Datasheet

I
CL
=
0.245V
R6
©
§
¹
·
+
-
OC
0.245V
17
R6
14
U1
Q5
V
OUT
V
IN
I
SW
-+
L = V
IN
= V
IN
D
'i x f
SW
©
§
¹
·
dt
di
©
§
¹
·
V
IN
= L
di
dt
©
§
¹
·
Board Features www.ti.com
I
IN
= 1830 mA @ V
IN
= 18V
Set inductor current ripple for 30% of average current.
ΔI
IN
= 915 mA x 0.30 = 275 mA
ΔI
IN
= 1830 mA x 0.30 = 550 mA
(10)
Therefore:
(11)
Inductor value @ V
IN-MIN
≊ 33 µH
5.4 Peak Current Limit
Due to its peak current-mode control architecture, the LM3423 has inherent cycle-to-cycle current limit
control. Inductor current flowing through the low-side power MOSFET (Q5) is sensed as a voltage
between IS (pin 17) and PGND (pin 14). This voltage is fed into an internal comparator which establishes
the peak current allowed during each switching cycle.
Two methods of switch current sensing are available on the evaluation board. The first is accomplished
through the use of an external sense resistor which allows for higher accuracy in sensing the peak current.
For the LM3423 evaluation board, the sense resistor R6 can be utilized using the jumper configuration as
described in Table 4.
Figure 4. External R
SENSE
I
SW
Current Sense
Table 4. External R
SENSE
Resistor Configuration
Jumper Operation
J2 Open Jumper
J3 Close Jumper
J4A, J4B Open Jumper
R6 Populate
The current limit (I
CL
) is calculated by the equation:
(12)
Substituting in the resistor value as listed in the board schematic gives a current limit I
CL
of approximately
4.1A.
6
AN-1907 LM3423 Buck-Boost Configuration Evaluation Board SNVA376A–December 2008–Revised May 2013
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