SMS3.3TCT Semtech, SMS3.3TCT Datasheet - Page 7

• Manufacturer Part Number: SMS3.3TCT
• Manufacturer: Semtech

Step 3: (Cont.)
Using our 6 LED example:
L
thus I
Step 4: Calculating the Current Limit Resistor for
Discontinuous Mode
The current limit resistor value is calculated based upon
the minimum ratio of the switch current to the current out
of the LIM pin. It also takes into account the fact that
there is a propagation delay during which time the
inductor current ramps beyond the current limit trip point.
Since I
maximum. The maximum current limit resistor, R
discontinuous mode is therefore:
Where:
V
reference = 385mV
Ratio
= 7100 A/A
t
trip point to the power switch turning off = 200ns
Using our 6 LED example:
I
thus R
Selecting this value or the next lower standard value in
this case gives us 7.5k . Of course a lower value resistor
may be used if desired, but may not necessarily be the
most efficient choice.
Step 5: Calculating the Current Set Resistor
The current set resistor is in series with the series LED
string. Thus the voltage developed across it is proportional
to the current flowing through the LEDs. The device will
regulate this voltage so that its average value equals the
feedback voltage, V
current set resistor value is given by the following
R
POWER MANAGEMENT
LIM(D)
plh
Applications Information (Cont.)
(D)
LIM(MIN)
LIM
2005 Semtech Corp.
= propagation delay from reaching the current limit
= 12µH
(
D
= 412mA
)
(MIN)
LIM(D)
LIM(D)
=
LIM
= the minimum value of the current limit voltage




I
LIM
= the minimum value of the I
increases as R
= 412mA
= 7.5k
(
D
V
)
LIM
−
t
(
MIN
plh
•
FB
)
•
(
, which is typically 350mV. Thus the
V
Ratio
IN
L
−
(
D
LIM
)
V
(
MIN
CE
decreases, this value is a
(
)
SAT
)
)




LX
to I
LIM(D)
LIM
ratio
, for
7
equation:
Using our 6 LED example:
I
thus R
We will select the 1% resistor value 23.2 .
Note: this calculation is applicable to both continuous and
discontinuous modes.
Step 6: Calculating the Inductor for Continuous Mode
Having determined that we need to be operating in
continuous mode, we next need to calculate the
maximum inductor value allowed that will permit the part
to output the correct power. The maximum continuous
inductor value, L
Where:
I
t
Selecting the next lower standard value gives us a safe
value for this inductor.
Step 7: Calculating the Current Limit Required with this
Inductor for Continuous Mode
Having determined the inductor value we are going to
use, we next need to calculate the current limit required
to meet the necessary output power. The continuous
mode current limit, I
Step 8: Calculating the Current Limit Resistor for
Continuous Mode
The current limit resistor value is calculated based upon
the minimum ratio of the switch current to the current
out of the LIM pin. It also takes into account the fact
that there is a propagation delay during which time the
I
R
L
OUT
OUT
OFF(MIN)
LIM
(
SET
C
(
)
C
= 15mA
= required output current
=
)
=
=
2
SET
I
= minimum switch off-time = 0.6µs
V
OUT
(
•
V
FB
V
= 23.3
OUT
IN
•
+
t
1
OFF
V
0
4 .
f
4 .
(C)
(
−
MIN
•
•
V
is given by:
V
)
L
IN
OUT
LIM(C)
•
(
C
)
(
•
)
V
•
t
OUT
, is given by:
OFF
I
OUT
(
+
MIN
V
)
f
−
V
IN
)
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