Datasheet
V
IN
- V
OUT
L
=
2'i
L
DT
S
t
L
i'
OUT
I
S
T
S
DT
L
V
OUT
L
- V
OUT
V
IN
D =
V
OUT
+ V
D
V
IN
+ V
D
- V
SW
D =
V
OUT
V
IN
LM2831
www.ti.com
SNVS422C –AUGUST 2006–REVISED APRIL 2013
CURRENT LIMIT
The LM2831 uses cycle-by-cycle current limiting to protect the output switch. During each switching cycle, a
current limit comparator detects if the output switch current exceeds 2.5A (typ), and turns off the switch until the
next switching cycle begins.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature
exceeds 165°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature
drops to approximately 150°C.
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (V
O
) to input voltage (V
IN
):
(1)
The catch diode (D1) forward voltage drop and the voltage drop across the internal PMOS must be included to
calculate a more accurate duty cycle. Calculate D by using the following formula:
(2)
V
SW
can be approximated by:
V
SW
= I
OUT
x R
DSON
(3)
The diode forward drop (V
D
) can range from 0.3V to 0.7V depending on the quality of the diode. The lower the
V
D
, the higher the operating efficiency of the converter. The inductor value determines the output ripple current.
Lower inductor values decrease the size of the inductor, but increase the output ripple current. An increase in the
inductor value will decrease the output ripple current.
One must ensure that the minimum current limit (1.8A) is not exceeded, so the peak current in the inductor must
be calculated. The peak current (I
LPK
) in the inductor is calculated by:
I
LPK
= I
OUT
+ Δi
L
(4)
Figure 25. Inductor Current
(5)
In general,
Δi
L
= 0.1 x (I
OUT
) → 0.2 x (I
OUT
) (6)
If Δi
L
= 20% of 1.50A, the peak current in the inductor will be 1.8A. The minimum ensured current limit over all
operating conditions is 1.8A. One can either reduce Δi
L
, or make the engineering judgment that zero margin will
be safe enough. The typical current limit is 2.5A.
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 9
Product Folder Links: LM2831