LM3552SDEV National Semiconductor, LM3552SDEV Datasheet - Page 10

• Manufacturer Part Number: LM3552SDEV
• Description: BOARD EVALUATION LM3552SD
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3552SDEV

• Current - Output / Channel: 700mA
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 4 V
• Features: Flash Timeout Protection
• Voltage - Input: 2.7 ~ 5.5V
• Utilized Ic / Part: LM3552
• Core Chip: LM3552
• Topology: Boost
• No. Of Outputs: 1
• Output Current: 1A
• Input Voltage: 2.7V To 5.5V
• Development Tool Type: Hardware - Eval/Demo Board
• Mcu Supported Families: LM3552
• Lead Free Status / RoHS Status: Not applicable / Not applicable

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Soft-Start
The LM3551 and LM3552 have a soft-start pin that can be
used to limit the inductor inrush current on start-up. The ex-
ternal SS pin is used to tailor the soft-start for a specific
application but is not required for all applications and can be
left open when not needed. When used, a current source
charges the external soft-start capacitor, C
ternal reference to ramp-up at a user determined rate.
V
IN
C
= 3.6V, T
SS
0.47
0.1
1.0
(µF)
A
= +25°C
Sharp LED @ 75mA Torch
Sharp LED @ 75mA Torch
Sharp LED @ 75mA Torch
Lumiled LED @ 200mA
Lumiled LED @ 700mA
Lumiled LED @ 200mA
Lumiled LED @ 700mA
Lumiled LED @ 200mA
Lumiled LED @ 700mA
Typical Start-Up Times
Sharp LED @ 250mA
Sharp LED @ 250mA
Sharp LED @ 250mA
Load (mA)
Torch
Torch
Torch
Flash
Flash
Flash
Flash
Flash
Flash
SS
Start-Up Time
, forcing the in-
(msec.)
1.6
12
35
35
25
75
30
70
3
8
6
6
10
Application Information
LM3551 AND LM3552 FUNCTIONALITY TRUTH TABLE
The LM3551 has a 1.2MΩ pull-up to V
LM3552 has a 1.2MΩ pull-down to GND on EN.
CURRENT SET EQUATIONS
The LM3551/2 utilize inline resistors to set the Torch and
Flash LED currents. The Torch-Mode current (continuous)
and the Flash-Mode current (pulsed) are programmed by
placing the appropriately selected resistors between the feed-
back pin (FB) and FET-T (torch FET) and FET-F (flash FET)
pins. Torch-mode is set by utilizes the current through one
resistor while Flash-mode is set by utilizes the currents
though both current set resistors. The following equations are
used to set the LED currents.
Note: Flash LEDs from different manufacturers can have very different con-
INDUCTOR SELECTION
Special care must be taken when selecting an inductor for use
in LM3551/2 applications. The inductor should have a current
saturation rating that is larger than the worst case peak in-
ductor current of the application to ensure proper operation.
Using an inductor with a lower saturation current rating than
is required can cause a dramatic drop in the inductance and
can derate the maximum output current levels severely. It is
worth noting that the output voltage ripple is also affected by
the total ripple current in the inductor. The following equations
can help give a good approximation as to what the peak in-
ductor current will be for a given application at room temper-
ature (T
V
V
Eff.
D
L
F
OUT-MAX
IN-MIN
SW
SD (LM3551)
EN (LM3552)
tinuous and pulse current ratings. See the manufacturers datasheets
to ensure that the proper current levels are used to avoid damaging
the flash LED.
I
L
R
or
A
(average) = [I
1
1
0
0
FLASH
= +25°C).
Maximum Output Voltage. Maximum output volt-
age over temperature with OVP used is 11V
(12.4V typically).
Minimum Input Voltage. Recommended minimum
input voltage is 3.0V. The LM3551/2 will work
down to 2.7V however, use at lower input voltages
will required an inductor with a higher saturation
current rating.
Converter Efficiency (approx. 85% over input volt-
age range).
Duty Cycle = 1 - [V
Inductance. Recommended inductance value is
4.7µH.
Switching Frequency = 1.25MHz
R
R
DSON-T
TORCH
= (1.265V / (I
I
L
ΔI
(peak) = I
= 0.98Ω and R
L
T/F
= (1.265V / I
0
1
0
1
= [V
LED
IN
× V
LM3551 Result LM3552 Result
× D] ÷ [L × F
L
Torch Mode
Flash Mode
Shut-down
Shut-down
FLASH
(ave) + [ΔI
OUT-MAX
IN
/ V
TORCH
- I
DSON-F
OUT
TORCH)
] ÷ [V
) - R
]
L
SW
÷ 2]
= 0.36Ω
IN
IN-MIN
) - R
DSON-T
]
on SD and the
Torch Mode
Flash Mode
Shut-down
Shut-down
DSON-F
× Eff.]