Datasheet
R
SN(MAX)
=
0.135 - 0.6 (0.135 - .025)
1.15 (3)
= 0.02:
R
SN(MAX)
=
V
CL(0)(MIN)
- D
MAX
(V
CL(0)(MIN)
- V
CL(100)(MIN)
)
1.15 x I
OUT(MAX)
|
V
CL(0)(MIN)
- D
MAX
(V
CL(0)(MIN)
- V
CL(100)(MIN)
)
V
OUT
(1-D
MAX
)
2 x L x f
S
I
OUT(MAX)
+
V
SEN(PEAK)
=
) (V), R
SL
= 0
V
OUT
(1-D
MAX
)
2 x L x f
S
R
SN
(I
OUT(MAX)
+
R
SN
x I
OUT(MAX)
(1+ 0.15) (V)
|
CONTROL VOLTAGE (mV)
DUTY CYCLE (%)
0 20 40 60 80
0
100
200
100
V
CL(0)
V
CL(D)
32
11
Blank-out
Time
V
SEN
V
HYS
(LM3477)
V
HYS
(LM3477A)
V
SEN(PEAK)
V
CL(100)
LM3477
SNVS141K –OCTOBER 2000–REVISED MARCH 2013
www.ti.com
PROGRAMMING THE CURRENT LIMIT/HYSTERETIC THRESHOLD
Definitions
Current Limit: The current limit is the point at which the LM3477/A begins to limit the peak switch current. The
current limit in the LM3477/A varies with duty cycle, which is a function of the V
IN
− V
OUT
differential.
Hysteric Threshold: Hysteretic threshold is the current at which the LM3477/A enters the hysteretic mode of
operation (see OVER VOLTAGE PROTECTION). The hysteretic threshold is with respect to the peak switch
current.
SETTING CURRENT LIMIT AND HYSTERETIC THRESHOLD
The adjustable current limit of the LM3477/A is set by the sense resistor R
SN
. The voltage across R
SN
is
compared to an internal control voltage V
C
. The onset of current limiting is when V
SEN(peak)
equals V
C(max)
, or V
CL
.
V
SEN
is defined here as the differential voltage from the V
IN
pin to the I
SEN
pin. V
CL
decreases as the duty cycle
increases, as shown in Figure 26. Therefore, it is important to know both V
SEN(peak)
and V
CL(min)
at the maximum
operating duty cycle, or lowest V
IN
condition.
Figure 26. Current Limit and Hysteretic Threshold vs Duty Cycle
(8)
V
CL(MIN)
= V
CL(0)(MIN)
− D
(MAX)
(V
CL(0)(MIN)
− V
CL(100)(MIN)
) (9)
where D
MAX
is the duty cycle at the lowest V
IN
condition.
To avoid current limit,
V
SEN(peak)
< V
CL(MIN)
(10)
Therefore,
(11)
Example: V
IN(MIN)
= 4.5V, V
OUT
= 2.5V, I
OUT(MAX)
= 3A
(12)
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