LM3503SQX-44 National Semiconductor Corporation, LM3503SQX-44 Datasheet - Page 17

• Manufacturer Part Number: LM3503SQX-44
• Description: Dual-display Constant Current Led Driver With Analog Brightness Control
• Manufacturer: National Semiconductor Corporation

Application Information
the LM3503’s proper operation, and reduces the effect of the
LM3503 on other devices sharing the same supply voltage.
To ensure low input voltage ripple, the input capacitor must
have an extremely low ESR. As a result of the low input
voltage ripple requirement multilayer ceramic capacitors are
the best choice. A minimum capacitance of 2.0 µF is required
for normal operation, so consult the capacitor manufactur-
er’s data curves to verify whether the minimum capacitance
requirement is going to be achieved for a particular applica-
tion.
OUTPUT CAPACITOR SELECTION
The output capacitor serves as an energy reservoir for the
white LED load when the internal power FET switch (Figure
1: N1) is on or conducting current. The requirements for the
output capacitor must include worst case operation such as
when the load opens up and the LM3503 operates in over-
voltage protection (OVP) mode operation. A minimum ca-
pacitance of 0.5 µF is required to ensure normal operation.
Consult the capacitor manufacturer’s data curves to verify
whether the minimum capacitance requirement is going to
be achieved for a particular application.
Some recommended capacitor manufacturers included but
are not limited to:
DIODE SELECTION
To maintain high efficiency it is recommended that the aver-
age current rating (I
larger than the peak inductor current (I
the average current rating of the diode should be larger than
the maximum LED current. To maintain diode integrity the
peak repetitive forward current (I
or equal to the peak inductor current (I
forward voltage ratings (V
magnitudes (C
The chosen diode must have a reverse breakdown voltage
rating (V
(V
not, certain selection criteria must be followed:
1. V
2. I
3. I
Some recommended diode manufacturers included but are
not limited to:
Taiyo-
Yuden
muRata
TDK
AVX
OUT
F
FRM
R
). No matter what type of diode is chosen, Schottky or
or I
and V
R
≥ I
O
and/or V
Lpeak
≥ I
GMK212BJ105MD
(0805/35V)
GRM40-035X7R105K
(0805/50V)
C3216X7R1H105KT
(1206/50V)
C3216X7R1C475K
(1206/16V)
08053D105MAT
(0805/25V)
08056D475KAT
(0805/6.3V)
1206ZD475MAT
(1206/10V)
RRM
J
LOAD
or C
>
RRM
F
T
or I
V
or C
or I
OUT
) that is larger than the output voltage
OUT
O
D
) of the selected diode should be
F
) are conducive to high efficiency.
) and low junction capacitance
FRM
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) must be greater than
Lpeak
Lpeak
(Continued)
). Diodes with low
). At the minimum
17
SHUTDOWN AND START-UP
On startup, the LM3503 contains special circuitry that limits
the peak inductor current which prevents large current
spikes from loading the battery or power supply. The
LM3503 is shutdown when both En1 and En2 signals are
less than 0.3V. During shutdown the output voltage is a
diode drop below the supply voltage. When shutdown, the
softstart is reset to prevent inrush current at the next startup.
THERMAL SHUTDOWN
The LM3503 stops regulating when the internal semiconduc-
tor junction temperature reaches approximately 140˚C. The
internal thermal shutdown has approximately 20˚C of hyster-
esis which results in the LM3503 turning back on when the
internal semiconductor junction temperature reaches 120˚C.
When the thermal shutdown temperature is reached, the
softstart is reset to prevent inrush current when the die
temperature cools.
UNDER VOLTAGE PROTECTION
The LM3503 contains protection circuitry to prevent opera-
tion for low input supply voltages. When Vin drops below
2.3V, typically, the LM3503 will no longer regulate. In this
mode, the output voltage will be one diode drop below Vin
and the softstart will be reset. When Vin increases above
2.4V, typically, the device will begin regulating again.
OVER VOLTAGE PROTECTION
The LM3503 contains dedicated ciruitry for monitoring the
output voltage. In the event that the LED network is discon-
nected from the LM3503, the output voltage will increase
and be limited to 15.5V(typ.) for the 16V version, 24V(typ.)
for the 25V version, 34V(typ.) for 35V version and 42V(typ.)
for the 44V version. (see electrical table for more details). In
the event that the network is reconnected regulation will
resume at the appropriate output voltage.
LAYOUT CONSIDERATIONS
All components, except for the white LEDs, must be placed
as close as possible to the LM3503. The die attach pad
(DAP) must be soldered to the ground plane.
The input bypass capacitor C
Application Circuit,, must be placed close to the IC and
connect between the V
copper trace resistance which effects input voltage ripple of
the IC. For additional input voltage filtering, a 100 nF bypass
capacitor can be placed in parallel with C
frequency noise to ground. The output capacitor, C
be placed close to the IC and be connected between the
V
C
Vishay
On
Semiconductor
Central
Semiconductor
OUT1
OUT
capacitor can increase the series resistance, which
and Pgnd pins. Any copper trace connections for the
SS12(1A/20V)
SS14(1A/40V)
SS16(1A/60V)
MBRM120E
(1A/20V)
MBRS1540T3
(1.5A/40V)
MBR240LT
(2A/40V)
CMSH1-40M
(1A/40V)
IN
and Pgnd pins. This will reduce
IN
, as shown in the Typical
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IN
to shunt any high
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OUT
, must