Datasheet
I
HYS
=
MAX (V
HYS
- 50 x 10
-6
x R
SL
x D
MAX
, 0)
R
SN
(A)
R
SN(MAX)
=
V
CL(0)(MIN)
- D
MAX
(V
CL(0)(MIN)
- (V
CL(100)(MIN)
- 50 x 10
-6
x
R
SL
))
V
OUT
(1-D
MAX
)
2 x L x f
S
I
OUT
+
|
1.15 x I
OUT
V
CL(0)(MIN)
- D
MAX
(V
CL(0)(MIN)
- (V
CL(100)(MIN)
- 50 x 10
-6
x
R
SL
))
CONTROL VOLTAGE (mV)
DUTY CYCLE (%)
0 20 40 60 80
0
100
200
100
V
CL(0)
V
SEN
V
SEN(PEAK)
V
CL(D)
32
11
V
SL
'
V
SL
'
Blank-out
Time
V
CL(100)
I
HYS
=
0.032
0.02
= 1.6A, LM3477
0.011
0.02
0.55A, LM3477A
|
=
0.032
R
SN
(A), LM3477
0.011
R
SN
(A), LM3477A
I
HYS
=
V
HYS
R
SN
=
=
LM3477
www.ti.com
SNVS141K –OCTOBER 2000–REVISED MARCH 2013
The hysteretic threshold is derived in a similar manner, the only difference being that V
SEN(peak)
is compared
V
C(min)
(V
HYS
). Notice that V
HYS
does not vary with the duty cycle. The hysteretic threshold is predetermined by
the selection of R
SN
above. The hysteretic threshold is:
(13)
Continuing with the example above,
(14)
If the peak switch current decreases below this threshold, the LM3477/A will operate in hysteretic mode (see
OVER VOLTAGE PROTECTION). In some designs, it will be desired to use R
SL
so that lower valued inductors
can be used (see DEFAULT/ADJUSTABLE SLOPE COMPENSATION and POWER INDUCTOR SECTION).
Using R
SL
will lower the current limit and the hysteretic threshold. See Figure 27. R
SL
effectively adds an
additional slope to the existing slope of the V
C
waveform.
Figure 27. Current Limit and Hysteretic Threshold vs Duty Cycle with R
SL
When R
SL
is used, the following equations apply:
(15)
(16)
where MAX(V
HYS
− 50x10
-6
x R
SL
x D
MAX
, 0) is the smaller of the two values in the parenthesis and V
HYS
is
0.032V and 0.011V for the LM3477 and LM3477A, respectively. R
SL
can be used creatively to intentionally lower
the hysteretic threshold, allowing for better performance at lower loads. However, when R
SL
is used, there may
be a minimum load requirement (see START-UP/SOFT-START).
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