURRENT LIMIT WITH EXTERNAL SLOPE COMPENSATION.
CURRENT LIMIT WITH EXTERNAL SLOPE COMPENSATION
R
SL
is used to add additional slope compensation when required. It is not necessary in most designs and R
SL
should be no larger than necessary. Select R
SL
according to the following equation:
where
• R
SEN
is the selected value based on current limit. With R
SL
installed, the control signal includes additional
external slope to stabilize the loop, which will also have an effect on the current limit threshold. Therefore, the
current limit threshold must be re-verified, as illustrated in the equations below : (20)
V
CS
= V
SENSE
– (D x (V
SL
+ ΔV
SL
))
where
• ΔV
SL
is the additional slope compensation generated and calculated as: (21)
ΔV
SL
= 40 µA x R
SL
(22)
This changes the equation for current limit (or R
SEN
) to:
(23)
The R
SEN
and R
SL
values may have to be calculated iteratively in order to achieve both the desired current limit
and stable operation. In some designs R
SL
can also help to filter noise on the ISEN pin.
If the inductor is selected such that ripple current is the recommended 30% value, and the current limit threshold
is 120% of the maximum peak, a simpler method can be used to determine R
SEN
. The equation below will
provide optimum stability without RSL, provided that the above 2 conditions are met:
(24)
POWER DIODE SELECTION
Observation of the boost converter circuit shows that the average current through the diode is the average load
current, and the peak current through the diode is the peak current through the inductor. The diode should be
rated to handle more than its peak current. The peak diode current can be calculated using the formula:
I
D(Peak)
= I
OUT
/ (1−D) + ΔI
L
(25)
In the above equation, I
OUT
is the output current and ΔI
L
has been defined in Figure 33.
The peak reverse voltage for boost converter is equal to the regulator output voltage. The diode must be capable
of handling this voltage. To improve efficiency, a low forward drop schottky diode is recommended.
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