Datasheet

ISW
LIMIT
=
I
OUT
(1-D)
+
(D x V
IN
)
(2 x f
S
x L)
R
SEN
=
V
SENSE
- (D x V
SL
)
ISW
LIMIT
DR
V
OUT
FB
L
D
Q
I
SEN
C
OUT
R
fb2
V
IN
LM3488
R
SEN
R
fb1
LM3488
LM3488-Q1
www.ti.com SNVS089M –JULY 2000–REVISED MARCH 2013
Figure 34. Adjusting the Output Voltage
SETTING THE CURRENT LIMIT
The maximum amount of current that can be delivered to the load is set by the sense resistor, R
SEN
. Current limit
occurs when the voltage that is generated across the sense resistor equals the current sense threshold voltage,
V
SENSE
. When this threshold is reached, the switch will be turned off until the next cycle. Limits for V
SENSE
are
specified in Electrical Characteristics. V
SENSE
represents the maximum value of the internal control signal V
CS
.
This control signal, however, is not a constant value and changes over the course of a period as a result of the
internal compensation ramp (see Figure 22). Therefore the current limit threshold will also change. The actual
current limit threshold is a function of the sense voltage (V
SENSE
) and the internal compensation ramp:
R
SEN
x ISW
LIMIT
= VCS
MAX
= V
SENSE
- (D x V
SL
)
where
• ISW
LIMIT
is the peak switch current limit, defined by the equation below. As duty cycle increases, the control
voltage is reduced as V
SL
ramps up. Since current limit threshold varies with duty cycle, the following equation
should be used to select R
SEN
and set the desired current limit threshold: (15)
(16)
The numerator of the above equation is V
CS
, and ISW
LIMIT
is calculated as:
(17)
To avoid false triggering, the current limit value should have some margin above the maximum operating value,
typically 120%. Values for both V
SENSE
and V
SL
are specified in Electrical Characteristics. However, calculating
with the limits of these two specs could result in an unrealistically wide current limit or R
SEN
range. Therefore, the
following equation is recommended, using the V
SL
ratio value given in Electrical Characteristics:
Copyright © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 19
Product Folder Links: LM3488 LM3488-Q1